DESCRIPTIVE AND SURGICAL ANATOMY

THE UPPER EXTREMITY

The Upper Extremity consists of four parts—the shoulder, the arm, the fore­arm, and the hand. The shoulder consists of two bones, the clavicle and the scapula.

THE CLAVICLE

The Clavicle (clavis, a 'key'), or collar-bone, is a long bone, curved somewhat like the italic letter^ and placed horizontally at the upper and lateral part of the thorax, immediately above the first rib. It articulates internally with the upper border of the sternum, and with the acromion process of the scapula by its outer extremity; serving to sustain the upper extremity in the various positions which it assumes, whilst at the same time it allows it great latitude of motion. The horizontal plane of the clavicle is nearly straight; but in the vertical plane it presents a double curvature, the convexity being in front at the sternal end, and behind at the scapular end. Its inner two-thirds are of a triangular prismatic form, and extend, in the natural position of the bone, from the sternum to the coracoid process of the scapula; the outer fourth being flattened from above downwards, and extending from the coracoid process to the acromion. It pre­sents for examination two surfaces, two borders, and two extremities.

The superior surface, for the inner three-fourths of its extent, is narrow, smooth, of equal diameter throughout, and presents near the sternal end impressions for the attachment of the Sterno-mastoid muscle behind, the Pectoralis major in front. Its outer fourth is broad, flat, uneven, perforated by numerous foramina, and covered by the fibres of the Deltoid and Trapezius muscles, which encroach upon it considerably in front and behind.

The inferior surface is also narrow for the inner three-fourths of its extent, broader and more flattened externally. Commencing at the sternal extremity, may be seen a small facet for articulation with the cartilage of the first rib, continuous with the articular surface at the sternal end of the bone. External to this a rough impression, the rhomboid, for the attachment of the costo-clavicular (rhomboid) ligament. The middle third of this surface is occupied by a longitu­dinal groove, the subclavian groove, broader externally than internally; it gives attachment to the Subclavius muscle, and by its anterior margin to the strong aponeurosis which encloses it; internally is a rough surface, the limit of the attach­ment of the Pectoralis major below. At the junction of the prismatic with the flattened portion of the bone, at its posterior border, may be seen a rough eminence, the tubercle of the clavicle. This, in the natural position of the bone, surmounts the coracoid process of the scapula, and affords attachment to the conoid ligament. From this tubercle an oblique line passes forwards and outwards on the under surface of the acromial extremity. It is called the oblique line of the clavicle, and affords attachment to the trapezoid ligament.

The anterior border is broad and convex for its sternal half, and presents a rough impression for the attachment of the Pectoralis major muscle. Its outer half is a narrow, concave margin, serving for the attachment of the Deltoid: a small interval is usually left between the attachments of these muscles, where this border is smooth, receiving no muscular fibres. This is the narrowest part of the clavicle, and hence the most common seat of fracture.

The posterior border is broad and deeply concave for the inner two-thirds of its extent, affording attachment internally by a small extent of surface, to the Sterno-hyoid; convex, narrow, and irregular in its outer third, for the attachment of the Trapezius. This border corresponds to the subclavian vessels and brachial plexus of nerves, and presents, towards its centre, the foramen for the nutritious artery of the bone.

The internal or sternal end of the clavicle is directed inwards, and a little downwards and forwards; it presents a large triangular facet, concave from before backwards, convex from above downwards, which articulates with the sternum through the intervention of an inter-articular fibre-cartilage; the circumference of the articular surface is rough, for the attachment of numerous ligaments. (rhomboid) ligament. The middle third of this surface is occupied by a longitu­dinal groove, the subclavian groove, broader externally than internally; it gives attachment to the Subclavius muscle, and by its anterior margin to the strong aponeurosis which encloses it; internally is a rough surface, the limit of the attach­ment of the Pectoralis major below. At the junction of the prismatic with the flattened portion of the bone, at its posterior border, may be seen a rough eminence, the tubercle of the clavicle. This, in the natural position of the bone, surmounts the coracoid process of the scapula, and affords attachment to the conoid ligament. From this tubercle an oblique line passes forwards and outwards on the under surface of the acromial extremity. It is called the oblique line of the clavicle, and affords attachment to the trapezoid ligament.

