102 ANNALS NEW YORK ACADEMY OF SCIENCES 



Mammalia 



The jaw muscles of the mammals are remarkably constant in character 

 throughout the group, except in the monotremes and some edentates 

 where extraordinary conditions occur. Throughout the group the muscles 

 are closely correlated with the dentition and with function. The chewing 

 muscles are limited to the digastric, masseter, temporal, pterygoideus 

 externus and pterygoideus internus. There is much individual variation 

 in these muscles, but they are always present, except the digastric, which 

 is absent in some monotremes and edentates, and the internal pterygoid, 

 which is sometimes not differentiated from the temporal. 



The temporal fossae of the mammals are opened out, so that there is 

 free play for the muscles, except where there is a secondary covering of 

 plates. 



The ramus mandibularis of the trigeminus nerve innervates all the 

 muscles of mastication except the posterior belly of the digastric and its 

 allies. The muscles innervated by Y 3 represent what remains of the 

 capiti-mandibularis of the reptiles. The temporal is the largest muscle 

 of this group in most of the mammals. It is closely associated with the 

 masseter, which is a slip that has separated completely in the mammals, 

 but still remains a part of the temporal mass in the reptiles. The tem- 

 poral is variable in its size, as the masseter is often larger, usurping its 

 functions in the rodents and in some other groups. It is partially divided 

 in the mammals, as we find it with a single head in Homo and with three 

 in the monotremes, but although the condition may vary the origin and 

 insertion remain quite constant. 



The masseter is much more variable than the temporal muscle. It is 

 double-headed in man, and also is divided into two muscles in the mono- 

 tremes, each with two slips, and we may have as many as five slips in the 

 rodents. Its size and condition depend on the special conditions and the 

 importance of the muscle. In the rodents, where it takes the place of the 

 temporal to a certain extent, it is large and very much specialized. Its 

 origin varies, as it may partly cover the temporal as in Didelphys, or may 

 extend forward, anterior to the eyes in the rodents, or divide into two 

 separate muscles as in the monotremes. This muscle in the mammals is 

 always associated with the development of the zygomatic arch and repre- 

 sents the outer portion of the temporal mass that may be traced from the 

 Pisces to reptiles, where the outer fibers of the capiti-mandibularis mass 

 are always slightly differentiated from the deep fibers by having a differ- 

 ent direction, although there is no separation in the reptiles to the extent 

 found in mammals. The pull of the masseter is usually slightly forward, 



