Part 20

The =scapula= is a flat bone, situated on the anterior part of the lateral wall of the thorax, and extending obliquely from the vertebral end of the seventh or eighth rib to the sternal end of the first rib. It is curved slightly and slopes outward in adaptation to the form of the thoracic wall. It is triangular in outline, and has two surfaces, three borders, and three angles.

The =external surface= or =dorsum= (Facies lateralis s. dorsalis) is divided into two fossæ by the =spine= (Spina scapulæ), which extends from the vertebral border to the neck of the bone, where it gradually subsides. The free edge of the spine is thick, rough, and in great part subcutaneous. A little above its middle is a variable prominence, the =tubercle= of the spine (Tuber spinæ), to which the trapezius muscle is attached. The =supraspinous fossa= (Fossa supraspinata) is situated in front of the spine, and the =infraspinous fossa= (Fossa infraspinata) behind it. The former is much the smaller of the two; it is smooth and is occupied by the supraspinatus muscle. The infraspinous fossa lodges the infraspinatus muscle; it is wide and smooth in its upper part, narrower below, where it is marked by several rough lines for the attachment of the teres minor muscle; near the neck is the nutrient foramen, and a little lower is a vascular groove.

FIG. 41.—LEFT SCAPULA OF HORSE, EXTERNAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =costal surface= or =venter= (Facies costalis) is hollowed in its length by the =subscapular fossa= (Fossa subscapularis); this occupies nearly the whole of the lower part of the surface, but is pointed above and separates two rough triangular areas (Facies serrata), to which the serratus magnus is attached. In the lower third there is a vascular furrow with several branches.

The =anterior= or =cervical border= (Margo cranialis) is convex and rough above, concave and smooth below.

The =posterior= or =dorsal border= (Margo caudalis) is slightly concave. It is thick and rough in its upper third, thin in its middle, and thickens again below.

The =superior= or =vertebral border= (Margo dorsalis s. basis) carries the =scapular cartilage= (Cartilago scapulæ). In the young subject the edge of the bone is thick, and is pitted by impressions into which the cartilage fits. The cartilage is the unossified part of the fœtal scapula. Its lower edge fits the depressions and elevations of the bone. It thins out toward the free edge, which is convex and lies alongside of the upper parts of the vertebral spines. In front it continues the line of the scapula, but behind it forms a rounded projection. The lower part of the cartilage undergoes more or less ossification, so that the vertebral border of the bone in old subjects is thin, irregular, and porous.

FIG. 42.—LEFT SCAPULA OF HORSE, COSTAL SURFACE.—(After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =anterior= or =cervical angle= (Angulus cranialis) is at the junction of the anterior and vertebral borders and lies opposite to the second thoracic spine. It is relatively thin and is about a right angle.

The =posterior= or =dorsal angle= (Angulus caudalis) is thick and rough; its position can be determined readily in the living animal.

The =inferior= or =articular angle= (Angulus glenoidalis) is joined to the body of the bone by the =neck= of the scapula (Collum scapulæ). It is enlarged, especially in the sagittal direction. It bears the =glenoid cavity= (Cavitas glenoidalis) for articulation with the head of the humerus. The cavity is oval in outline, and its margin is cut into in front by the =glenoid notch= (Incisura glenoidalis), and is rounded off externally; just above its postero-external part is a tubercle to which a tendon of the teres minor is attached. The =bicipital tuberosity= or =tuber scapulæ= is the large rough prominence in front, to which the tendon of origin of the biceps brachii is attached; projecting from its inner side is the small =coracoid process= (Processus coracoideus), from which the coraco-brachialis muscle arises.

=Development.=—The scapula has four centers of ossification, viz., one each for the body of the bone, the bicipital and coracoid processes, the anterior part of the glenoid cavity, and the tuber spinæ. The last ossifies after birth and fuses with the spine about the third year. The bicipital tuberosity and coracoid fuse with the body of the bone about the end of the first year.

