ANATOMY AND PHYSIOLOGY 



21 



page, and observe the result. Examine a model of the human eye or 

 dissection of a mammalian eye if available. 



Exercise 19. — The Spinal Nerves and Sympathetic Trunks. 



(c) The spinal nerves are visible on the dorsal wall of the body 

 cavity as pairs of whitish cords running laterally from the region of 

 the spinal column, or backbone. They arise from the spinal cord 

 (Fig. 21) and emerge in pairs between the vertebrse which make up 

 the spinal column. Surrounding the place of exit of each nerve is a 

 light-colored mass known as the periganglionic gland. The sympa- 



cbm 



Fig. 21. — Nervous system of the frog, from lateral view, cbl, cerebellum; 

 cbm, cerebral hemisphere or cerebrum; ol.L, olfactory lobes; op.L, optic 

 lobes; r.c, ramus communicans; sp.c, spinal cord; sp.n., spinal nerve; symp, 

 part of the sympathetic system; I, II, V, VII, VIII, IX, and X, cranial nerves. 

 The third, fourth, and sixth cranial nerves are small and not shown in 



this figure. 



thetic trunks consist of a delicate pair of nerves running parallel to 

 the spinal column on each side and ventral to the spinal nerves. 

 Each spinal nerve is attached to the neighboring sympathetic trunk 

 by a ramus communicans. Nerves which have been removed extended 

 ventrally from these main trunks of the sympathetic system to the 

 organs of the body cavity. Parts of the sympathetic trunks will be 

 found in your specimen and should be distinguished clearly from the 

 spinal nerves. There are ten pairs of spinal nerves, designated by 

 numbers beginning anteriorly. The large nerves which were seen 

 in the examination of the subclavian artery are the second pair. Trace 

 one of them as far as you can, separating the muscles of the fore leg 



