640 



BIRDS. 



fl 



to the forearm, and the sacral plexus to the leg, should be noticed. 

 In the lumbar region the halves of the cords diverge for a short dis- 

 tance, forming a wide space the rhomboidal sinus roofed only by 

 membrane. The cervical part of the sympathetic nervous system is 



double on each side. 



Sense organs. --The sense of smell is not 

 well developed in Birds. The nostrils are 

 longitudinal slits overhung by the swollen, 

 more or less tactile, cere. Apart from the 

 cere there is only a diffuse sense of touch, 

 and the sense of taste is also slightly 

 developed. 



The sense of hearing is acute. Ex- 

 ternally the ear is marked by an open 

 tube the external auditory meatus ; the 

 aperture of which lies behind the eye, 

 concealed beneath the feathers. Within 

 the tube, a little beneath the surface, lies 

 the drum or tympanum ; connecting this 

 with the fenestra ovalis of the inner ear 

 is the columella ; the tympanic chamber is 

 continued past the ear as the Eustachian 

 tube, which unites with that of the opposite 

 side, and opens into the mouth cavity 

 in front of the basisphenoid bone. The 

 cochlea, or curved protuberance of the 

 FIG. 317. Bones sacculus, which is incipient in Amphibians, 

 of hind-limb of an( } i ar ger in Reptiles, is yet more marked 



easjle. -r* i 



in Birds. 



The eye has an upper, a lower, and a 

 third eyelid or nictitating membrane. The 



joint; inf., tarso- J i i 



metatarsus; wz"., last is frequently twitched across the eye, 



fifree). metatarsal an( j ^^ tQ keep the frQnt dean . jj- ig 



present in many Reptiles and most Mam- 

 mals. The front of the sclerotic protrudes in a rounded 

 cone, and is strengthened by a ring of little bones. Into 

 the vitreous humour a vascular pigmented pecten protrudes 

 from the region of the blind spot where the optic nerve 

 enters. Birds have remarkable powers of optic accom- 

 modation. 



Alimentary system.- -The jaws are ensheathed in horn, 



/. . l<ermir; 1. 1. , 



tibio-tarsus ;//'., 

 ; a. ankle- 



