2 44 ELEMENTARY BIOLOGY. [CHAP. 



cord, with enlargements (ganglia) on it at in- 

 tervals. 



b. Note the branches passing between its ganglia 

 and the nerves of the sciatic plexus. 



c. 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 

 employment 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 re- 

 tina will however be found below (J. h). 



a. The Eye. 



i. 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 top of the head, but if touched it is withdrawn into 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 roof, and this is more prominent when the eye-ball 

 is retracted. 



a. Gently touch the eye and observe how it is closed, 

 by the pulling over it of the lower transparent 

 eye-lid. The upper eye-lid is very small and 

 hardly moveable. 



