162 THE FROG CHAP. 



The seventh, eighth and ninth nerves supply the 

 muscles and skin of the hind-limbs. The seventh' (7/7) 

 leaves the neural canal between the seventh and eighth 

 vertebrae, the eighth (77/7) between the eighth and ninth, 

 and the ninth (IX) between the ninth vertebra and 

 the urostyle. They all pass almost directly back- 

 wards, and are united with one another by oblique 

 cross branches so as to form the sciatic plexus (sci. pi), 

 from which are given off, amongst others, two nerves 

 to the leg, the largest of which, the sciatic nerve (sci), 

 being that already mentioned in the chapter on the 

 muscular system (p. 62). 



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

 through a small aperture in the side of the urostyle 

 (p. 39), and supplies the cloaca, urinary bladder, and 

 adjacent parts. It is connected by cross -branches with 

 the ninth. 



It will be noticed that while the large ventral branch 

 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, or mixed nerves. They all 

 branch out in a complex manner, and are traceable to 

 the remotest parts of the body. 



The Sympathetic Nerves. On either side of the dorsal 

 aorta is a very delicate nerve, having at intervals little ; 

 swellings called ganglia, each of which is connected with 

 a spinal nerve by a communicating branch (Figs. 51 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 continued 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. 



