THE RIBS 45 
reduced and much closer together in the back, larger and interlocking in the lumbar 
region. 
The transverse processes are large and outstanding in the neck, where they 
form the lateral boundary of a ventral groove occupied by the longus colli muscle. 
In the back they are short and stout, and are characterized by the facets for the 
tubercles of the ribs. On the first thoracic vertebra this facet is large, deeply con- 
eave, and situated almost directly outward from the cavity for the head of the rib; 
traced backward it becomes smaller and flatter, and gradually comes to lie behind 
the cavity for the head of the rib, with which it is fused on the last and often also on 
the next to the last thoracic vertebra. The processes in the lumbar region have a 
characteristic elongated plate-like form. In the sacral region they are fused to 
form the wings and lateral parts of the sacrum. In the coccygeal region they are 
at first of considerable size relatively, but undergo rapid reduction, and disappear 
at the fifth or sixth vertebra. 
The cavities for the heads of the ribs diminish progressively in size and depth 
from first to last. 
The mammillary processes are usually distinct on the fourteenth to the seven- 
teenth thoracic vertebree. In front of these they blend with the transverse, behind 
with the anterior articular, processes. 
The length of the vertebral column (including the intervertebral fibro-cartilages) in a horse 
of medium size is about nine feet (ca. 2.7 meters). The relative lengths of the various regions ap- 
pear to vary most in the neck and back. The following average lengths of the several regions 
were obtained by measurement of several subjects: Cervical, 70 cm.; thoracic, 86 em.; lumbar, 
34 cm.; sacral, 20 cm.; coccygeal, 60cm. The percentage values are approximately 26, 32, 12.5 
THE RIBS 
There are usually eighteen pairs of ribs in the horse, but a nineteenth rib on 
one side or both is not at all rare. Eight are sternal ribs, the remainder asternal. 
Ribs from different parts of the series vary much in length, curvature, and other 
characters. We will therefore consider as a type a rib from the middle of the series 
first, and afterward note the chief serial differences; such a rib has the following 
characters: 
The shaft or body (Corpus cost) is elongated, relatively very narrow, and 
strongly curved; the curvature is most pronounced in the dorsal third, and the 
ventral part is twisted and inclined inward, so that when a rib is laid with its lateral 
surface on the table, the sternal end is raised. The lateral surface is convex in 
its length and also transversely; its anterior part is, however, grooved longitudin- 
ally. A distinct angle, 7. e., a point at which the curve of the rib changes rather 
suddenly, as in man, can scarcely be said to exist in the horse. The term is some- 
times applied, however, to a corresponding rough elevation which gives attachment 
to the longissimus muscle. The medial surface is smooth, concave in its length, 
and rounded from side to side; the costal groove (Sulcus cost), situated posteriorly, 
is very distinct above and fades out about the middle; it contains the intercostal 
vein. The anterior border is concave, the posterior convex. 
The vertebral extremity (Extremitas vertebralis) consists of the head, neck, 
and tubercle. The head (Capitulum cost) has an articular surface (Facies 
articularis capituli costs) which is composed of two convex facets, anterior and 
posterior, separated by a groove (Sulcus capituli) for the attachment of the con- 
jugal ligament. It articulates with the cavity formed by facets on the bodies of 
two adjacent thoracic vertebre and the intervertebral fibro-cartilage. The neck 
(Collum cost) is roughened above and in front. The tubercle (Tuberculum cost) 
is placed above and behind the junction of neck and shaft; it has an oval surface 
