ENCEPIIALON. 19 



and the inflation of the folds in the anterior membrane 

 gives the canal a beaded appearance when distended. The 

 lens itself is a transparent double convex body, the poste- 

 rior convexity being greater than the anterior: its consist- 

 ency is greater at the centre or nucleus than it is externally. 

 By allowing it to lie in water for a little while, each surface 

 will separate into three parts by lines radiating from the 

 mtre, those of one side being intermediate in position to 

 lose of the other. If the lens be hardened either in alco- 

 )1 or by boiling, it will be easy to separate it into laminae, 

 like those of an onion, each lamina being divided into three 

 segments by the diverging lines, just spoken of. 



The VITREOUS HUMOR is a jelly-like, transparent sub- 

 stance, filling the greater part of the eyeball behind the 

 iris ; it is contained in a thin and delicate membrane called 

 the hyaloid, which not only envelops it externally, but pene- 

 trates into its interior, and, forming a sort of trabeculated 

 arrangement, helps to support it and retain it in place. 





DISSECTION XIII. 



ANATOMY OF THE ENCEPHALON. 



The density of the cerehral substance increasing with age, the brains 

 of persons advanced in life are the most desirable for examination. 

 The autopsy-room is the only, place where they can be obtained fresh 

 enough for dissection, and the firmness of their tissue may be improved 

 by several days' immersion in alcohol. For inspection, the brain is 

 to be placed with its base uppermost, and with its convexity resting 

 in a coil of cloth, to maintain it in position. 



MEMBRANES AND VESSELS. 



The dura mater has been previously described (Dissec- 

 tion IV.); the arachnoid and pia mater are still connected 

 with the brain after its removal ; all of these membranes' 

 are prolonged into the vertebral canal, and can be seen in 

 dissecting the spinal cord. 



The ARACHNOID, a thin serous membrane, the parietal part 

 of which has been already observed lining the inner surface 

 of the dura mater, is reflected over the pia mater, and may 

 be demonstrated at almost any point where it passes across 

 the intervals between one convolution of the brain and 



