184 ELEMENTARY BIOLOGY. [XIII. 



nerves for the limbs. In transverse sections, the cord is seen 

 to be not truly cylindrical and to be indented by two 

 longitudinal grooves, one dorsal and one ventral, which leave 

 but a small connecting bridge between its two halves. In 

 the centre of this is a canal, the canalis centralis, the cavity 

 of which is continued forwards into the fourth ventricle. 



Ten symmetrically disposed pairs of nerves come off from 

 the sides of the cord, each nerve having two roots, one from 

 the dorsal surface of the lateral half of the cord and one 

 from the ventral half. The dorsal root presents a small 

 ganglionic enlargement, beyond which it joins the ventral root 

 t3 form the common trunk of the spinal nerve. The roots of 

 the hinder spinal nerves are very long and lie, side by side, 

 for some distance, in the spinal canal. 



The first spinal nerve leaves the neural canal by the 

 interspace between the arches of the first and second 

 vertebrae, so that there is no suboccipital nerve in the Frog. 

 It gives a branch to the muscles which move the head upon 

 the atlas, but the main trunk of it descends behind the 

 mandible, along with the glossopharyngeal nerve, and is 

 distributed to the muscles of the tongue. It therefore 

 answers to the hypoglossal nerve in the higher Vertebrata. 



The second and third spinal nerves, of which the second 

 is the larger, unite to form a ' brachial plexus,' and are dis- 

 tributed chiefly to the fore-limb. 



The fourth, fifth and sixth spinal nerves go to the middle 

 parietes of the body. 



The seventh, eighth and ninth, are large nerves which 

 unite to form the lumbosacral plexus, whence nerves are 

 given off to the posterior parietes of the body, and to the 

 hind-limb. The nerves of the latter are the crural to the 

 front part of the thigh, and the sciatic, which passes to the 

 back of the thigh and ultimately divides into the peronceal 

 and tibial nerves which supply the leg and foot. 