The anterior border is broad and convex for its sternal half, and presents a rough impression for the attachment of the Pectoralis major muscle. Its outer half is a narrow, concave margin, serving for the attachment of the Deltoid: a small interval is usually left between the attachments of these muscles, where this border is smooth, receiving no muscular fibres. This is the narrowest part of the clavicle, and hence the most common seat of fracture.

The posterior border is broad and deeply concave for the inner two-thirds of its extent, affording attachment internally by a small extent of surface, to the Sterno-hyoid; convex, narrow, and irregular in its outer third, for the attachment of the Trapezius. This border corresponds to the subclavian vessels and brachial plexus of nerves, and presents, towards its centre, the foramen for the nutritious artery of the bone.

The internal or sternal end of the clavicle is directed inwards, and a little downwards and forwards; it presents a large triangular facet, concave from before backwards, convex from above downwards, which articulates with the sternum through the intervention of an inter-articular fibre-cartilage; the circumference of the articular surface is rough, for the attachment of numerous ligaments.

The Calvicle

The outer extremity, directed forwards and outwards, presents a small oval facet, for articulation with the acromion process of the scapula.

Peculiarities of this Bone in the Sexes and in Individuals. In the female, the clavicle is less curved, smoother, longer, and more slender than in the male. In those persons who perform considerable manual labour, which brings into constant action the muscles connected with this bone, it acquires considerable bulk, becomes shorter, more curved, its ridges for muscular attachment become prominently marked, and its sternal end of a prismatic or quadrangular form.

Structure. The shaft as well as the extremities consists of cancellous tissue, invested in a compact layer much thicker in the centre than at either end. The clavicle is highly elastic, by reason of its curves. From the experiments of Mr. Ward, it has been shewn that it possesses sufficient longitudinal elastic force to raise its own weight nearly two feet on a level surface; and sufficient transverse elastic force, opposite the centre of its anterior convexity, to raise its own weight about a foot. This extent of elastic power must serve to moderate very consider­ably the effect of concussions received upon the point of the shoulder.

Development. By two centres: one for the shaft, and one for the sternal end of the bone. The centre- for the shaft appears very early, before any other bone; the second centre makes its appearance about the eighteenth or twentieth year, and unites with the rest of the bone a few years after.

Articulations. With the sternum, scapula, and cartilage of the first rib.

Attachment of Muscles. The Sterno cleido-mastoid, Trapezius, Pectoralis major, Deltoid, Subclavius, and Sterno-hyoid.

THE SCAPULA

The Scapula is a large flat bone, triangular in shape, which forms the back part of the shoulder. It is situated at the posterior aspect and side of the thorax, between the first and seventh ribs, and presents for examination two surfaces, three borders, and three angles.

The anterior surface, or venter, presents a broad concavity, the sub-scapular fossa. It is marked, in the posterior two thirds, by several oblique ridges, which pass from behind obliquely forwards and upwards, the anterior third being smooth. The oblique ridges above-mentioned, give attachment to the tendinous intersections, and the surfaces between them, to the fleshy fibres of the Subscapu-laris muscle. The anterior third of the fossa, which is smooth, is covered by, but does not afford attachment to, the fibres of this muscle. This surface is sepa­rated from the posterior border, by a smooth triangular margin at the superior and inferior angles, and in the interval between these, by a narrow margin which is often deficient. This marginal surface affords attachment throughout its entire extent to the Serratus magnus muscle. The subscapular fossa presents a trans­verse depression at its upper part, called the subscapular angle; it is in this situ­ation that the fossa is deepest, and consequently the thickest part of the Subscapularis muscle lies in a line parallel with the glenoid cavity, and must consequently operate most effectively on the humerus which is contained in it.

