i62 THE FROG chap. 



and eighth vertebra, the eighth ( VIII) between the eighth 

 and ninth, and the ninth {IX) between the ninth vertebra 

 and the urostyle. They all pass almost directly backwards, 

 and are united with one another by oblique cross branches 

 so as to form the sciatic plexus {sci.pl), from which are 

 given off, amongst others, two nerves to the leg, the largest 

 of which, the sciatic tterve {Sci) being that already mentioned 

 in the chapter on the muscular system. 



The tenth (X) is a very small nerve. It emerges through 

 a small aperture in the side of the urostyle, and supplies 

 the cloaca, urinary bladder, and adjacent parts. It is con- 

 nected by cross branches with the ninth. 



It will be noticed that while the large ventral part of the 

 first spinal nerve — the hypoglossal — supplies muscles only, 

 and is therefore a motor nerve, all the others go to both 

 muscles and skin, and are therefore both motor and sensory, 

 ox mixed nerves. They all branch out in a complex manner, 

 and are traceable to the remotest parts of the body. 



Tie Sympathetic Nerves. — On either side of the dorsal 

 aorta is a very delicate nerve, having at intervals little swell- 

 lings called ganglia, each of which is connected with a spinal 

 nerve by a communicating branch (Figs. 5T and 53 Sy, Sy.g, 

 Sy. c). In front of the point where the dorsal aorta {D. Ao) 

 is formed by the union of the two systemic trunks {Ao), 

 these two sympathetic nerves, as they are called, are con- 

 tinued forward, one on either side of the vertebral column, 

 towards the head, when they enter the skull and become 

 connected with certain of the cerebral nerves. 



Each sympathetic nerve has altogether nine or ten ganglia, 

 each connected with one of the spinal nerves, and from 

 the ganglia branches are given off which supply the heart 

 and blood-vessels, the stomach, liver, kidneys, reproductive 

 organs, rectum, and urinary bladder. 



