THE POSTERIOR LIMBS. 95 



forms, with that of the other ischium, the summit of the triangular space 

 which constitutes the ischiatic arch, or pubic arch of some species. 



THE COXA IN GENERAL. The bone whose three constituent parts we have 

 just been studying, presents for consideration, as a whole, a middle portion 

 and two extremities. The middle, very much contracted, offers, outwards 

 and downwards, the coiyloid cavity (or acetabulum), which has not yet been 

 described, because its study does not properly pertain to either of the three 

 regions of the coxa. This cavity is intended to receive the articulating head 

 of the femur, and represents the segment of a hollow sphere ; it is circum- 

 scribed by a very salient rim which is thin at its free margin and widely 

 notched on the inner side. The deeper portion is occupied by the rough- 

 ened and depressed surface already designated as the bottom of the cotyloid 

 cavity (fundus acetabuli), and which communicates by the internal notch of 

 the rim with the inferior groove of the pubis. The anterior extremity, 

 flattened on both sides, and formed by the ilium, rests, as has been shown, 

 on the sacrum. The posterior extremity, flattened in an inverse sense to the 

 preceding, is constituted by the pubis and the ischium, and traversed, from 

 above to below, by the sub-pubic (or obturator) foramen, the large oval 

 aperture which separates these two bones from one another, and perforates 

 the floor of the pelvis ; this opening is closed in the fresh state by muscles. 



The two coxae, by uniting in their posterior part, form the articulation 

 to which has been given the name of ischio-pubic or pelvic symphysis ; thus 

 united, the two bones represent something like a V with the opening in 

 front ; a circumstance which makes the lateral diameter of the pelvis greater 

 in front than behind. 



STRUCTURE AND DEVELOPMENT OF THE COXA. To the three centres of 

 ossification which constitute the coxa, are added two complementary 

 centres : one for the anterior spinous process and spine of the ilium, another 

 for the ischiatic tuberosity. 



In youth, the different 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 reservoir. As the animal advances in age, however, 

 the layers of compact tissue increase in thickness, approaching each 

 other as the spongy substance is lessened. The pubis becomes thinnest, and 

 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 communicated to the trunk by the posterior limbs. It is also in this 

 cavity that ossification commences. 



B. The Pelvis in General 



1. EXTERNAL AND INTERNAL CONFORMATION OF THE PELVIS. The pelvis is a 

 Idnd of rear cavity in the form of a cone, which prolongs the abdominal cavity. 



It occupies the posterior part of the trunk, and with regard to its 

 conformation, presents for study an external and an internal surface. 



External surface. This may be resolved into four regions or faces. 



The superior region 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 supersacral canals. 



