386 PROCEEDINGS OF THE ACADEMY OF [1880. 



tendon at tlie origin from tlie malar bone. The fibres beneath this 

 are continuous, in most mammals, with the superficial layer of 

 fibres of the temporal muscle, including the supra-zygomatic slip, 

 which, in some animals, is distinct in great part from the fibres 

 arising from the temporal aponeurosis. The masseter presents a gene- 

 ral resemblance to the internal pterygoid muscle, which, wherever 

 exampled, has shown these imperfect attempts at planal cleavage. 



This outline being borne in mind, it may be well to turn to the 

 descriptions emplo3'ed by writers on comparative anatomy. 



The descriptions of the muscles in Meckel (Vergleich. Anat., 

 iv, 495) are ver}- general. The temporal is said to be covered by 

 a conspicuous aponeurosis ; the muscle to be more or less fan- 

 shaped, and gradually narrowed from above downward. The mas- 

 seter is said to be divided ordinarily into an outer, longer, stouter 

 and straight layer and an inner, shorter, weaker layer, in which 

 the fibres are more or less obliquelj^ placed from above downward 

 and before backward. 



Cuvier (Legons d'Anat. Comp., 2d Ed., iv, Ime Part, 64 infra) 

 describes the temporal in the apes, bats, insectivora, rodents, two- 

 toed ant-eater, hog, ruminants and the cony. None of these includes 

 the arrangement of fibres above given. It is true that in the ant- 

 eater the masseter and temporal muscles are united, but no detail 

 of the character of the union is presented. Mivart (Elements 

 of Anatomy, 310) repeats this statement. It is evident that the 

 union of the muscles is here thought to be exceptional. Cuvier 

 and Laurillard further describe the masseter in the bats, rodents, 

 artiodactyles, ant-eater and the cony as composed of two portions, 

 a zygomatic and a maxillary. The former is present in all ; the 

 latter is seen in the rodents, artiodactyles, the ant-eater and the cony. 



Mivart (1. c, p. 309) describes the masseter in Lagostomus and 

 Dasyprocta as follows, as of " great development :" "" The masseter 

 is divided into three portions, and traverses the singularly en- 

 larged infra-orbital foramen spoken of in describing the skeleton." 

 According to the interpretation used in this paper, the masseter in 

 rodents has even fewer subdivisions than in some other mammals. 

 Of these, at least one only passes in such direction as to permit 

 the expression that it " traverses the infra-orbital foramen;" and 

 this part is not separable from all the fibres lying on a plane lower 

 than that of the zygoma. 



The descriptions of Cuvier and Laurillard of animals I have not 

 dissected, may be liere epitomized : 