Left Scapula, Anterior Surface

The posterior surface, or dorsum is convex from above downwards, alternately convex and concave from side to side. It is subdivided unequally into two parts by the spine; that portion above the spine is called the supra spinous fossa, and that below it, the infra spinous fossa.

Left Scapula, Posterior Surface

The supra spinous fossa, the smaller of the two, is concave, smooth, and broader towards the vertebral than at the humeral extremity. It affords attachment by its inner two-thirds to the fibres of the Supra spinatus muscle.

The infra spinous fossa is much larger than the preceding; towards its inner side a shallow concavity is seen at its upper part; its centre presents a prominent convexity, whilst towards the axillary border is a deep groove, which runs from the upper towards the lower part. The inner three-fourths of this surface affords attachment to the Infra-spinatus muscle; the outer fourth is only covered by it, without giving origin to its fibres. This surface is separated from the axillary border by an elevated ridge, which runs from the lower part of the glenoid cavity, downwards and backwards to the posterior border, about an inch above the infe­rior angle. This ridge serves for the attachment of a strong aponeurosis, which separates the Infra-spinatus from the two Teres muscles. The surface of bone between this line and the axillary border is narrow for the upper two-thirds of its extent, and traversed near its centre by a groove for the passage of the dorsalis scapulae artery; it affords attachment to the Teres minor. Its lower third presents a broader, somewhat triangular surface, which gives origin to the Teres major, and over which glides the Latissimus dorsi muscle; sometimes this muscle takes origin by a few fibres from this part. The broad and narrow portions of bone above alluded to are separated by an oblique line, which runs from the axillary border, downwards and backwards; to it is attached the aponeurosis separating the two Teres muscles from each other.

The Spine is a prominent plate of bone, which crosses obliquely the inner three-fourths of the dorsum of the scapula at its upper part, and separates the supra from the infra spinous fossa: it commences at the vertebral border by a smooth triangular surface, over which the trapezius glides, separated by a bursa; and, gradually becoming more elevated as it passes forwards, terminates in the acromion process which overhangs the shoulder joint. The spine is triangular and flattened from above downwards, its apex corresponding to the posterior border, its base, which is directed outwards, to the neck of the scapula. It presents two surfaces and three borders. Its superior surface is concave, assists in forming the supra-spinous fossa, and affords attachment to part of the Supra-spinatus muscle. Its inferior surface forms part of the infra-spinous fossa, gives origin to part of the Infra-spinatus muscle, and presents near its centre the orifice of a nutritious canal. Of the three borders, the anterior is attached to the dorsum of the bone; the posterior, or crest of the spine, is broad, and presents two lips, and an intervening rough interval. To the superior lip is attached the Trapezius, to the extent shown in the figure. A very rough prominence is generally seen occupying that portion of the spine which receives the insertion of the middle and inferior fibres of this muscle. To the inferior lip, its whole length, is attached the Deltoid. The interval between them is also partly covered by the fibres of these muscles. The external border, the shortest of the three, is slightly concave, its edges thick and round, continuous above with the under surface of the acromion process, below with the neck of the scapula. The narrow portion of bone external to this border, serves to connect the supra and infra spinous fossas.

The Acromion process, so called from forming the summit of the shoulder joint (aicpov, a summit, the shoulder), is a large, and somewhat triangular process, flattened from behind forwards, directed at first a little outwards, and then curving forwards and upwards, so as to overhang the glenoid cavity. Its upper surface directed upwards, backwards, and outwards, is convex, rough, and partly covered by some of the fibres of origin of the Deltoid. Its under surface is smooth and con­cave. Its outer border, which is thick and irregular, affords attachment to the Deltoid muscle. Its inner margin, shorter than the outer, is concave, gives attach­ment to a portion of the Trapezius muscle, and presents about its centre a small oval surface, for articulation with the scapular end of the clavicle. Its apex, formed at the point of meeting of these two borders in front, is thin, and has attached to it the coraco-acromion ligament.

