1884.] NATURAL SCIENCES OP PHILADELPHIA. 265 



At several points the pia and ependyma unite to form the sole 

 elements of the brain-wall, giving the transparent effect, in 

 external view, which has been mentioned. A striking instance of 

 this is seen in the dorsal wall of the infundibular cavity, which 

 is extremely delicate, the ependyma consisting of a single row 

 of cells. The vascular plexuses above the medulla and between 

 the hemispheres are instances of such union, elaborated by the 

 introduction of vascular plexuses from the pia. Three varieties 

 of the cells of the ependyma can be distinguished. The cells of 

 the first variety form a general investment of the inner brain- 

 wall ; they are from one to three deep, cylindrical or much 

 elongated, crowded between them are yellowish oily granules, 

 and many of the cells remotely resemble ordinary fat cells in the 

 possession of a proto-plasmic nucleated centre, lying between 

 yellowish, unstained terminations. It is the innermost of these 

 cells which give rise to thread-like processes which radiate out- 

 wards in the brain-wall, but the latter never make such beautiful 

 displays as are seen in the frog's brain, and figured by Stieda.^ 

 The cells of the second variety lack the fatty granules ; they are 

 found coating the prsecommissura, but are principally observed 

 wherever the brain-wall is reduced to a single row of cells as in 

 the roof of the infundibulum, and in that part of the floor to 

 which the hypophysis is attached; they are small, rounded cells, 

 at one point becoming very much elongated, namely, in the sides 

 of the processus pineahs. The transition from this to the third 

 variety is beautifully shown in the forward portion of the roof of 

 the third ventricle. Here the rounded passes into the beaded 

 character of the single cell layer which follows the elaborate 

 foldings of the diaplexus. 



The consecutive series of sections in three planes afford fine 

 material for the study of the nerve-fibre courses, and much has 

 already been ascertained that throws light upon the relations of 

 the brain segments. I will here describe only the fibre courses 

 which have a transverse direction, considering under this head 

 the relations of the cerebellum,^ the origin of the optic nerves, 

 and the various commissures. 



^ Zeitschrift fiir wiss. Zool., Band xx. 



* Compare E. C. Spitzka. The relations of the Cerebellum, Alienist 

 and Neurologist. New York, January, 1884. 



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