30 INTRODUCTION. 



advances anteriorly, and sends back but a small zygomatic branch, the 

 zygoma itself being greatly contracted. 



With the extent and character of the zygomatic arch are connected, 

 not only the position and expression of the eye, but also the habits and 

 manners of the animal. In carnivorous animals, the force of whose jaws 

 is very great, the volume of the temporal and masseter muscles is 

 immense : the temporal muscles occupy the whole of the sides of the 

 skull, beginning from the mesial line down the cranium, which in many, 

 as in the Wolf, Hyaena, &c., is elevated into a strong ridge, for their 

 firmer and more voluminous attachment. (See fig. 18, representing the 

 zygomatic arch and temporal muscle of a carnivorous animal.) Now, as 

 the space beneath the zygomatic arch is filled up by the temporal muscle 

 18 passing beneath, it must be evident that the 



span and spring of the arch will be an index 

 ^^^^ of the volume of this muscle, while its extent 



* "SlR'^^^B^ Mm an ^ stre ngth will be in accordance with those 

 of the masseter ; at least, generally : for, in 

 the Horse, a projecting ridge is carried for- 

 ward from the zygoma, across the lower edge 

 of the malar bone, and for the space of nearly two inches on the maxillary 

 bone, in order to allow of the development of this important muscle of 

 mastication. In the Cow, this muscle is less developed than in the Horse, 

 and there is no projecting ridge of the malar bone. 



In Man the zygomatic arch is small, the temporal muscles are mode- 

 rate, and the force of the jaws is comparatively limited. In the Ape 

 tribe, the zygomatic arch is larger, and the temporal fossa deeper. 



In the Elephant the zygomatic arch is of moderate stoutness, but 

 spans a vast concavity, filled by temporal muscle of enormous volume. 



In the Hippopotamus the zygomatic arch is short and strong, and 

 stretches over a deep temporal fossa; this is also the case in the Horse: 

 but, as already said, a spinous ridge runs from it anteriorly, for the more 

 extensive attachment of the masseter. This arch is broad and strong in the 

 Kangaroo. In the Chlamyphorus and the Armadillo it is slender and 

 weak ; but, at the same time, it affords an extensive line for the attach- 

 ment of the masseter. The same form occurs in the Mole, in which the 

 zygomatic arch assumes the appearance of a straight osseous thread. 



In the Ruminantia the zygomatic arch is short, and the temporal 

 fossa circumscribed ; but the masseter is not confined to the zygoma, 

 being also attached to the greater part of the side of the superior maxil- 

 lary bone, beginning close below the orbit. In rodents the anterior 

 part of this arch consists, as previously observed, of a branch, or some-\ 

 times (as in the Coipus) of a double branch, surrounding a large foramen, 

 projecting from the superior maxillary bone ; the malar bone, reduced 



