MUSCULAE SYSTEM OF VERTEBEATA. 491 



structure it is still a foot, so long as we hold to the anatomical 

 conception of what hand and foot are, and do not put functional 

 relations iuto the fore-ground ; and if we do, then the proboscis 

 of the elephant is a " hand " also. 



This change in the character of the foot is seen in many Mar- 

 supials, Prosimiee, and Primates. The chief change lies in the 

 development of the first toe in a manner similar to that in which 

 the thumb of the hand is developed. In Man also there are indica- 

 tions of the foot having once been a prehensile organ. When this 

 function is lost, the hallux is shortened in proportion to the extent to 

 which the whole of the sole of the foot ceases to take part in loco- 

 motion. The shorter hallux is then functionally inoperative (Digiti- 

 grade Carnivora). It disappears altogether in the Ungalata, where 

 the limbs do nothing but aid in locomotion and support the body. 

 The characters of the metatarsus and digits are parallel in character 

 to those of the fore-limb in the Artio- and Perissodactyla. In the 

 latter, the gradual conversion of the foot from a four-toed to a one- 

 toed condition has been recognised as obtaining in the same paljeon- 

 tological series as that which has already been pointed oiit in 

 reference to the fore-limb. 



Muscular System. 

 § 370. 



The muscular system of the Vertebrata is differentiated from the 

 mesoderm in the embryo, and is divided in a way corresponding to 

 the metamerism of the whole body. Before the skeleton is differen- 

 tiated the musculature below the integument unites with it to form 

 a dermo-muscular tube, similar to that of segmented Invertebrata in 

 many points, although not immediately derivable from one of those 

 forms. 



Its relations to the skeleton, and the formation of a muscular 

 system connected with the skeleton, are therefore acquired in pro- 

 portion to the extent to which they are concerned in the develojD- 

 ment of the skeleton. In Amphioxus, where the skeleton is 

 essentially formed by the chorda dorsalis, the musculature, in the 

 region of the trunk at any rate, has none of these relations ; it is 

 only in that portion of the body which encloses the respiratory 

 cavity that it seems to have any relations to the visceral skeleton. 

 The whole of the musculature is arranged in two lateral longitudinal 

 masses, which are separated by connective tissue into dorsal and 

 ventral masses. These longitudinal bands of muscle are separated 

 by septa of connective tissue into a series of metameres (myocom- 

 mata) ; and each septum serves for the origin as well as for the 

 insertion of the fibres, which take a straight course between them. 

 While this mass of muscle extends along the whole of the dorsal 

 region of the body, it is modified in the ventral surface of the 



