II: Proceedings of the Royal Physical Society. 



the question of Expression, and yet, according to their nerve- 

 supply, that is their original position. 



As regards those muscles concerned in Mastication, Deglu- 

 tition, and the movements of the Eyeball, very few variations 

 of a kind to suggest the method of their evolution are found. 

 A similar statement may be made regarding the muscles of 

 the trunk, although, in the case of the rectus abdominis, the 

 tendinous intersections (linese transversae), which are its con- 

 stant characteristic, are believed to represent those abdominal 

 ribs which are an unfailing feature of the crocodile. 



The entire absence of the pannicuhts carnosus muscle from 

 the trunk of man's body is a feature which, it seems to me, 

 has not received sufficient attention, from, as I take it, an 

 imperfect appreciation of the function of this important 

 sheet of muscle. That this muscle enables an animal to 

 perform twitching movements of its skin appears clear 

 enough, but this explanation is not sufficient to account for 

 its complete development among Cetacea and other marine 

 mammals, whose ability to twitch a blubber-distended skin 

 must be feeble, as well as of doubtful necessity. So far as I 

 am aware, this muscle is found among all mammals, with the 

 exception of the Anthropoid Apes and Man. In the latter 

 groups, their departure from the horizontal attitude common 

 to mammals requires to be taken into account, and I am 

 strongly of opinion that the chief function of the panniculm 

 carnosus is that of an Expiratory muscle, and that its dis- 

 appearance is coincident with the changed attitude which 

 enables the weight of the chest wall, together with that of 

 'the superimposed anterior, or upper limbs, to mechanically 

 replace the expulsive power of the muscular sheet. 



Such variations as are shown by the vascular and visceral 

 contents of the trunk are referable to embryonic conditions 

 which stereotype certain forms of the historic development 

 common to all Vertebrata lower in the scale than the one 

 which may be under consideration, and a discussion of these 

 evidences would be too wide for my present purpose. 



Associated with the abdominal viscera, however, there is 

 one pair of muscles, viz., the levatores ani, which require 

 special notice from the part they perform in relation to the 



