176 . BIRDS. 



greater number the female differs from the male in her colours 

 being less bright. The young generally resemble the female. 



The brain of birds has the same general characters as that 

 of the other oviparous vertebrated animals, but is distinguished 

 by its proportionally greater volume, which often exceeds that 

 of the Mammalia. But this apparent magnitude is caused by 

 tubercles analogous to the corpora striata, and not by the hemi- 

 spheres, which are very small and without circumvolutions. The 

 cerebellum is of considerable size, without lateral lobes, and al- 

 most completely formed by the vermiform process. 



The trachea or windpipe in birds is formed of complete rings. 

 At its bifurcation is a glottis provided with muscles, termed the 

 inferior larynx. This is the organ which produces the voice of 

 birds ; and it is afterwards modified by the length, breadth, and 

 elasticity of the trachea and its orifice in the throat. The upper 

 larynx is simple. 



The cavity of the thorax is not in Birds separated by a fleshy 

 partition from the abdomen. The lungs adhere to the spine, and 

 communicate with many membranous sacs situate in the ab- 

 domen, under the axilla, and even in the cavities of the larger 

 bones, the substance of the bill, and in the fistulous portions of 

 the quills. The great quantity of air which birds respire ap- 

 pears to have effect upon all their motions. They respire, it 

 may be said, as well by the branches of the aorta as by those of 

 the pulmonary artery. It is believed that the temperature to 

 which the bodies of birds is raised in hatching, and the great 

 muscular force which they sometimes exert in almost continued 

 flight for many days, depends upon the action of the air upon 

 the blood. 



Birds, like all the other vertebrated animals, possess five sen- 

 ses ; but in this class that of touch is the least perfect. Their 

 feathers prevent them from receiving by immediate contact the 

 impressions of the objects which they touch ; and their feet are 

 enveloped in corneous laminae or scales, which materially blunt 

 sensation. All enjoy the organ of sight ; and by a particular 

 mechanism in the structure of the eye they are enabled to per- 

 ceive objects at a distance with the same facility as when near 

 the body. Besides the two ordinary eyelids there is always 

 a third semitransparent one placed at the internal angle of the 



