IOO ELEMENTARY BIOLOGY. [CHAP. 



a. Median or axial (forward continuations of the 

 central canal of the spinal cord); in order of 

 succession from behind forwards 



a. The fourth ventricle ; tongue-shaped and ex- 

 panded in front ; enclosed by the medulla 

 and cerebellum. 



(3. The iter or aqueduct of Sylvius ; between and 

 below the optic lobes. 



y. The third ventricle; lying within the thala- 

 mencephalon and bounded in front by the 

 lamina terminalis (front wall of the median 

 portion of the cerebro-spinal axis). 



b. Paired. 



a. The optic ventricles ; lying within the optic 

 lobes and derivative of the iter. 



ft. The lateral ventricles (central cavities of the 

 hemispheres) in communication, on either 

 side, with the third ventricle through a wide 

 passage (foramen of Monrd). Each is seen 

 to lodge a thickened fold of pia-mater (cho- 

 roid plexus) which enters it immediately in 

 front of the above-named foramen. (Cf. p. 27.) 



y. The olfactory ventricles ; forward prolongations 

 of /?, extending into the bases of the olfactory 

 lobes. 



v. Longitudinal vertical section. Obtain the brain of 

 the largest frog accessible, and, when fully hard- 

 ened in spirit, remove one half cutting to the near 

 side of the middle line. 



Examine in water under a hand lens, and note 

 the following 