Of the three borders or costae of the scapula, the superior is the shortest and thinnest; it is concave, terminating at its inner extremity at the superior angle, at its outer extremity at the coracoid process. At its outer part is a deep semicircular notch, formed partly by the base of the coracoid process. This notch is converted into a foramen by the transverse ligament, and serves for the passage of the supra scapular nerve. The adjacent margin of the superior border affords attachment to the Omo-hyoid muscle. The external, or axillary border, is the thickest of the three. It commences above at the lower margin of the glenoid cavity, and inclines obliquely downwards and backwards to the inferior angle. Imme­diately below the glenoid cavity, is a rough depression about an inch in length, which affords attachment to the long head of the Triceps muscle; to this succeeds a longitudinal groove which extends as far as its lower third, and affords origin to part of the Subscapularis muscle. The inferior third of this border which is thin and sharp, serves for the attachment of a few fibres of the Teres major behind, and of the Subscapularis in front. The internal, or vertebral border, also named the base, is the longest of the three, and extends from the superior to the inferior angle of the bone. It is convex, intermediate in thickness between the superior and the external, and that portion of it above the spine bent considerably outwards, so as to form an obtuse angle with the lower part. This border presents an ante­rior lip, a posterior lip, and an intermediate space. The anterior lip affords attachment to the Serratusmagnus; the posterior lip, to the Supra-spinatus above the spine, the Infra-spinatus below; the interval between the two lips, to the Leva-tor auguli scapulas above the triangular surface at the commencement of the spine, the Rhomboideus minor, to the edge of that surface; the Rhomboideus major being attached by means of a fibrous arch, connected above to the lower part of the triangular surface at the base of the spine, and below to the lower part of the pos­terior border.

Of the three angles, the superior, formed by the junction of the superior and internal borders, is thin, smooth, rounded, somewhat inclined outwards, and gives attachment to a few fibres of the Levator anguli scapulae muscle. The inferior angle thick and rough, is formed by the union of the vertebral and axillary borders, its outer surface affording attachment to the Teres major, and occasionally a few fibres of the Latissimus dorsi. The anterior angle is the thickest part of the bone, and forms what is called the head of the scapula. The head presents a shallow, pyriform, articular surface, the glenoid cavity (yXijvi), a superficial cavity; et&»?, like); its longest diameter is from above downwards, and its direction outwards and forwards. It is broader below than above; at its apex is attached the long tendon of the Biceps muscle. It is covered with cartilage in the recent state; and its margins, slightly raised, give attachment to a fibro-cartilaginous structure, the glenoid ligament, by which its cavity ib deepened. The neck-of the scapula is the slightly depressed surface which surrounds the head, it is more distinct on the posterior than on the anterior surface, and below than above. In the latter situation, it has, arising from it, a thick prominence, the coracoid process.

The Coracoid process, so called from its fancied resemblance to a crow's beak (/copal;, a crow; ?(?09, like), is a thick curved process of bone, which arises by a broad base from the upper part of the neck of the scapula; it ascends at first upwards and inwards, then becoming smaller, it changes its direction and passes forwards and outwards. The ascending portion, flattened from before backwards, presents in front a smooth concave surface, over which passes the sub-scapularis muscle. The horizontal portion is flattened from above downwards; its upper surface is convex and irregular; its under surface is smooth; its anterior border is rough, and gives attachment to the Pectoralis minor, its posterior also rough to the coraco-acromion ligament, while the apex is embraced by the conjoined tendon of origin of the short head of the Biceps and Coraco-brachialis muscles. At the inner side of the root of the coracoid process is a rough depression for the attachment of the conoid ligament, and running from it obliquely forwards and outwards on the upper surface of the horizontal portion, an elevated ridge for the attachment of the trapezoid ligament.

