XIII 



PHYLUM CHORDATA 



393 



rhomboidalis (s. rhb.), 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 turbinals, 

 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 ; 

 its posterior portion, including the middle and posterior turbinals, 

 is invested by the one-layered epithelium of the Schneiderian 

 membrane to which the fibres of the olfactory nerve are distributed. 



The eye (Fig. 1058) is not even approximately globular, but has 



sc 



en 



cl.p 



pet 



FIG. 1058. Columba livia. The eye. A, in sagittal section ; B, the entire organ, external 

 aspect, en. cornea ; eh. choroid ; cl.pr. ciliary processes ; ir. iris ; /. lens ; opt. nv. optic- 

 nerve ; pet. pecten ; rt. retina ; sol. sclerotic ; scl. pi. sclerotic plates. (After Vogt and 

 Yung.) 



the form of a biconvex lens. Sclerotic bony plates (B, scl. pi.) 

 are present, and there is a large pecten (pet.) in the form of a plaited 

 and strongly pigmented membrane projecting into the cavity of 

 the eye from the entrance of the optic nerve. The pecten is 

 stated to be of nervous character, and is in all probability 

 a sensory organ having some function connected with the process 

 of accommodation. 



The auditory organ (Fig. 1059) is chiefly distinguished from that 

 of Reptiles by the great development of the cochlea (lag). The 

 anterior canal (SB) is of great size, and the whole membranous 

 labyrinth is closely invested by a layer of dense ivory-like bone, 

 which can be isolated by cutting away the surrounding spongy 

 bone, and is then seen to form a sort of model of the contained 



