408 AVES. 
webbed stem, hollowed at its base; these webs or horizontal feathers are 
again themselves webbed by still smaller ones; and the texture, strength, 
justre, and general form of these feathers are infinitely varied. The feathers 
fall off twice a year, and this change of plumage is termed moulting. In 
some species, the winter plumage differs from that of the summer; and in 
the greater number the female differs from the male, in her colors 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 1s 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 hemis- 
pheres, which are very small and without circumvolutions. The cerebel- 
lum is of considerable size, without lateral lobes, and almost 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 com- 
municate with many membranous sacs, situate in the abdomen, under tne 
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, appears 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 exert in almost continued flight for many days, depends upon the action 
of the air upon the blood. Birds, like all the vertebrated animals, possess 
five senses; but in this class, that of touch is least perfect. Their feathers 
prevent them from receiving, by immediate contact, the impressions of the 
object which they touch; and their feet are enveloped in corneous lamina, 
or scales, which materially blunt sensation. All enjoy the organ of sights. 
and by a particular mechanism in the structure of the eye, they are enabled 
to perceive objects at a distance with the same facility as when near the 
body. Besides the two ordinary eyelids, there is always a third semi-trans- 
parent one placed at the internal angle of the eye, which, by the assistance 
of a remarkable muscular apparatus, may be drawn before the eye like a 
curtain. The cornea is very convex. Although birds have no external 
cartilaginous ear, all appear to enjoy the faculty of feeling. Some of the 
nocturnal birds have the auditory opening surrounded by feathers. The 
organs of smell are concealed in the base of the bill, and the breadth of the 
nasal openings determine their form. The sense of smell in the vulture 
and raven, is said to be so very acute, that they can smell carrion at a very 
