THE RABBIT 301 



the superior mesenteric artery. These ganglia are joined with the 

 longitudinal sympathetic nerve by several nerves, of which the 

 great splanchnic nerve is the largest. This nerve branches from 

 the sympathetic in the thorax, pierces the diaphragm, and goes 

 directly to the coeliac ganglion. 



The lumbosacral plexus is composed of the ventral rami of the 

 fourth to the seventh lumbar spinal nerves and the four sacral 

 spinals ; the nerves issuing from it supply the muscles of the hind 

 legs and the hinder part of the trunk. This plexus is partly hidden 

 by muscles and may be best studied by rinding the nerves which 

 issue from it and following them back to the plexus. The largest 

 of these nerves are the great sciatic and the femoral nerve. The 

 former, which is the largest nerve in the body, will be seen pass- 

 ing straight back through the pelvis into the leg. Trace it forward, 

 removing the muscles which cover it ; it will be seen to be formed 

 mainly by the union of the seventh lumbar and the first sacral 

 nerve. The femoral nerve is a large nerve which is formed by the 

 union of the fifth and sixth lumbar nerves. Trace it into the leg ; 

 it will be seen to send branches into the muscles of the thigh, and 

 to continue its course, as the saphenous nerve, into the lower leg. 

 Lay bare the entire plexus and follow the main nerves issuing from it. 



Exercise 26. Draw a diagram of the lumbosacral plexus and the 

 nerves issuing from it, so far as observed. 



The Special Sense Organs; the Eye. The eyeball, together with 

 its muscles, glands, and other accessory organs, lies within the 

 bony orbit. Toward the outside the eye is protected by the 

 upper and lower eyelids and the nictitating membrane, the third 

 eyelid. The eyelids are folds of the skin ; they have no eyelashes, 

 but the upper and lower lids are covered with hair, like the rest 

 of the body. The nictitating membrane is situated at the inner 

 corner of the eye, over which it moves. Pull it forward with 

 forceps. A thin membrane called the conjunctiva covers the inner 

 surface of the eyelids, including both sides of the nictitating mem- 

 brane, and the exposed surface of the eyeball. 



Observe the exposed portion of the eyeball, which is called the 

 cornea. The cornea is convex in form and transparent, permitting 



