242 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the lower jaw may be depressed is ve^ much greater than in the 

 lierbivora ; in the latter, as in the horse, eight to ten centimeters being 

 the extent of separation of the lower from the upper incisors. The 

 digastric muscle is comparatively feeble, and would appear to pull the 

 jaw back, but really it tends to advance it, since it is a lever of the 

 third class. In the hare, rabbit, and ox the digastric muscle has only one 

 bell} 7 , and in the ox is joined to the same muscle of the opposite side 

 by transverse muscular fibres ; in the dog there is no intermediary 

 tendon in the digastric muscle. The development of the digastric de- 

 pends upon the character of the food of animals. In the horse, sheep, 

 and ox, where it is small, it is a double muscle, and is inserted more 

 anteriorly in the lower jaw than in carnivorous animals, where it is 

 large. The conditions, therefore, are most favorable for its action in 

 lierbivora on account of its different insertion. This muscle antagonizes 

 the temporals, masseters, and pterygoid muscles. 



FIG. 92. HEAD OF SOLJPEDE HORSE. (B&clard.) 



The representative of the digastric in the lower vertebrates, as in 

 reptilia, according to Mr. G. E. Dobson, is a bundle of muscular fibres 

 arising from the occiput and inserted into the posterior extremity of 

 the mandibular ramus, its functions being simply those of drawing the 

 angle of the mandible backward and upward, and so separating the 

 jaws in front. This is also its form and function in birds and most 

 mammals, though in man, monke^ys, and rodents the muscle is made up 

 of two bellies with an intermediate tendon, which is often connected 

 by ligament with the hyoid bone. Mr. Dobson has traced an interesting, 

 connection between the mode of feeding and the type of the digastric 

 muscle. In the group of animals in which this muscle is connected 

 with the hyoid bone, the species swallow their food while in the erect po- 

 sition, with the head bent forward on the chest and the long axis of the 

 cavity of the mouth at right angles with the oesophagus ; in the other 

 this muscle is free, and all the species feed while resting on their 



