MUSCULAR SYSTEM. II5 



sponds to this origin (see cranial nerves). These muscles move 

 the eye, and in many forms are re-enforced by a retractor bulbi, 

 apparently derived from the third head segment. 



Beneath the skin of mammals there occurs a general mus- 

 cular layer, the panniculus carnosus, concerning which our 

 information is none too extensive. From this layer are devel- 

 oped in the facial region ' muscles of expression,' which serve 

 to move the skin, especially that around the mouth and eyes. 

 The fact that these muscles of expression are innervated by the 

 facial nerve would apparently indicate their point of origin as 

 behind the jaws. 



The muscles which move the limbs are divided into intrinsic 

 (those which have their origin and insertion on the bones of 

 the limb or of the supporting girdle) and extrinsic (which arise 

 from the trunk and are inserted on the girdle or on the limb). 

 In the fishes neither series acquires extensive development ; but 

 with the more varied movements necessary in a terrestrial life 

 both sets, and especially the intrinsic, attain a high grade of 

 differentiation. Both series may be grouped as dorsal and ven- 

 tral, and these divisions again may be considered accordingly as 

 they are preaxial or postaxial in position ; i.e., accordingly as 

 they are in front of or behind the axis of the limb. The proxi- 

 mal extrinsic and intrinsic preaxial muscles act as protractors, 

 serving to move the limb forwards, the postaxial as retractors, 

 which move it in the opposite direction. The other intrinsic 

 muscles are divided between flexors, which bend the limb upon 

 itself, and extensors, which straighten it after flexion. For 

 the details of these muscles reference must be made to special 

 works. 



Eletrical Organs. — In certain fishes. Torpedo, electrical eel 

 (Gymnotus), Malapteriis, and to a less degree in some skates 

 (^Raia), certain of the muscles become metamorphosed into an 

 electrical organ. This organ lies in the Torpedo on either side 

 of the head ; in the others in the trunk or tail near the back- 

 bone. In all the organ consists of a series of capsules of con- 

 nective tissue filled with a gelatinous substance in which are 

 the 'electrical plates,' in which the nerves terminate, and which 

 are apparently the modified motor end-plates of the muscle. 



