498 COMPARATIVE ANATOMY. 



Vertebrates. They are divided into those which pass to the girdle, 

 and those which belong to the appendage itself. 



When the appendages are metamorphosed, the musculature 

 undergoes changes also; the muscles are, indeed, simplified in 

 number, but are entrusted with more functions, in consequence of 

 the greater freedom and independence of the skeletal parts. 



As compared with the Fishes the most important change is the 

 extension of the musculature of the shoulder-girdle and 

 of the anterior extremity over the dorsal and ventral 

 surface of the body; this obtains in all the higher Vertebrata. 

 The muscles developed from the superior lateral trunk-muscles are 

 covered over by muscles which go to the limbs, and which are sub- 

 stituted in Pishes by a mass of muscle which arises from the head. 

 These are slightly differentiated in the Perennibranchiata, and more 

 so in the Caducibranchiata ; they form those muscles, which repre- 

 sent the cucullaris and the sterno-cleidomastoid in the higher 

 divisions. They are supplied with nerves from the head. Other 

 muscles, which are probably derived from the muscles of the trunk, 

 and which pass to the appendages, partly from the back, and partly 

 from the thorax, are added on to them. 



The other muscles, which belong to the limbs themselves, are 

 derived from the layers, which in Fishes are more similar to one 

 another, and which cover the dorsal and ventral faces of the skeleton 

 of the thoracic fin. The musculature undergoes great changes 

 owing to the reduction of this fin, and to the modifications under- 

 gone by the parts that are persistent; the changes, therefore, in 

 the anatomical characters of the musculature in the various divi- 

 sions run parallel to the functional changes in the value of the 

 appendages. 



In the hinder limbs the relations of the pelvic girdle to the axial 

 skeleton are, at first, the factors which affect the characters of the 

 musculature. The absence of any connection between these skeletal 

 parts is the cause of the greater independence of the pelvic girdle 

 in Fishes ; so far as this affects the musculature it is made up for by 

 its more indifferent character. The more intimate connection be- 

 tween the pelvic girdle and the axial skeleton in the Amniota 

 diminishes its powers of movement, and, consequently, the develop- 

 ment of the muscles by which this is effected. The muscles which 

 belong to the limb itself have their origin in the pelvic girdle, or in 

 the skeleton of the limb ; they seem, on the whole, to be divided 

 into the same groups as those of the fore-limb, with the exception 

 of such modifications as are due to the difference in the function of 

 the two limbs. 



§ 377. 



The subvertebral muscles form a special group. They lie 

 below the vertebras and their lateral processes, so that in the thoracic 

 region they lie within the thorax. 



The musculus longus forms an anterior portion of the muscles 



