Eighth Day. 



Nervous System {continued). 



B. The Peripheral Nervous System. 



1. The Spinal Nerves. 



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 



vertebra?. 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 



Platan a. 



[In the Frog, the seventh, eighth, arid 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.] 



2. The Sympathetic Nervous System. 



This system consists of a cord on each side closely following the course 

 of the systemic arches and the dorsal aorta. It has on it six ganglionic 

 swellings, and there are connections between the ganglia and all the spinal 

 nerves. Anteriorly each of the cords is joined to the vagus nerve ; it ends 

 posteriorly as a broad swelling on the ninth spinal nerve. 



[In the Frog the system consists of a chain of ganglia on each side of the 

 body closely following the course of the systemic arches and the dorsal aorta. 

 The cords have connections with the spinal nerves. Anteriorly each cord ends 

 in the Gasserian ganglion of the trigeminal nerve and posteriorly it is united 

 with the coccygeal nerve by a varying number of branches.] 



Make a complete sketch showing the spinal nerves and the sympathetic 

 nervous system, 



44 