In old subjects the spongy substance disappears at the middle part of the fossæ, so that the bone consists here of a thin layer of compact substance. Considerable ossification of the cartilage is usual, the borders become much rougher, the muscular lines are more pronounced, and a medullary cavity may appear in the neck. Much variation occurs in dimensions and slope. The average ratio between the length and breadth (scapular index) is about 1 ∶ 0.5, but in many cases the base is relatively wider. The inclination on a horizontal plane varies from 50 to 65 degrees. Exceptionally the coracoid process reaches a length of an inch or more (2½ to 3 cm.), and the chief nutrient foramen may be on the posterior border or in the subscapular fossa.

FIG. 43.—DISTAL END OF LEFT SCAPULA OF HORSE, END VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE HUMERUS

The =humerus= is a long bone which extends from the shoulder above, where it articulates with the scapula, to the elbow below and behind, where it articulates with the radius and ulna. It is directed obliquely downward and backward, forming an angle of about 55 degrees with a horizontal plane. It may be divided into a shaft and two extremities.

FIG. 44.—PROXIMAL END OF LEFT HUMERUS OF HORSE, END VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =shaft= or =body= (Corpus humeri) is irregularly cylindrical and has a twisted appearance. It may be regarded as having four surfaces. The =external surface= is smooth and is spirally curved, forming the =musculo-spiral groove= (Sulcus musculi brachialis), which contains the brachialis muscle; the groove is continuous with the posterior surface above and winds around toward the front below. The =internal surface= is nearly straight in its length, rounded from side to side, and blends with the anterior and posterior surfaces. Just above its middle is the =internal= or =teres tubercle= (Tuberositas teres), to which the tendon of the latissimus dorsi and teres major muscles is attached. The =nutrient foramen= is in the lower third of this surface. The =anterior surface= is triangular, wide and smooth above, narrow and roughened below. It is separated from the external surface by a distinct border, which bears on its upper part the =deltoid tuberosity= (Tuberositas deltoidea). From the latter a rough line curves upward and backward to the outer surface of the neck, and gives origin to the external head of the triceps muscle. Below the tuberosity the border inclines forward, becomes less salient, and ends at the coronoid fossa. The =posterior surface= is rounded from side to side and smooth.

FIG. 45.—LEFT HUMERUS OF HORSE, INTERNAL SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

FIG. 46.—LEFT HUMERUS OF HORSE, EXTERNAL SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =proximal extremity= consists of the head, neck, two tuberosities, and the bicipital groove. The =head= (Caput humeri) presents an almost circular convex articular surface, which is about twice as extensive as the glenoid cavity of the scapula, with which it articulates. In front of the head is a fossa, in which are several foramina. The neck (Collum humeri) is well defined behind, but is practically absent elsewhere. The =external tuberosity= (Tuberculum majus) is placed antero-externally, and consists of two parts; the anterior part forms the outer boundary of the bicipital groove and gives attachment to the external branch of the supraspinatus muscle; the posterior part gives attachment to the short insertion of the infraspinatus, while its outer surface is coated with cartilage, over which the chief tendon of the same muscle passes to be inserted into a triangular facet on the outer aspect of the anterior part. The =internal tuberosity= (Tuberculum minus) is less salient, and consists of anterior and posterior parts; the anterior part forms the inner boundary of the bicipital groove, and furnishes insertion to the inner branch of the supraspinatus above, and the posterior deep pectoral muscle below; the posterior part gives attachment to the subscapularis muscle. The =bicipital= or =intertubercular groove= (Sulcus intertubercularis) is situated in front; it is bounded by the anterior parts of the tuberosities, and is subdivided by an intermediate ridge. The groove is covered in the fresh state by cartilage, and lodges the tendon of origin of the biceps brachii muscle. Just below the intermediate ridge is a small fossa in which several foramina open.

