DIGASTRIC MUSCLE OF THE MACAQUES. 439 



111 six cases the space between the arch and the hyoid bone 

 was occupied by a triradiate fibx-ous band, the vertical limb of 

 the Y being attached to the bone, and the two lateral limbs to 

 the posterior edge of the arch anterior to the posterior extreme 

 of the anterior bellies. In the fresh specimen the contrast 

 between the white band and the subjacent red mjlo-hyoid muscle 

 was very marked (text-fig. 4). 



In the single example of the Common Macaque {Macacus 

 fascicularis) which I examined (text-fig. 5), the arch was connected 

 to the hyoid bone by a broad band of connective tissue into which 

 two slender bands, from the tendons of the posteriar bellies, were 

 attached. 



The condition present in the Common Macaque (text-fig. 5) 

 difi;ers only in degree from the variety of the Rhesus Macaque 

 present in text-fig. 3 a. 



As there is no gap ]:)etween the two anterior bellies of the 

 digastric, and no solution in the continuity of the tendinous 

 arch, it is not easy, at first sight, to give a rational explanation 

 of the nljove appearances. I was enabled, fortunately, to examine 

 a series of monkeys which exhibited transitions between Parson's 

 first and third types of muscle. Each of these had one or more 

 of the structural elements shown in text-figs. 2-5. 



The variable degree of adhesion between the digastric anterior 

 bellies and the subjacent mylo-hyoid muscle indicates the origin 

 of the former from the latter, and is the beginning of the meta- 

 morphosis. This is followed b}^ the appearances in text-figs. 2-5, 

 and the condensation of the latei-al fibres of the anterior bellies, 

 constituting the nearest approach to a true digastric muscle, is 

 the last stage. 



In my opinion : — 



(1) The anterior belly arises from the mylo-hyoid muscle by 

 splitting. There then ensues : — 



(2) Two muscles joined to the hyoid bone by inembrane (text- 

 figs. 2 and 3 h). 



(3) The pull of the anterior and posterior bellies separates the 

 latter into bands. 



(4) These bands disappear leaving a muscle of Parson's first 

 type. 



(5) The tendinous arch splits and the anterior bellies separate 

 leaving a true digastric muscle. These I have not seen in 

 Macaques, but they are present in Cercopithecus cetkiojys, Cerco- 

 cebus cethiojnciis, and Cehus cdbifrons. 



(6) The muscle in the Macaques has stopped short of being a 

 true digastric condition in which there are two muscles, each 

 with an anterior and posterior belly (Parson's third type). 



(7) There is no variation in the posterior bellies. 



