OSSEOUS SYSTEM OF MAMMALIA. 35 



In Man and the Quadrumana alone do we find the orbits walled 

 around, internally, so as to form isolated recesses. In some herbivorous 

 quadrupeds, as the Deer, the Horse, &c., only the external ring, or margin, 

 of the orbit, is complete. It is incomplete, however, in the Rodentia, as the 

 Beaver, the Rat, the Hare, &c. The same observation applies, also, to the 

 Carnivora, at least, as a general rule. In the Horse, the Deer, and the An- 

 telope, the external angular process of the frontal bone joins an angle of 

 the malar bone, so as to perfect the ring ; but this ring stretches over a 

 concavity, by which the orbit fully communicates with the temporal fossa, 

 the wing of the sphenoid failing to complete the wall of separation. 

 In the human skull, the large spheno-maxillary fissure* in the orbit, 

 which opens into the temporal fossa, is the first step toward throwing 

 the orbit and temporal fossa into one. In the Monkeys this fissure is 

 less considerable, it advances less forward, and separates between the 

 alveolar ridge of the superior maxillary bone and the pterygoid process 

 of the sphenoid, including only a small portion of its wing. The orbits 

 are, in fact, proportionally deeper in the Simiae than in the human subject, 

 and are separated from each other by a much narrower space, occupied 

 by the nasal cavities. 



In the human skull, the malar bone, carried backward, unites with a 

 long process of the temporal bone (the zygomatic process), in order to 

 form the zygomatic arch, beneath which passes the temporal muscle, to 

 be inserted into the coronoid process of the lower jaw ; and the lower edge 

 of this arch gives rise to the masseter muscle, which also acts on the same 

 lever. In Man, the zygomatic process of the malar bone is short, so that 

 the arch is principally formed by the zygomatic process of the temporal 

 bone ; but, in many of the Mammalia, the reverse is the case. In the 

 Monkey, the two branches, which form the arch, are nearly equal. In 

 the Dog, the malar bone is reduced to a narrow strip, sending backward 

 a long acute branch, which is overlaid by the corresponding branch 

 of the temporal bone : in other animals the malar bone is reduced 

 still more, and thrown more backward, a process of the superior maxil- 

 lary bone taking its place j so that, in the form of a narrow slender slip, 

 it constitutes the centre of the zygomatic arch, between the suborbital 

 branch of the maxillary and the zygomatic branch of the temporal bone. 

 This is seen in the Rat, and many other Rodentia ; in the Hedgehog, 

 Mole, and other Insectivora. In some of the Insectivora, as the genera 

 Centetes, Ericulus, and Echinops, and in the great Ant-eater among the 

 Edentata, it is absent ; the zygomatic arch being incomplete. In the 

 Antelope, Deer, and Horse, especially the latter, the malar bone is large, 



* This fissure intervenes between the orbital plates of the sphenoid and the superior maxillary bones ; 

 but the malar and palate bones also enter into it ; the former bounding it anteriorly, the latter pos- 

 teriorly. 



