88 THE SKELETON OF THE HORSE 
of the biceps brachii is attached; projecting from its medial 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 tuber scapulze and coracoid process, the anterior part of 
the glenoid cavity, and the tuber spine. The last ossifies after birth and fuses with 
the spine about the third year. The tuber scapulze and coracoid process 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 fosse, 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 medul- 
lary cavity may appear in the neck. Much variation occurs in dimensions and slope. The aver- 
age 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. Ex- 
ceptionally the coracoid process reaches a length of an inch or more (21% to 3 em.), and the chief 
nutrient foramen may be on the posterior border or in the subscapular fossa. 
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, form- 
ing an angle of about 55 degrees with a horizontal plane. It consists of a shaft 
and two extremities. 
The shaft (Corpus humeri) is irregularly cylindrical and has a twisted appear- 
ance. It may be regarded as having four surfaces. The lateral surface (Facies 
lateralis) is smooth and is spirally curved, forming the musculo-spiral groove 
(Suleus musculi brachialis), which contains the brachialis muscle; the groove is 
continuous with the posterior surface above and winds around toward the front 
below. The medial surface (Facies medialis) is nearly straight im its length, 
rounded from side to side, and blends with the anterior and posterior surfaces. 
Just above its middle is the teres tuberosity (Tuberositas teres), to which the 
tendon of the latissimus dorsi and teres major muscles is attached. The nutrient 
foramen is in the distal third of this surface. The anterior surface (Facies crani- 
alis) is triangular, wide and smooth above, narrow and roughened below. It is 
separated from the lateral surface by a distinct border, the crest of the humerus 
(Crista humeri), which bears above its middle the deltoid tuberosity (Tuberositas 
deltoidea). From the latter a rough line curves upward and backward to the 
lateral surface of the neck, and gives origin to the lateral head of the triceps muscle. 
Below the tuberosity the border inclines forward, becomes less salient, and ends at 
the coronoid fossa. The posterior surface (Facies caudalis) is rounded from side 
to side and smooth. 
The proximal extremity (Extremitas proximalis) consists of the head, neck, 
two tuberosities, and the intertuberal 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 lateral tuberosity (Tuberositas 
lateralis)? is placed antero-laterally, and consists of two parts: the anterior part 
forms the lateral boundary of the intertuberal or bicipital groove and gives attach- 
ment to the lateral 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 
1 The proper “anatomical neck”’ (Collum anatomicum) is, however, indicated by the shallow 
depression which separates the head from the tuberosities, and gives attachment to the joint capsule. 
2 The term “tubercle”? as used in human anatomy does not apply well to the domestic ani- 
mals. 
