THE SPINAL NERVES. 85 



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 hack : cut through and jom out the hody- 

 ivalls, and remove the abdominal viscera. Note the spinal nerves, 

 seen as white 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. 



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

 vertebral canal between the first and second ver- 

 tebrse, 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. 15.) 



2 and 3. The second and third spinal nerves leave 

 the canal between the second and third, and 

 third and fourth vertebrae respectively : they 

 unite together to form the brachial nerve, which 

 gives off a large cor aco clavicular 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 

 between the fourth and fifth, fifth and sixth, and 

 sixth and seventh vertebrae respectively. 



7, 8, and 9. The seventh, eighth, and ninth spinal 

 nerves together form the sciatic plexus. The 

 roots of these three nerves within the vertebral 



