ON THK DIGASTRIC MUSCLE OF THli MACAQUES. 437 



29. The Variations in the Digastric Muscle of the Rhesus 

 Macaque and the Common Macaque. Bj Chas. F, 

 SoNNTAG, M.D., Ch.B., F.Z.S., Anatomist to the Society. 



[Received October 24, 1919 : Eead November 18, 1919.] 

 (Text-figures 1-5.) 



In the course of several dissections to explore the extrinsic 

 muscles of the tongue of the Rhesus Macaque, I have been struck 

 with the various appearances of the digastric muscle. I decided, 

 therefore, to examine the muscle of every monkey brought to 

 the Prosectorium, and the present communication is based on the 

 dissections of fifteen animals. 



The muscle belongs to Parson's first type*. In it the posterior 

 bellies of the two dii^astric muscles terminate in long; slender 

 tendons which, passnig forwards and inwards, unite to form an 

 arch anterior and siiperficial to the hyoid bone. From the arch the 

 fibres of the united anterior bellies pass to the lower edge of the 

 inferior maxilla (text-fig. 1). This arch is closely applied to the 

 mylo-hyoid muscle and compels it to have a dome-like appearance. 

 In some cases a probe can be passed under the arch, but in most 

 of them they are fused and can only be torn apart. Moreover, 

 in aboiTt fifty per cent, of cases there is no connection between 

 the arch and the hyoid bone. 



The extent and nature of the attachment of the anterior 

 bellies to the inferior maxilla vary. In some cases the muscle 

 extends right back to the angles on both sides, but more commonly 

 the extreme posterior limit is about half an inch anterior to the 

 angle, and lymphatic or submaxillary glands are lodged in the 

 small I'ecess. In the majority of cases the attachment to the 

 maxilla is muscular, but in one case there were a number of 

 tendinous fibres interspersed among the muscles. In no case did 

 I find any exception to the rule that the two anterior bellies are 

 joined in the middle line. 



In a little more than fifty per cent, of cases, the space between 

 the tendinous arch and the hj^oid bone is occupied by a membi-ane, 

 by fibrous bands, or by both together. In one case, a membrane 

 alone filled the space (text-fig. 2), and there was no fusion be- 

 tween it and the mylo-hyoid muscle underneath. The anterior 

 bellies did not reach the posterior angles of the jaw. 



In one case (text-fig. 3 a) the space was filled up by a membrane, 

 and the tendons of the posterior bellies were connected to it by 

 short fibrous bands. This specimen was interesting in another 

 way, for the membrane between the arch and the hyoid bone was 

 common to the digastric and mylo-hyoid muscles, so that the two 

 could not be separated. The mylo-hyoid muscle was defective in 

 front, there being a V-shaped gap which was filled by membrane 

 (text-fig. 3 b). 



* F. G. Parsons, Jourii. Anat. Physiol. 1898, p. 436. 



30* 



