76 THE NERVOUS SYSTEM OF TUB FROG 



between the fourth and fifth, fifth and sixth, and 

 sixth and seventh vertebrae respectively. 



7| 8, and 9. The seventh, eighth, and ninth spinal 

 nerves together form the sciatic plexus The 

 roots of these three nerves within the vertebral 

 canal form the main part of the cauda equina. 

 The seventh nerve leaves the canal between 

 the seventh and eighth vertebrae, the eighth 

 nerve between the eighth and ninth vertebrae, 

 and the ninth nerve between the ninth or 

 sacral vertebra and the urostyle. Outside the 

 vertebral canal the three nerves unite together 

 opposite the middle of the urostyle to form 

 the sciatic plexus, from which branches are 

 given to the large intestine, bladder, oviducts, 

 etc. Just before joining the plexus the seventh 

 nerve gives off the ileohypogastric and crural 

 nerves, supplying the muscles and skin of the 

 abdomen and thigh. Beyond the plexus is the 

 large sciatic nerve, which runs down the thigh, 

 giving branches to it, and dividing a short distance 

 above the knee into the tibial and peroneal nerves 

 supplying the leg and foot. 



The relative sizes of the nerves forming the 

 sciatic plexus, and the mode of their union with 

 one another are subject to considerable individual 

 variation in different frogs. 



10. The coccygeal or tenth spinal nerve emerges 

 through a small hole in the side of the urostyle 

 near its anterior end. It gives branches to the 

 bladder, cloaca, and other adjacent parts, and 

 contributes a branch to the sciatic nerve. 



ii. The dorsal branches of spinal nerves 2 to 8 inclusive 

 are to be found traversing the dorsal lymph-sac in 

 their passage to the skin. 



b. The spinal nerves within the vertebral canal. 



Cut away with scissors the centra of the vertebrae one by one, to 

 expose the spinal cord and the roots of the spinal nerves from t/te 

 ventral surface. Note the following points : 



