THE EXTREMITIES 351 



pelvic bones posterior to them are attache 1 to the vertebi'pe, and though they 

 inclose oi^gans of less vital importance, yet they are perfectly analogous to 

 these parts in their types and in the offices waich they perform. 



OF THE THORACIC ARCH AND ANTERIOR 

 EXTREMITIES 



Lying in the horse at some distance posteriorly to the three first 

 segments of the hcemal arch (the bones of the face, lower jaw, and os hyoides), 

 and separated from them by the neck, where there is a hiatus, the thoracic 

 arch and anterior extremities depend from the vertebrae corresponding to 

 them. In many of the higher vertebrates the fore extremity is firmly 

 united by a joint to the thorax, and may be considered with it ; but in the 

 horse it is only attached by muscles, the thorax being slung between the 

 upper edges of the blade-bones by means of two broad sheets of muscular 

 fibres. Hence the collar-bone is entirely absent in this animal ; and thus, 

 while he is free from dislocations and fractures of that bone, to which he 

 would be constantly subject if it were present, he is rendered more liable to 

 strains and rheumatic inflammations of the muscular sling, by which freedom 

 of action is impaired. 



In the articulated skeleton it is usual to consider 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 

 cartilages, 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. This will be hereafter more fully explained in examining these 

 parts at page 456 e^ seq. 



THE PELVIC ARCH AND HIND EXTREMITIES 



Behind the thorax occurs a second interval corresponding to the loins, 

 where the hsemal arch is deficient ; but at the pelvis the circle is completed 

 by the bones of the ischium, ilium, and pubes, imited to the sacrum above, 

 and having the hind extremities firmly articulated to them at the hip joints. 

 The pelvis constitutes 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 balancing the body 

 when rapidly moving in any new direction. It is made up of from fifteen 

 to eighteen bones, which will be described in the next chapter. 



