XIII.] THE FROG. 235 



9. The brachial nerve. 



Lay it bare in the arm-pit and follow it back to the 

 spinal cord : it is formed by the union of the 2nd and 

 3rd spinal nerves. 



10. The sympathetic system. 



a. Gently raise the aorta : along each side of it will 

 be found the main sympathetic trunk. 



b. A slender cord, with enlargements (ganglia) on 

 it at intervals. 



c. Note the branches passing between its ganglia 

 and the nerves of the sciatic plexus. 



d. Carefully dissect out the gangliated cord for its 

 whole length : ten ganglia, each provided with 

 communicating branches to other (spinal) nerves, 

 will be found on it. 



H. THE ORGANS OF SPECIAL SENSE. 



The complete examination of these, especially as re- 

 gards their histology,[is difficult, and necessitates the em- 

 ployment of niceties in manipulation which it would be 

 beyond the scope of this work to describe, so that in the 

 following account attention is mainly given to those 

 points which can be made out without the microscope. 

 A brief account of the microscopic structure of the retina 

 will however be found below (J. h). 



a. The Eye. 



1. Take an uninjured frog and examine its eye. It will 

 normally be found to project considerably above the 

 top of the head, but if touched it is withdrawn into a 

 sort of socket. If the animal's mouth be opened, an 

 elevation, caused by the eye-ball, will be seen on its 