Structure. In the head, processes, and all the thickened parts of the bone, it is cellular in structure, of a dense compact tissue in the rest of its extent. The centre and upper part of the dorsum, but especially the former, ia usually so thin as to be semi-transparent; occasionally the bone is found wanting in this situ­ation, and the adjacent muscles come into contact.

Development. By seven centres; one for the body, two for the coracoid process, two for the acromion, one for the posterior border, and one for the inferior angle.

That for the body makes its first appearance at about the same period that osseous matter is deposited in the vertebrae, and forms the chief part of the bone. At birth, all the other centres are cartilaginous. About the first year after birth, osseous deposition occurs in the middle of the coracoid process; which usually becomes joined with the rest of the bone at the time when the other centres make their appearance. Between the fifteenth and seventeenth years, osseous matter is deposited in the remaining centres in quick succession, and in the following order: first, near the base of the acromion, and in the upper part of the coracoid process, the latter appearing in the form of a broad scale; secondly, in the inferior angle and contiguous part of the posterior border; thirdly, near the extremity of the acromion; fourthly, in the posterior border. The acromion process, besides being formed of two separate nuclei, has its base formed by an extension into it of the centre of ossification which belongs to the spine, the extent of which varies in different cases. The two separate nuclei unite, and then join with the extension carried in from the spine. These various epiphyses become united to the bone between the ages of twenty-two and twenty-five years.

Development of Scapula

Articulations. With the humerus and clavicle.

Attachment of Muscles. To the anterior surface, the Subscapularis; posterior surface, Supra-spinatus, Infra-spinatus; spine, Trapezius, Deltoid; superior border, Omo-hyoid; vertebral border, Serratus magnus, Levator anguli scapulae, Rhomboideus minor and Major; axillary border, Triceps, Teres minor, Teres major, glenoid cavity; long head of the Biceps, coracoid process; short head of Biceps, Coraco-brachialis, Pectoralis minor; and to the inferior angle occasionally a few fibres of the Latissimua dorsi.

THE HUMERUS

The Humerus is the longest and largest bone of the upper extremity; it pre­sents for examination a shaft and two extremities.

The Superior Extremity is the largest part of the bone; it presents a rounded head, a constriction around the base of the head, the neck, and two other emi­nences, the greater and lesser tuberosities.

Humerus

The head, nearly hemispherical in form, is directed inwards, upwards, and a little backwards; its surface is smooth, coated with cartilage in the recent state, and articulates with the glenoid cavity of the scapula. The circumference of its articular surface is slightly constricted, and is termed the anatomical neck, in contradistinction to the constriction which exists below the tuberosities, and is called the surgical neck, from its being the seat of the accident called by surgeons, 'fracture of the neck of the humerus.'

The neck, which is obliquely directed, forming an obtuse angle with the shaft, is more distinctly marked in the lower half of its circumference, than in the upper half, where it presents a narrow groove, separating the head from the tuberosities. Its circumference affords attachment to the capsular ligament, and is perforated by numerous vascular foramina.

The greater tuberosity is situated on the outer side of the head and lesser tuberosity. Its superior surface is rounded and marked by three flat facets, separated by two slight ridges, the most anterior giving attachment to the tendon of the Supra-spinatus; the middle, to the Infra-spinatus; the posterior, to the Teres minor. The external surface of the great tuberosity is convex, rough, and continuous with the outer side of the shaft.

The lesser tuberosity is more prominent, although smaller than the greater; it is situated in front of the head, and is directed inwards and forwards. Its

summit presents a prominent facet for the insertion of the tendon of the Subsca-pularis muscle. These two tuberosities are separated from one another by a deep groove, the biclpital groove, so called from its lodging the long tendon of the Biceps muscle. It commences above between the two tuberosities, passes obliquely downwards and a little inwards, and terminates at the junction of the upper with the middle third of the bone. It is deep and narrow at its commencement, and becomes shallow and a little broader as it descends. In the recent state it is covered with a thin layer of cartilage, lined by a prolongation of the synovial membrane of the shoulder joint, and receives part of the tendon of insertion of the Latissimus dorsi about its centre.

