CHARACTERS OF VERTEBRATE ANIMALS 



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 



is 

 marked 



OLFACTORY 



LOBES CEREBRAL 



HEMISPHERES 



OPTIC LOBE 



SPINAL CORD 



CEREBELLUM 



SPINAL CORD 



/ MEDULLA 

 lOBLONCATA 



EUSTACHIAN 

 TUBE 



by 



PECTEN 



OPTIC NERVE 



SCLEROTIC 



CHOROID 



RETINA 



Fig. 103. Structure of Pigeon 



A, Brain from above and from left side: i, olfactory nerve; n, optic 

 nerve. B, Organs of hearing (much enlarged); c, columella: M, position V)r f ^\r\f* 

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



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, which 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 \hefenestra 

 ovalis. The external ear 

 consists simply of a pas- 

 sage leading down from 

 the outside of the head 

 to the tympanic mem- 



t-r^ 



K^inrr 

 lllg 



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 



