252 THE BIOLOGY OF THE ANIMAL 



glion. How do you distinguish it? What is its 

 shape? Size? Color? Immediately after the 

 two roots unite, the nerve divides again into two 

 branches, one of which runs dorsally, the other 

 ventrally. How do the two branches compare in 

 size ? The dorsal branches run mainly to the 

 muscles and skin of the back. 



Trace, in order, the following ventral branches 

 of the spinal nerves. 



These nerves receive their number from the 

 number of that vertebra back of which they leave 

 the neural canal. 



1. The first spinal nerve or hypoglossal runs 

 forward from behind the first vertebra to the 

 tongue, passing in its course under the mylo- 

 hyoid muscle and into the substance of the ge- 

 niohyoid. 



2. The second and third spinal nerves unite 

 to form the brachial nerve. This gives off a 

 branch to the shoulder muscles and then con- 

 tinues down the arm to form the radial and 

 ulnar nerves. 



3. The fourth, fifth, and sixth spinal nerves 

 run to the body-wall and to the skin. 



4. The seventh, eighth, and ninth spinal 

 nerves unite to form the sciatic plexus, from 

 which arises the sciatic nerve, which contin- 

 ues down the hind limb and divides into the 

 tibial and peroneal nerves. 



5. The tenth spinal nerve or coccygeal 

 emerges from a foramen in the urostyle, and 

 sends branches to the cloaca, bladder, and sur- 

 rounding parts. 



Compare aU of these nerves and note the striking dif- 



