CHAP. IV] THE GENERAL ANATOMY OF GORILLA 181 



sheet, which is lost on the facial aspect of the symphysial region. 

 The tendinous " arcade " from which the anterior muscle arises in 

 the Cercopithecidae, is thus vestigial only. No decussation of 

 the anterior bellies was noticed, though I found this condition 

 in an adult male Orang-utan. On the whole, the anterior belly 

 of the M. digastricus is to be contrasted strongly with that of 

 Man, in whom it has become so massive that some notable 

 accession of function must have occurred. I would suggest that 

 of the various possible factors, two are worthy of particular notice. 

 Thus we should remark that the muscle varies in size inversely 

 with the mass of the mandible. And again, the increase in the 

 muscle is accompanied by an increase in the angle at which the 

 two halves of the mandible are inclined to one another. Conse- 

 quently we may suppose that with a lighter mandible, the 

 employment of a more powerful depressor of the chin is advanta- 

 geous : in Man the M. digastricus has met this demand. Again, 

 the widened space within the jaw necessitates a more powerful 

 sublingual support for the tongue. And the human M. digastricus 

 serves this purpose also. 



3. The sternal and clavicular parts of the M. sterno-mastoid 

 resemble those of the Cercopitheci in their distinctness. But the 

 clavicular part is weaker than the sternal component, so that a 

 contrast here exists between the Gorilla and the lower monkeys. 

 As regards the cranial attachment of the muscles, a marked exten- 

 sion over the occipital bone is noteworthy. This is provided by 

 the sternal component of the conjoint muscle, and the mechanical 

 advantage of such an extension is quite clearly appropriate to the 

 enormous skull. 



4. The M. trapezius has also a wide area of cranial attachment, 

 but on the other hand it extends down the spine to the tenth 

 thoracic vertebra only, and does not overlap the M. latissimus 

 dorsi. It appears therefore that the lower part of this muscle is 

 defective in comparison with that of Man, while the upper part 

 is more extensive. This suggests that the latter (the cranial 

 portion) is influenced by the great weight of the cranium, while 

 the lower part ha's assumed in Man new functions related to 

 the erect attitude. The general characters of the Mm. rhom- 

 boideus, omo-cervicalis, levator anguli scapulae, and deltoideus are 



