246 THE BIOLOGY OF THE ANIMAL 



not injure the pigmented membrane covering the 

 brain. "When the roof of the skull has been re- 

 moved, cut away in the same manner the tops of 

 the neural arches and urostyle. Prepare another 

 specimen by fastening it down on its back and 

 removing all of the viscera. 



I. Oeneral structure. 

 a. Compare the two specimens and note that the 

 nervous system consists of two portions, a central 

 portion, enclosed in the cerebro-spinal canal, 

 and a peripheral portion, consisting of nerves 

 running to various parts of the body. ISTotice also 

 that the central nervous system is divisible into 

 two regions, an anterior portion, the enceplialon 

 or brain, lying in the skull, and a posterior por- 

 tion, the myelon or spinal cord, lying in the 

 neural canal. How do these two regions compare 

 in length? In diameter? Are they distinctly 

 separated from one another? How do you dis- 

 tinguish the one from the other ? 



h. The membranes. — Examine the pigmented mem- 

 brane, the pia mater, which covers these parts of 

 the nervous system. "What is its color ? Is it even- 

 ly colored ? If not, where are there noticeable 

 collections of pigment cells ? Does the pia mater 

 cover all parts lying within the cerebro-spinal 

 canal ? Note the large blood-vessel running along 

 the middle of the dorsal surface of the pia mater. 

 Examine the membrane with a lens, and note the 

 numerous blood-vessels. Divide the pia mater 

 immediately behind the posterior lobes of the 

 brain, and examine that part of it which, form- 

 ing a choroid plexus, covers a triangular cav- 



