142 VERTEBRATE ZOOLOGY 



Look for the sympathetic nerves on the sides of the great re- 

 tractor muscles of the head and neck and on the surface of the 

 vertebral column. The delicate white cord will be seen on each 

 side, with ganglionic enlargements. Follow it forward to the 

 brachial plexus; then across this plexus and on the side of the 

 neck, where it will be seen to lie along the inner side of a larger 

 nerve, which is the vagus. Follow it to the head. 



Observe carefully the branches which connect each sympathetic 

 ganglion with a spinal nerve ; also those which pass off from the 

 longitudinal cord and the ganglia to the various organs of the 

 trunk. Expose the whole plexus and note the spinal nerves of 

 which it is composed. Find the nerves which go to the shoulder 

 and arm, and follow them as far as possible. Note the relation of 

 the sympathetic nerve to the brachial plexus. 



Exercise 15. Draw a diagram of the brachial plexus and the nerves 

 proceeding from it, and the anterior portion of the sympathetic 

 system, so far as observed. 



Follow the longitudinal sympathetic nerve posteriorly. Expose 

 the posterior spinal nerves and the lumbo-sacral plexus. This 

 plexus is usually made up of the last six spinal nerves of the trunk 

 and gives rise to the nerves which supply the hind legs and the 

 muscles of the pelvis. Follow these nerves as far as possible. 



Exercise 16. Draw a diagram of this plexus and the nerves springing 

 from it, and of the posterior portion of the sympathetic system, so 

 far as observed. 



The Special Sense Organs. The ear contains two regions, the 

 inner ear, or membranous labyrinth, and the middle ear, or tym- 

 panic cavity, — the former being the essential organ of hearing 

 which receives the auditory nerves, and the latter the space 

 bounded externally by the tympanic membrane. 



Carefully disarticulate and remove the head, taking care not to 

 cut any of the bones of the skull or the cervical vertebrae. 



Remove the skin which covers the tympanic membrane. This 

 will be done easily if the head is first immersed a short time in 

 hot water. Now will be seen the circular tympanic membrane,— 

 a thin plate of cartilage filling the external opening of the tym- 



