XIlI 



Phylum chordata 



405 



developed olfactory organ : on the other hand, the optic nerves and 

 tracts are of unusual size. 



The spinal cord (Fig. 1040, sp. ed.) presents large brachial and 

 lumbar enlargements from which the nerves of the fore- and 

 hmd-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 simcs 



Fig. 1046.— Coluxnba livia. Ttte brain. A, with the cavities opened from ahove ; B, in 

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

 c.s. corpus striatum ;/. m. foramen of Monro; 171/. infundibulum ; m,o, medulla oblongata; 

 0. c. commissure of optic lobes ; 0. ck. optic chiasma ; 0. 1, optic lobes ; oif. olfactory bvilbs ; 

 0. V. optoccele ; p peduncles of cerebellum ; p, e, posterior commissure ; 2>7t. pineal body ; 

 the, dieneephalon ; v. S, diactfile ; v.h, metaccele. (From Parker's Zootomy.) 



rhomioidalis (s. rhh.), bounded above only by the membranes 

 of the cord. 



Sensory Organs. — The olfactory organs are paired chambers in 

 the base of the beak, separated from one another by the meseth- 

 moid and bounded externally by the ecto-ethmoid. The latter 

 is produced inwards into three scroll-like processes, the turhinals, 

 which greatly increase the surface of mucous membrane. The 

 anterior portion of the cavity, including the anterior turbinal, 

 is covered by laminated epithelium and serves as a vestibule; 



VOL, II C C 



