86 THE NERVOUS SYSTEM OF THE FROG. 



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 vertebrte, 

 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 Avhich branches are 

 given to the large intestine, bladder, oviducts, 

 etc. Just before joining the plexus the seventh 

 nerves give 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. 



b. The spinal nerves within the vertebral canal. 



Cut away with scissors the centra of the vertehrre one hy one, to 

 expose the spinal cord and the roots of the spinal nerves from the 

 ventral surface. Note the folloidng points : 



i. The roots of the nerves : dorsal and ventral. 



ii. The obliquity of the middle and posterior roots. 



iii. The Cauda equina, formed by the roots of the 

 hinder nerves together with the filum terminale. 



iv. The ganglia on the dorsal roots as they pass 

 through the intervertebral foramina : these are 

 covered on their ventral surfaces by whitish cal- 

 careous patches, which form conspicuous objects 