The Shaft of the humerus is almost cylindrical in the upper half of its extent; prismatic and flattened below, it presents three borders and three surfaces for examination.

The external border runs from the back part of the greater tuberosity to the external condyle, and separates the external from the posterior surface. It is rounded and indistinctly marked in its upper half, and serves for the attachment of the external head of the triceps muscle; its centre is traversed by a broad but shallow oblique depression, the musculo-spiral groove; its lower part is marked by a prominent rough margin, a little curved from behind forwards, which presents an anterior lip for the attachment of the Supinator longus above, the Extensor carpi radialis longior below, a posterior lip for the Triceps, and an interstice for the attachment of the external intermuscular aponeurosis.

The internal border extends from the lesser' tuberosity above to the internal condyle below. Its upper third is marked by a prominent ridge, forming the inner lip of the bicipital groove, and giving attachment from above downwards to the tendons of the Latissimus dorsi, Teres major, and part of the origin of the inner head of the Triceps. About its centre is a rough ridge for the attachment of the Coraco-brachialis, and just below this is seen the entrance of the nutritious canal directed downwards. Its inferior third is raised into a slight ridge, which becomes very prominent below; it presents an anterior lip for the attachment of the Brachialis anticus, a posterior lip for the internal head of the Triceps, and an intermediate space for the internal intermuscular aponeurosis.

The anterior border runs from the front of the great tnberosity above, to the coronoid depression below, separating the internal from the external surface. Its upper part is very prominent and rough, forms the outer lip of the bicipital groove, and serves for the attachment of the tendon of the Pectoralis major. About its centre is seen the rough deltoid impression; below, it is smooth and rounded, affording attachment to the Brachialis anticus.

The external surface is directed outwards above, where it is smooth, rounded, and covered by the Deltoid muscle; forwards below, where it is slightly concave from above downwards, and gives origin to part of the Brachialis anticus muscle. About the middle of this surface, is seen a rough triangular impression for the insertion of the -Deltoid muscle, and below it the musculo-spiral groove, directed obliquely from behind, forwards and downwards; it transmits the musculo-spiral nerve and superior profunda artery.

The internal surface, less extensive than the exterrial, is directed forwards above, inwards and forwards below: at its upper part it is narrow, and forms the bicipital groove. The middle part of this surface is slightly rough for the attachment of the Coraco-brachialis; its lower part is smooth, concave, and gives attachment to the Brachialis anticus muscle.

The posterior surface, appears somewhat twisted, so .that its superior part is directed a little inwards, its inferior part backwards and a little outwards. Nearly the whole of this surface is covered by the external and internal heads of the triceps, the former being attached to its upper and outer part, the latter to its inner and back part, their origin being separated by the musculo-spiral groove.

