260 ME. G-. E. DOBSON ON THE DIGASTEIC MUSCLE. 



a. With the anterior bellies united across the space between the rami of the 



mandible, the mylo-hyoid muscles feeble or absent. 



Ex. Gymnura rafflesii, Epomophorus macrocephalus. 



b. With the anterior bellies separate, the mylo-hyoid muscles well developed. 



Ex. Erinaceus ev/ropceus and other species, Cavia 

 apercea, Epomophorus franqueti, &c. 

 B. Connected by ligament or by tendinous fibres with the hyoid bone ; tendinous 

 intersection well developed, oblique or longitudinal. 



a. With the anterior bellies united across the space between the rami of the 



mandible, the mylo-hyoid muscles feeble. 



Ex. Tupaia ellioti, Mm decuman/as, Ilyoxus avel- 

 lanarius, Cercopithecus caUithricus, &c. 



b. With the anterior bellies separate, the mylo-hyoid muscles well developed. 



Ex. Lor is gracilis, Homo sapiens. 



In the first example cited above, Gymnura rafflesii, I found the digastric condition 

 of this muscle well developed. About the middle of the muscle, at its narrowest part, it 

 is traversed by an oblique tendinous intersection more marked internally, arising from the 

 upper margin, which is continued inwards and slightly forwards from its lower and 

 internal margin as a tendinous band across the mylo-hyoid muscle to unite in front of 

 the hyoid bone with the corresponding band from the opposite side. From this 

 tendinous raphe thus formed muscular fibres arise, which, extending forwards and 

 inwards, cover the anterior three fourths of the mylo-hyoid and part of the genio-hyoid 

 muscles, and passing above the margins of the anterior bellies of the digastric muscles of 

 opposite sides, are inserted with them and for some distance in front of them into the 

 rami of the mandible. 



This horizontal muscular expansion which takes its origin thus from the tendinous 

 intersection and its median continuation might also be described as a deep division of 

 the anterior belly of the muscle, commencing at the tendinous intersection, and uniting 

 with its fellow of the opposite side along the middle line between the jaws. Its posterior 

 margin, the tendinous raphe above described, is concave, quite free from the hyoid bone, 

 but attached to a fascial aponeurosis, which passes backwards over it and the sterno- 

 hyoid muscles. On removing this muscular expansion the mylo-hyoid muscles are found 

 beneath, very thin, not extending half the distance between the hyoid bone and the 

 symphysis menti. 



In Epomophorus macrocephalus, minor, labiatus, and gambianus the anterior bellies 

 of the digastric are double, and the deep lamina> united not only across the space 

 between the rami of the mandible, as in Gymnura rafflesii, but their union extends so 

 far backwards that when the head is bent forwards then' posterior free margin passes 

 behind the hyoid bone. The mylo-hyoid muscles are completely absent, their functions 

 being evidently performed altogether by these greatly developed digastrics. It is most 

 remarkable, as showing what extraordinary structural differences may occur in the 

 anatomy of species to judge from external characters very closely allied, that in 

 Epomophorus franqueti, comptus, and pusillus the digastrics arc single, and not united 



