I.] THE FROG. 31 



In the centre of this is a canal, the canalis ceiitralis, 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 surface. The dorsal root presents a 

 small ganglionic enlargement, beyond which it is bound up 

 in a common sheath with the ventral root to form the com- 

 mon trunk of the spinal nerve. The roots of the hinder 

 spinal nerves are very long and lie, side by side, 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 nerve in the Frog answering to 

 the suboccipital. It gives a branch to the muscles which 

 move the head upon the backbone, but the main trunk of it 

 descends behind the mandible, along with the glosso- 

 pharyngeal nerve, and is distributed to the muscles of the 

 tongue. Its distribution therefore answers to that of the 

 hypoglossal nerve in the higher Vertebrata. 



The second and third spinal nerves unite to form a 

 ^hracJiial plexus,^ and are distributed 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 hcmbosacral plexus, whence nerves are 

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

 hind-limb. 



The tenth spinal nerve leaves the neural canal by the 

 coccygeal foramen, and is distributed to the adjacent 

 parts. 



