Remove the abdominal viscera carefully so as to expose the spinal
nerves which are seen as white cords on each side of the vertebral
column as follows:--
(_a_) The first or hypoglossal nerve arising between the 1st and 2nd
vertebræ. It runs forwards on the under surface of the head beneath
the mylo-hyoid muscle and supplies the tongue. It was noted in the
third day's work.
(_b_) The second and third nerves unite immediately after escaping
from the spinal cord to form the brachial plexus, which supplies the
muscles of the shoulder, and then extends along the arm.
(_c_) The fourth, fifth, and sixth nerves are small and supply the
muscles and the skin of the body wall.
(_d_) The seventh, eighth, and ninth nerves run obliquely backwards,
the eighth and ninth uniting to form the sciatic plexus, which gives
rise to the large sciatic nerve which extends along the thigh, and
down the leg. The sciatic plexus also receives a branch from the
seventh nerve.
(_e_) The tenth or coccygeal nerve, which escapes through a small
aperture in the side of the urostyle, and is distributed to the
walls of the cloaca and urinary bladder. It is connected by a branch
with the sciatic nerve. This nerve is frequently small or absent in
the Platana.
[_In the Frog, the seventh, eighth, and ninth nerves unite to form
the sciatic plexus which gives rise to the large sciatic nerve
extending along the thigh, and dividing above the knee into the
tibial and peroneal nerves to the leg and the foot. The tenth nerve
is present._]