THE SKELETON. 255 



In the artioulated skeleton it is usual to vonsider the 

 thorax as made up by the eighteen dorsal vertebrae superiorly, the 

 eighteen ribs and their cartilages on each side, and the sternum 

 with its cartilages below. But the cavity of the thorax, as bounded 

 by the diaphragm posteriorly, is not nearly so large as would be 

 supposed from a consideration of the dry skeleton, for though the 

 diaphragm is attached to the twelve posterior ribs near their car- 

 tilages, yet its surface is so convex towards the thoracic cavity, 

 that a very large space within the bony thorax is really occupied 

 by the abdominal organs. 



THE PELVIC ARCH AND HIND EXTREMITIES. 



Behind the thorax occurs a second interval corresponding 

 to the loins, where the haemal arch is deficient ; but at the pelvis 

 the circle is completed by the bones of the ischium, ilium, and 

 pubes, united to the sacrum above, and having the hind extremi- 

 ties firmly articulated to them at the hip joints. The pelvis con- 

 stitutes not only a firm and solid case for the protection of the 

 large blood-vessels, and of the urinary and genital organs, but it is 

 also intimately connected with locomotion, to which the posterior 

 extremities largely contribute. 



THE TAIL. 



This organ appears to be intended chiefly to protect the body 

 from insects ; but it also serves to some extent as an aid in bal- 

 ancing the body when rapidly moving in any new direction. It is 

 made up of from fifteen to eighteen bones. 



THE FORE AND HIND EXTREMITIES CONSIDERED AS 

 ORGANS OE SUPPORT AND LOCOMOTION. 



Regarding the limbs as means op support, it must be re- 

 membered that the fore limbs are nearer the centre of gravity, 

 and, therefore, sustain more weight than the hind. The fore 

 quarter is suspended between the bases of the two shoulder-blades, 

 chiefly by the sen-ati magni, and in such a way as to require no 

 special muscular contraction. The four parts of which the limb 

 itself is composed being bent at various angles, are prevented from 

 giving way by the muscular actions of the extensors of the hu- 

 merus and ulna, the carpus (or knee) having little tendency to 

 yield, and the pastern being supported by the flexor muscles and 

 suspensory ligament. The hinder limbs, though sustaining less 

 weight, are not so favorably circumstanced for this purpose, the 

 angles between their several parts being generally more acute- 

 But if these are attentively regarded, there is not so much differ- 

 ence as is generally supposed. Thus, the first joint, the ilio-fe- 

 moral, forms a less acute angle t^an its analogue, the shoulder 



