392 



ZOOLOGY 



SECT. 



a.c 



tJic 



- o.c 



and the latter is solid, having no epicoele. The hemispheres (c.h.) 

 extend backwards to meet the cerebellum, and the optic lobes (o. I.) 

 are thereby pressed outwards so as to take up a lateral instead of 

 the usual dorsal position : these are of rounded form, and each 

 contains an optoccele (Fig. 1057, o. v.) opening from a narrow 

 passage, the iter, which represents the original cavity of the 

 mid-brain. A further result of the extension of the hemispheres 

 and cerebellum respectively backwards and forwards is that no 

 part of the diencephalon (the.) appears externally except on the 



ventral surface : else- 

 where it is seen only 

 when the hemispheres 

 are pressed aside. It 

 contains a narrow ver- 

 tical cavity, the dia- 

 cale (v 3 .), bounded 

 laterally by the optic 

 thalami, and com- 

 municating on each 

 side by the foramina 

 of Monro (f.vn.) with 

 the paracceles or 

 cavities of the hemi- 

 spheres. The corpora 

 striata (c. s,) are of 

 immense size, and 

 form the great mass 

 of the hemispheres : 

 the dorsal portions of 

 the latter, forming 

 the roofs of the para- 

 cceles, are very thin. 

 Hippocampi are ab- 

 sent. The anterior 

 commissure is, as in 

 lower Vertebrates, 

 the chief commissure 

 of the fore - brain. 

 The olfactory bulbs 

 (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. 1051, 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 

 the central canal into a wide, diamond-shaped cavity, the sinus 



9?f- 



il.fi. 



L 



m.o . 



FiQ. 1057. Columba livia. The brain. A, with the cavi- 

 ties opened from above ; B, in sagittal section, a.c. 

 anterior commissure ; cb. cerebellum ; c. h. cerebral hemi- 

 spheres ; c.s. corpus striatum : f.m. foramen of Monro ; inf. 

 infundibulum ; m.o. medulla oblongata ; o.c. optic commis- 

 sure ; o.ch. optic chiasma ; o.l. optic lobes ; olf. olfactory 

 bulbs; o.v. optpccele ; p. peduncles of cerebellum; p.c. 

 posterior commissure ; pn. pineal body ; the. diencephalon ; 

 e 3 ., diacoele ; v 4 ., metaccele. (From Parker's Zootomy.) 



