15° 



CHARACTERS OF VERTEBRATE ANIMALS 



OLFACTORY 



LOBES CEREBRAL 

 .HEMISPHERES 



OPTIC LOBE 



SPINAL CORD 



CEREBELLUM 



SPINAL CORD 



r MEDULLA 

 I OBLONGATA 



EUSTACHIAN 

 TUBE 



These last are two rounded elevations situated one on each 

 side, while the corresponding parts in a mammal are four in 

 number, and have a dorsal situation. The cerebellum is very- 

 large, and marked by 

 deep transverse furrows. 

 Sense Organs (fig. 103). 

 — Only the ear and eye 

 deserve special mention 

 under this heading. The 

 organs of hearing con- 

 sist, as in a mammal (see 

 p. 56), of internal, middle, 

 and external ears, but 

 there are a number of 

 differences in detail. The 

 internal ear or mem- 

 branous labyrinth differs 

 most as regards the 

 cochlea,^\{\c^ is a slightly- 

 curved tube (lagena) in- 

 stead of being spirally 

 coiled. In the middle ear 

 it may be noted that in- 

 stead of a chain of auditory 

 ossicles there is a minute 

 rod, the columella, which 

 runs across the tympanic 

 cavity from its external 

 membrane to the fenestra 

 ovalis. The external ear 

 consists simply of a pas- 

 sage leading down from 

 the outside of the head 

 to the tympanic mem- 

 brane, there being no 

 external flap or pinna. 

 It will be remembered, however, that the pinna is also absent in 

 certain aquatic mammals, such as the whales and the true seal. 



The eye of a bird is not, as in a mammal, approximately 

 spherical, but consists of outer and inner halves possessing very 



CHOROID 



PECTEN 



OPTIC NERVE^v^y 



Fig. 103. — Structure of Pigeon 



A, Brain from above and from left side: I, olfactory nerve; II, optic 

 nerve. B, Organs of hearing (much enlarged) ; c, columella: m, position 

 of tympanic membrane; c. Diagrammatic section of eye (enlarged). 



