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VIII. On the Digastric Muscle, its Modifications and Functions. 

 By G. E. Dobson, M.A., M.B., F.L.S. 



(Plate XXV.) 



Read December 15th. 1881. 



IN a paper published in the ' Proceedings of the Royal Society ' for March 1881, I have 

 traced the origin of the oblique tendinous intersection of the Digastric Muscle to an 

 originally united condition of the anterior bellies of the muscles of opposite sides in 

 front of the hyoid bone, the superficial tendinous inscription still found traversing the 

 surfaces of the digastrics of many species in which the muscles are not united being 

 shown to be the rudiment of the origin of a tendinous band which once formed the 

 posterior limit and support of the united muscles. I shall now endeavour to trace the 

 leading modifications of this muscle and their relations to its functions. 



The representative of the digastric in the lower Vertebrates, as in R,eptilia, is a bundle 

 of muscular fibres arising from the occiput and inserted into the posterior extremity of 

 the mandibular ramus, its functions being simply those of drawing the angle of the man- 

 dible backwards and upwards, and so separating the jaws in front. Such is its form and 

 such its functions also throughout the class Aves and in most of the species of Mammalia. 

 In many species, however, of the latter class, notably in certain Orders — the Primates and 

 Rodentia — we find it no longer of the simple form above described ; its anterior attach- 

 ment has advanced nearer to the anterior than to the posterior extremity of the man- 

 dibular ramus, and the muscle is found to be made up of two bellies, an anterior and a 

 posterior, with an intermediate tendon or tendinous intersection, thus forming a true 

 musculus digastricus. 



In the Primates, in many species of Rodents, and in a few species of other Orders, 

 the intersecting tendon is connected by ligament or by tendinous fibres with the hyoid 

 bone, the anterior bellies of the muscles of opposite sides uniting in the middle line 

 across the mylo-hyoids, or connected only by the fascial expansion known as the supra- 

 hyoid aponeurosis which extends between their inner margins. 



The leading modifications of the muscle may therefore be arranged as follows : — 



I. Single (31. depressor mandibular), inserted into the posterior extremity of the 



mandible, as in reptiles, birds, and many mammals. 



II. Double (31. digastricus), consisting of an anterior and posterior belly, with an 



intermediate tendon or tendinous intersection, inserted more or less in front into 

 the side of the mandible, as in the Primates, many Rodents, and some species 

 of other orders. 

 A. Not connected with the hyoid bone ; tendinous intersection oblique or 

 transverse. 



SECOND SERIES. — ZOOLOGY, VOL. II. 3G 



