34 On the Tendinous Intersection of the Digastric. [Mar. 31, 



in all tliose now snowing a trace of tendinous inscription in that mnscle, 

 were such as we still find it in Gymnura rafflesii and Epomophorus 

 minor, its modified form or apparent total absence in most species 

 remains to be accounted for. 



The leading modifications of the muscle, including what I consider 

 its primary form, may be arranged as follows : — 



A. With the anterior bellies united across the space between the 

 rami of the mandible, the mylo-hyoid muscles feeble or absent. 



a. Anterior belly double. Ex., Gymnura. 



b. Anterior belly single. Ex., Tupaia. 



B. With the anterior bellies separate, scarcely or not exceeding the 

 posterior in calibre. 



c. Tendinous intersection distinct. Ex., Homo. 



d. Tendinous intersection rudimentary. Ex., Erinaceus. 



e. Tendinous intersection absent. Ex., Ganis. 



And with respect to its attachments to the hyoid bone : — 



C. Connected by ligament or by tendinous fibres with the hyoid 

 bone. Ex., Homo, Tupaia. 



D. Unconnected. Ex., Ganis, Gymnura. 



Where the digastric is directly connected by ligament with the greater 

 cornu of the hyoid bone, as in man, other Primates, and the Bodentia, 

 tension on the middle of the muscular mass would tend to the dis- 

 appearance of the muscular fibres in the part directly acted upon, and 

 so the tendinous intersection would alone remain forming a tendon ; # 

 where, on the other hand, as in the dog and most other mammals, there 

 is no such connexion, or the connexion is by fascial aponeurosis equally 

 distributed between the sides of the anterior bellies of the muscles and 

 the hyoid bone, as in Erinaceus, the character of the digastric would 

 be mainly muscular throughout. Now, the special development of 

 the central tendon, or its absence, appears to me to be directly related 

 to the attitude of the head of the animal with respect to its body 

 when engaged in swallowing its food. In the Primates, in most of the 

 Bodentia, and in the arboreal Insectivora, the food is swallowed while 

 the body is in the erect or semi-erect posture, and the head being bent 

 forwards the cavity of the mouth is at right angles with the oeso- 

 phagus. f In this position the mylo-hyoid and genio-hyoid muscles 

 are relaxed, and cannot act efficiently in drawing the hyoid bone 

 upwards and forwards, so as to allow the masticated mass of food to 

 pass into the oesophagus ; this duty, therefore, partly devolves on the 



* The intermediate condition is well seen in Loris gracilis and in Tupaia ellioti, 

 as described above. 



f The attitude of the bead with respect to tbe body is of course mainly connected 

 with the position of the foramen magnum, and the latter with the development of 

 the brain. 