FIG. 47.—LEFT HUMERUS OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =distal extremity= has an oblique surface for articulation with the radius and ulna, which consists of two condyles of very unequal size, separated by a ridge. The =internal condyle= (Condylus medialis) is much the larger, and is crossed by a sagittal groove, on the anterior part of which there is usually a synovial fossa. Posteriorly the groove extends upward considerably above the rest of the articular surface and reaches the olecranon fossa, and this part articulates with the semilunar notch of the ulna. The =external condyle= (Condylus lateralis) is much smaller and is placed somewhat lower and further back, giving the extremity an oblique appearance; it is marked by a wide shallow groove. The =coronoid fossa= (Fossa coronoidea) is situated in front above the groove on the internal condyle; it furnishes origin to part of the extensor carpi, and external to it is a rough depression from which the anterior or common extensor of the digit arises. Behind and above the condyles are two thick ridges, the epicondyles. The =internal= or =flexor epicondyle= (Epicondylus medialis s. flexorius) is the more salient; it furnishes origin to flexor muscles of the carpus and digit, and presents internally a tubercle for the attachment of the internal lateral ligament of the elbow joint. The =external= or =extensor epicondyle= (Epicondylus lateralis s. extensorius) bears externally the =external supracondyloid crest= (Crista condyloidea lateralis), which forms here the outer boundary of the musculo-spiral groove, and gives origin to the extensor carpi. Below this is a rough excavation in which the external lateral ligament is attached. The lower border of the epicondyle gives attachment to the flexor carpi externus. Between the epicondyles is the deep =olecranon fossa= (Fossa olecrani).

=Development.=—The humerus ossifies from six centers, viz., three primary centers for the shaft and epiphyses, and three secondary centers for the external tuberosity, the deltoid tuberosity, and the internal condyle. The proximal end fuses with the shaft at about three and one-half years, the distal at about one and a half years of age.

THE RADIUS

The =radius= is much the larger of the two bones of the forearm in the horse. It extends in a vertical direction from the elbow, where it articulates with the humerus, to the carpus below. It is gently curved, the convexity being anterior. It consists of a shaft and two extremities.

The =shaft= (Corpus radii) is curved in its length, somewhat flattened from before backward, and expanded at its ends. It presents for description two surfaces and two borders. The =anterior surface= (Facies dorsalis) is smooth, slightly convex in its length, and rounded from side to side. The =posterior surface= (Facies volaris) is correspondingly concave in its length and is flattened in the transverse direction. At its upper part there is a smooth shallow groove, which concurs with the ulna in the formation of the =interosseous space= of the forearm; the nutrient foramen is in the lower part of this groove. Below this there is in the young subject a narrow, rough, triangular area to which the ulna is attached by an interosseous ligament; in the adult the two bones are fused here. A variable rough elevation below the middle of the surface and close to the internal border gives attachment to the superior check ligament. The =internal border= (Margo medialis) is slightly concave in its length and is largely subcutaneous; at its proximal end there is a smooth area on which the tendon of insertion of the brachialis muscle lies, and a small rough area just below gives attachment to that muscle and the long internal lateral ligament of the elbow joint. The =external border= (Margo lateralis) is more strongly curved, but presents no special features.

The =proximal extremity= or =head= (Capitulum radii) is flattened from before backward and wide transversely. It presents an articular surface (Fovea capituli) which corresponds to that on the distal end of the humerus; it is crossed by a central sagittal ridge, which has a synovial fossa on its posterior part, and ends in front at a prominent lip, the =coronoid process= (Processus coronoideus). Just below the posterior border there are two concave facets for articulation with the ulna, and between these and the interosseous space is a quadrilateral rough area at which the two bones are united by an interosseous ligament. At the inner side of the anterior surface is the =bicipital tuberosity= (Tuberositas radii), into which the biceps tendon is inserted. The =internal tuberosity= is continuous with the preceding eminence, and furnishes attachment to the short part of the internal lateral ligament. The =external tuberosity= is more salient; it gives attachment to the external lateral ligament and to the anterior and lateral extensor muscles of the digit.