262 ME. G. E. DOBSON ON THE DIGASTEIC MUSCLE. 



niger, and in many other species of Primates, the anterior bellies of the digastrics are 

 united, and the intermediate tendon is well developed and connected with the hyoid 

 bone. 



In Man, as an anomaly, the fibres of the two anterior bellies have been found blended 

 together, or the anterior belly has been found double, as in Gymnura, the deeper part 

 uniting with its fellow in a median raphe, covering the mylo-hyoid muscle, or the inter- 

 secting tendon is not unfrequently continued from one side across to the other, forming 

 a zone immediately above the hyoid bone, to the body of which it is tied down by fibrous 

 tissue ; from the upper surface of this the anterior bellies arise. 



This united condition of the digastrics in man is very interesting, as it indicates rever- 

 sion to a condition common in lower species of Primates. 



If now we arrange the species of Mammals referred to above according to the relations 

 of the digastric to the hyoid bone, we shall find that they fall into two groups : — 'in one 

 the muscle is connected with the hyoid, and the species swallow their food while in the 

 erect position, with the head bent forwards upon the chest and the long axis of the 

 cavity of the mouth at right angles with the oesophagus ; in the other this muscle is free, 

 and all the species feed while resting on their anterior extremities, having the long axis 

 of the mouth in a line with the oesophagus. In the latter, where a tendinous intersection 

 exists, it is, as I have shown, either the origin of a tendinous band, as in Gymnura raf- 

 flesii, which, passing obliquely inwards and forwards across the mylo-hyoid, unites with 

 its fellow of the opposite side, or it is very small, forming a mere superficial tendinous 

 inscription, as in Erinaceits, indicating a united condition of the muscle in ancestral 

 forms. 



Thus among certain Rodents and arboreal Insectivora, as the species of Tvpaia, which 

 habitually sit erect when feeding, holding their food between their fore feet, the anterior 

 bellies of the digastrics are large and united, and the intermediate tendons well developed 

 and connected by fascial bands with the hyoid bone and by their deep surfaces with the 

 mylo-hyoid muscles, as may be seen in the species of Muridce and Ifyoxidce, of which ex- 

 cellent examples are readily available in the rat and common dormouse. In the water- 

 vole (Arvicola amphibitts), however, the digastrics are connected together in front by fascia 

 alone, and the upper margin only of their intermediate part is tendinous and not con- 

 nected with the hyoid bone. These animals live on vegetable substances obtained while 

 swimming, and habitually hold the head stretched out in a line with the body. Again, 

 an instructive example is met with in the great rodent moles (Bathyerges marltimus) 

 of South Africa, and in the species of Spalax and Siphneus, which feed chiefly on the 

 roots of grasses, and while feeding necessarily hold the head in a line with the body, 

 or raised upwards ; in them, accordingly, the digastric is quite simple, without tendinous 

 intersection of any kind, and unconnected with the hyoid bone, a true depressor niandibulce. 



In those animals, then, which habitually swallow their food while the head is bent 

 forwards at a right angle with the neck, the digastric has functions other than those of 

 a simple depressor mandibuloe ; it is an important agent in the act of deglutition, such, in 

 fact, as we find it in man. Where, as in reptiles, birds, and most mammals, the position 

 of the mouth with respect to the oesophagus during the act of swallowing the food is 



