XIII 



PHYLUM CHORDATA 



377 



that no part of the diencephaloii (the.) appears externally except 

 on the ventral surface : elsewhere it is seen only when the 

 hemispheres are pressed aside. It contains a narrow vertical 

 cavity, the diaccde ( V. 3), bounded laterally by the optic thalami, 

 and communicating on each side by the foramina of Monro (/. m.) 

 with the paracaeles or cavities of the hemispheres. The corpora 

 stmata (c. s.) are of immense size, and form the great mass of the 



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C.S 



o.c 



ITlf 0.0. 



m.o. 



FIG. 987. Columba livia. The brain. A. with the cavities opened from above; B, in 

 sagittal section, a. c. anterior commissure ; cb. cerebellum ; c. h. cerebral hemispheres ; 

 c. .*. corpus striatum ; f. m. foramen of Monro ; inf. infundibulum ; m. o. medulla oblongata ; 

 o. c. optic commissure ; o. ch. optic chiasma ; o. 1. optic lobes ; o. c. optoccele ; p. peduncles 

 of cerebellum ; p. c. posterior commissure ; pn pineal body ; the. diencephaloii ; r. 

 diaccele ; c. 4, rnetaccele. (From Parker's 



hemispheres : the dorsal portions of the latter, forming the roofs of 

 the paracceles, are very thin. The olfactory lobes (olf.) are extremely 

 small, in correspondence with the poorly developed olfactory 

 organ : on the other hand the optic nerves and tracts are of 

 unusual size. 



The spinal cord (Fig. 981, sp. cd.) presents large brachial and 

 lumbar enlargements from which the nerves of the fore and 

 hind limbs respectively are given off. In the lumbar enlargement 

 there is a divergence of the dorsal columns of the cord converting 



