XIII 



PHYLUM CHORDATA 



163 



is composed externally of obliquely running fibres, and represents 

 one of the two oblique muscles of the abdomen of higher forms. 

 Mesially this passes into a median band of longitudinally running 

 fibres corresponding to a primitive rectus. The muscles of the limbs 

 are distinguishable into two main sets those inserted into the 

 limb arch and those inserted into the free part of the appendage. 

 The latter, according 

 to their insertion, act r *> -t 



as elevators, depress- 

 ors, and adductors. 

 A series of circular 

 muscles pass between 

 the cartilages of the 

 visceral arches, and 

 when they contract 

 have the effect of con- 

 tracting the pharynx 

 and constricting the 

 apertures. A set of 

 muscles pass between 

 the various arches 

 and act so as to ap- 

 proximate them ; and 

 a broad sheet of longi- 

 tudinal fibres divided 

 into myomeres ex- 

 tends forwards from 

 the shoulder girdle to 

 the visceral arches. 



Electric organs 

 organs in which 

 electricity is formed 

 and stored up, to be 

 discharged at the will 



of the Fish OCCUr FIG. TOO. A Torpedo-Ray with the electric organs dissected 



in Severil Flismo ? 11 ^' ^ n *^~ right the surface only of the electric organ (oe) 



branchs. They are 

 best developed in 

 the Electric Kays 

 (Torpedo and Hypnos) 

 (Fig. 790) in which they form a pair of large masses running 

 through the entire thickness of the body, between the hea< 

 and the margin of the pectoral fin. A network of strands of fibrou 

 tissue forms the support for a number of vertical prisms, each 

 divided by transverse partitions into a large number of com- 

 partments or cells. Numerous nerve-fibres pass to the various 

 parts of the organ. These are derived mainly from four nerves, 



M 2 



is shown. On the left the nerves passing to the organ are 

 dissected out. The roof of the skull is removed to bring the 

 brain into view. br. branchiae ; /, spiracle ; o, eyes ; tr, 

 trigeminal ; tr', its electric branch ; v. vagus ; /, fore-brain ; 

 //, mid-brain ; ///, cerebellum ; IV, electric lobe. (From 

 Gegenbaur.) 



