ix.] THE NERVOUS SYSTEM. 403 



limbs, above Fishes. It may be, however, as e.g. in Meao- 

 poma, that the posterior tibial is divided into two nerves, 

 thus making the correspondence between the nerves of the 

 pectoral and pelvic limbs more unmistakable than it is in 

 man, in whom but a single leg-nerve (the posterior tibial) 

 answers to both the median and ulnar of the arm. 



In Fishes we find great variety as to the nerve supply of 

 the hinder members (ventral fins). Thus, small branches 

 from the ventral nerves of the fourth pair of spinal nerves 

 may be distributed to the ventral fins, as is the case in 

 thoracic and jugular Fishes ; or from the seventh and eighth 

 spinal nerves, as in the Carp ; or finally, as in the Sharks and 

 Rays, the ventral fins may be supplied by more posterior 

 spinal nerves, and by as many as six of them. 



In man the nerves for the extensor muscles pass out post- 

 axially as regards the scapula, pre-axially as regards the 

 pelvis ; but both the corresponding set of nerves may pass 

 out post-axially, as e.g. in Menopoma. 



30. The SYMPATHETIC SYSTEM is made up of an immense 

 number of small nerves and ganglia closely connected with 

 the arteries and with the viscera, and proceeding from two 

 longitudinal gangliated cords which extend upwards and 

 downwards one on each side of the ventral aspect of the ske- 

 letal axis from the pre-sphenoid to the coccyx. The visceral 

 nerves in passing to the various organs they supply, traverse 

 those folds of membrane (the mesenteries) which, as we 

 shall see, suspend the viscera from the backbone. 



In the head, filaments of this system communicate with all 

 the true cranial nerves (i.e. with all but the so-called " optic " 

 and " olfactory " nerves), and below the head the successive 

 ganglia unite with the corresponding spinal nerves by the 

 filaments which, as before mentioned, pass from the latter to 

 the sympathetic. 



Thus, in fact, the sympathetic system may be but a series of 

 internal branches of the spinal nerves of each side of the body. 

 In this case each spinal nerve must be considered as dividing 

 into three branches. One of these branches follows the line 

 of the ascending dorsal lamina of the embryo as a dorsal 

 nerve. Another of these follows the line of the outer plate 

 of the bifurcating ventral lamina of the embryo as an 

 abdominal nerve. The third branch viz. the filament to 

 the sympathetic and its continuation follows the line of the 

 internal plate of the bifurcating ventral lamina of the embryo. 

 If, then, this view is correct, the sympathetic system essen- 

 D D 2 



