THE HEAD AND NECK 



103 



of the muscles (temporal) which close the jaw, but its 

 anterior part contains much fat, the loss of which in 

 old horses gives rise to the characteristic depression 

 seen in them above the eye. Farther forward is the 

 cavity (o), of almost circular outline, in which the eye- 

 ball is lodged, called the ' orbit,' with very sharply- 

 defined and complete outer and hinder margin, formed 

 by a bridge of bone passing upwards from the zygo- 

 matic arch to join the 4 frontal,' or forehead-bone. This 

 is a point in the anatomy of the horse to be especially 

 noticed, as it gives a very characteristic appearance to 

 his skull. The interest of this bridge of bone, dividing 

 the orbit from the temporal fossa, is that it did not 

 exist in any of the primitive Ungulates of the Eocene 

 period, in which, consequently, these two cavities, or 

 fossae, were freely continuous (see skull of Phenacodus 

 in fig. 2, p. 21). Moreover, it does not exist in any other 

 of the Perissodactyles of the present time (tapir or rhino- 

 ceros), but is a special and recently-acquired character, 

 developed only in the later stages of the horse group, 

 not being met with in any of the ancestral forms until 

 after the close of the Miocene period. 



The horse, however, is by no means alone among 

 mammals in possessing it ; but whenever it occurs, it 

 appears to be an evidence of advance in structure, being 

 found in the higher and more recent forms of several 

 groups, the lower and more ancient members of which 



