THE POSTERIOR LIMBS. 131 



It must be added that there is, within aud iu front of the cotyloid cavity, a 

 cotyloid nucleus analogous to the g-lenoid nucleus of the scapula. This nucleus, 

 comprised between the three bones of the coxa, has been named by Serres the 

 Y-shaped bone. 



In youth, the diiferent parts of the coxa are very thick, and the spongy 

 tissue is abundant, while the compact is rare. The pubis is always convex on 

 its two faces, and the middle part of the coxa — that adjoining the cotyloid 

 cavity — is of considerable thickness, a feature which much diminishes the extent 

 of the pelvic cavity. As the animal advances iu age, however, the layers of 

 compact tissue increase in thickness, approaching each other as the spongy 

 substance is lessened. The pubis becomes thinnest, aud at an advanced period 

 of life is sometimes even translucid. 



The compact tissue is always abundant in the neighbourhood of the cotyloid 

 cavity, as this is the centre on which converge all the impulsive efforts com- 

 mimicated to the trunk by the posterior limbs. It is also in this cavity that 

 ossification commences. 



B. The Pelvis in Geneeal. 



1. External and Internal Conformation of the Pelvis. — The pelvis 

 is a kind of rear cavity iu the form of a cone, which prolongs the abdominal 

 ca\ity between the sacrum and coccygeal vertebra?. 



It occupies the posterior part of the trunk, aud, with regard to its conforma- 

 tion, presents for study an external and internal surface. 



Extermil surface. — This may be resolved into four planes or faces. 



The superior plane is slightly oblique from above to below, and before to 

 behind ; its degree of obliquity varies. It is contracted from before to behind, 

 and shows : 1. On the median line, the spinous processes of the sacral and the 

 first coccygeal vertebrae. 2. On each side the sacral grooves, at the bottom of 

 which open the supra-sacral canals. 



The inferior plane is nearly horizontal. Formed by the pubes and ischial 

 bones, it presents from before to behind : 1. In the middle, the ischio-pubic 

 symphysis. 2. On each side the subpubic groove, the oval foramina, and the 

 inferior face of the ischial bones. 3. Quite externally, the cotyloid cavities, by 

 which the pelvis rests upon the posterior limbs. 



The latei-al faces are oblique downwards and outwards, and are wider in front 

 than behind. They exhibit : 1, The spine of the iliimi and the two anterior 

 spinous processes. 2. The external iliac fossa. 3. The ischial arch. 4. The 

 supra-cotyloid crest or ischiatic spine, which presents, outwardly, the surface of 

 insertion for the internal or deep gluteus muscles. 5. The lesser ischiatic notch. 

 6. The ischial tuberosity. 



Internal surface. — The internal surface of the Horse's pelvis cannot be 

 divided into two portions as in Man, because the inner surface of the iliac bones 

 is not hollowed out to form an anterior cavity. 



The pelvis of Sohpeds is, therefore, a simple conoid cavity, in which are 

 distinguished four planes or faces, and two apertures called the inlet and 

 outlet. 



The anterior openincj, or inlet, is nearly circular, especially in the Mare, and a 

 little obUque downwards and backwards. It is limited above by the inferior face 

 of the first vertebra of the sacrum ; inferiorly, by the anterior border of the 



