378 



ZOOLOGY 



SECT. 



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

 rhomboidalis (s. rhb.) bounded above only by the membranes 

 of the cord. 



Sensory Organs.- -The olfactory or (/cms are paired chambers in 

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

 moicl and bounded externally by the ectoethmoid. 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, 



en 



pet 



FIG. 988. Columba livia. The eye. A, in sagittal section; B, the entire organ, external 

 aspect, en. cornea ; ch. choroid ; cl. pr. ciliary processes; ir. iris; I. lens; opt. ne. optic 

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

 Yung.) 



is invested by the one-layered epithelium of the Schneiderian 

 membrane to which the fibres of the olfactory nerve are dis- 

 tributed. 



The eye (Fig. 988) is not even approximately globular, but has 

 the form of a biconvex lens. Sclerotic plates (B. scl.pl.) 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 auditory organ (Fig. 989) is chiefly distinguished from that 

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

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

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



