192 ELEMENTARY BIOLOGY. [CHAP. 



tinued back as a slender filament, \hefilum terminate^ to the 

 commencement of the canal of the urostyle. The diameter 

 of the cord is somewhat enlarged opposite the origin of the 

 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 ven- 

 tricle. 



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 to 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 



