THE SPINAL NERVES 75 



within the vertebral canal some distance before reaching their 

 foramina of exit : the bundle formed by these roots, together 

 with the filum terminale, is spoken of as the cauda equina. 



a. The spinal nerves outside the vertebral canal. 



Lay the frog on its back : cut through and pin out the body- 

 walls, and remove tJie abdominal viscera. Note the spinal nerves, 

 seen as u'hite cords at the sides of the vertebral column. Clean 

 the nerves on one side and follow them to their distribution. 



Each nerve divides, directly after the union of its two roots, 

 into a small dorsal branch, and a much larger ventral branch. 



i. The ventral branches. 



1. The hypoglossal, or first spinal nerve, leaves the 

 vertebral canal between the first and second ver- 

 tebrae, and then runs forwards on the under sur- 

 face of the head beneath the mylohyoid and in 

 the substance of the geniohyoid muscle to the 

 chin, where it enters the tongue, in which it 

 ends. It supplies the muscles of the tongue and 

 floor of the mouth, and also some of the muscles 

 of the back and shoulder. (Fig. 10.) It fre- 

 quently sends one or two branches to the brachial 

 plexus. 



2 and 3. The second and third spinal nerves leave 

 the canal between the second and third, and 

 third and fourth vertebrae respectively. The 

 brachial plexus is formed by the fusion of (a) 

 one or two branches from the hypoglossal nerve, 

 (b) the second spinal nerve, (c) a branch from 

 the third spinal nerve. These unite together to 

 form the brachial nerve, which gives oft* a large 

 coracoclavicular branch to the shoulder muscles 

 and then runs down the arm, supplying it with 

 muscular and cutaneous branches, and divides 

 just above the elbow into the radial and ulnar 

 nerves, supplying the forearm and hand. 



4, 5, and 6. The fourth, fifth, and sixth spinal nerves 

 .are small and supply the muscles and skin of 

 the body-wall. They leave the vertebral canal 