The Lower Extremity is flattened from before backwards, and curved slightly forwards; it terminates below in a broad articular surface, which is divided into two parts by a shallow groove; on either side of the articulate surface are the ex­ternal and internal condyles. The articular surface extends a little lower than the con-dyles, and is curved slightly forwards, so as to occupy the more anterior part of the bone; its greatest breadth is in the trans­verse diameter, and it is obliquely directed, so that its inner extremity occupies a lower level than the outer. The outer portion of this articular surface presents a smooth rounded eminence, which has received the name of the lesser or radial head of the humerus; it articulates with the cup-shaped depression on the head of the radius, is limited to the front and lower part of the bone, and does not extend as far back as the other portion of the articular surface. On the inner side of this eminence is a shallow groove, in which is received the inner margin of the cup-like cavity of the head of the radius. The inner or trochlear portion of the articular surface presents a deep depression between two well-marked borders. This surface is curved from before backwards, concave from side to side, and occupies the anterior lower and posterior part of the bone. The external border, less prominent than the internal, corresponds to the interval between the radius and ulnar. The internal border is thicker, more prominent, and, consequently, of greater length than the external. The grooved portion of the articular surface fits accurately within the greater sigmoid cavity of the ulna; it is broader and deeper on the posterior than on the anterior aspect of the bone, and is directed obliquely from behind forwards, and from without inwards. Immediately above the back part of the trochlear surface, is a deep triangular de­pression, the olecranon depression, in which is received the summit of the olecranon process in extension of the fore-arm. Above the front part of the trochlear surface, is seen a smaller depression, the coronoid de­pression; it receives the coronoid process of the ulna during flexion of the fore-arm. These fossae are separated from one another by a thin lamina of bone, which is some­times perforated; their margins afford at­tachment to the anterior and posterior ligaments of the elbow joint, and they are lined in the recent state by the synovial membrane of this articulation. Above the front part of the radial tuberosity, is seen a slight depression which receives the anterior border of the head of the radius when the fore-arm is strongly flexed. The external condyle is a small tubercular eminence, less prominent than the internal, curved a little forwards, and giving attachment to the external lateral ligament of the elbow joint, and to a tendon common to the origin of some of the extensor and supinator muscles. The internal condyle, larger and more promi­nent than the external, is directed a little backwards, it gives attachment to the internal lateral ligament, and to a tendon common to the origin of some of the flexor muscles of the fore-arm. These eminences are directly continuous above with the external and internal borders.

Structure. The extremities consist of cancellous tissue, covered with a thin compact layer; the shaft is composed of a cylinder of compact tissue, thicker at the centre than at the extremities, and hollowed out by a large medullary canal.

Development of Humerus

Development. By seven centres; one for the shaft, one for the head, one for the greater tu-berosity, one for the radial, and one for the trochlear portion of the articular surface, and one for each condyle. The centre for the shaft ap­pears very early, soon after ossifica­tion has commenced in the cavicle, and soon extends towards the extremi­ties. At birth, it is ossified nearly in its whole length, the extremities re­maining cartilaginous. Between the first and second years, ossification com­mences in the head of the bone, and between the second and third years the centre for the tuberosities marks its appearance usually by a single ossific point, but sometimes, according to Beclard, by one for each tuberosity, that for the lesser being small, and not appearing until after the fourth year. By the fifth year, the centres for the head and tuberosities have enlarged and become joined, so as to form a single large epiphysis.

The lower end of the humerus is developed in the following manner: At the end of the second year, ossifi­cation commences in the radial portion

of the articular surface, and from this point extends inwards, so as to form the chief part of the articular end of the bone, the centre for the inner part of the articular surface not appearing until about the age of twelve. Ossification commences in the internal condyle about the fifth year, and in the external one not until between the age of thirteen or fourteen. About sixteen or seventeen years, the outer condyle and both portions of the articulating surface (having already joined) unite with the shaft; at eighteen years, the inner condyle becomes joined, whilst the upper epiphysis, although the first formed, is not joined until about the twentieth year.

Articulations. With the glenoid cavity of the scapula, and with the ulna and radius.

Attachment of Muscles. To the greater tuberosity, the Supra-spinatus, Infra-spinatus, and Teres minor; to the lesser tuberosity, the Subscapularis; to the ante­rior bicipital ridge, the Pectoralis major; to the posterior bicipital ridge and groove, the Latissimus dorsi and Teres major; to the shaft, the Deltoid, Coraco-brachialis, Brachialis anticus, External and Internal heads of the Triceps; to the internal

condyle, the Pronator radii teres, and common tendon of the Flexor carpi radialis, Palmaris longus, Flexor digitorum sublimis, and Flexor carpi ulnaris ; to the external condyloid ridge, the Supinator longus, and Extensor carpi radialis longior; to the external condyle, the common tendon of the Extensor carpi radialis brevior, Extensor communis digitorum, Extensor minimi digiti, and Ex­tensor carpi ulnaris, the Anconeus, and Supinator brevis.

The Fore-arm is that portion of the upper extremity, situated between the elbow and wrist. It is composed of two bones, the Ulna and Radius. NEXT PAGE