THE SPINAL NERVES. 79 
arches of the vertebree one by one, according to the directions given 
for the dissection of the central nervous system ; or else lay the frog 
on its back and cut away with stout scissors the bodies of the ver- 
tebree so as to expose the spinal cord and nerves from the ventral 
surface. Note :— 
i. The roots of the nerves: dorsal and ventral. 
ii, The obliquity of the middle and posterior roots. 
iii, The cauda equina: formed by the roots of the 
hinder nerves together with the filum terminale. 
iv. The ganglia on the posterior roots as they pass 
through the intervertebral foramina: they are 
best seen from the ventral surface, where they 
are covered by whitish calcareous patches, which 
form conspicuous objects on either side of the 
vertebral column. Hemove these patches carefully 
to see the ganglia. 
b. The spinal nerves outside the vertebral canal : 
Lay the frog on its back: cut through and pin out the body- 
walls, 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- 
tebree, and then runs forward 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. 13.) 
2 and 3: the second and third spinal nerves leave 
the canal between the second and third, and 
third and fourth vertebre respectively: they 
unite together to form the brachial nerve, which 
gives off a large coracoclavicular branch to the 
shoulder muscles and then runs down the arm, 
supplying it with muscular and cutaneous 
