CHARACTERS OP THE CLASSES OF VERTEBRATE ANDIALS. 17 



gullet, being co-extensive with the neck, is of great length ; the 

 stomach is always divided into two cavities, the first glandular, the 

 second muscular ; and the distinction between small and large in- 

 testines is usually marked by the presence of two caica. The in- 

 testine terminates, as in the Reptiles, in a common cloaca. 



The cerebral hemispheres have acquired a large proportional size in 

 Birds as compai'ed with Reptiles, and the cerebellum is complicated by 

 many, trans verse folds: but Birds are peculiarly distinguished by the 

 inferior and lateral position of the optic lobes ; and the whole brain 

 presents a more compact form and larger size, in proportion to the 

 spinal cord and nerves, than in Reptiles. The partial enlargements of 

 the spinal maiTow, corresponding to the brachial and lumbar nervous 

 plexuses, are more marked than in Reptiles, and the lumbar en- 

 largement is distinguished by a ventricle. The sense of sight is 

 peculiai-ly keen and perfect in the class of Birds, and the eye has 

 some structures which are not found in other Vertebrata. The 

 organisation of the ear has likewise advanced, a cochlea, though of 

 simple form, being added to the semicircular canals. A circle of 

 feathers radiates from the outer aperture of the ear, forming a con- 

 cave disc or conch, to catch and concentrate the vibrations of sound ; 

 and such an advance is in harmony with the varied power of ex- 

 pressing the feelings and the passions with which Birds are gifted. 

 We may still, indeed, hear in the aquatic members of the class the 

 hiss of the serpent or the croak of the frog ; but as Birds rise in the 

 scale their vocal powers rapidly develope : the cock " with shrill 

 clarion sounds the silent hours," and the nightingale, bursting forth 

 in song, fills all the grove with her varied melody. With regard to 

 the sense of smell we estimate its improvement by observing the 

 more extensive and complex turbinated cartilages in the present 

 Class. But taste must still be dull ; there is no true gustatory nerve, 

 and the tongue is commonly sheathed with horn. The beak is in 

 some birds modified to communicate a delicate faculty of touch, but 

 elsewhere this sense is very limited, the general surface of the body 

 being defended by the dense, imbricated, insensible plumage. 



All birds are oviparous : only the aquatic birds enjoy intromission. 

 The female constructs a nest and incubates her eggs, and, after ex- 

 clusion, cherishes, feeds, and educates her young. 



The class MammaUd, which crowns the vertebrated scries and the 

 animal kingdom, is characterised by a double circulation, a quick 

 respiration by lungs subdivided into minute cells, warm blood, and, 

 with few exceptions, a covering of hair. But the lungs are not fixed 

 in the interspaces of the ribs, as in birds ; nor do they connnunicate 

 with abdominal air-cells ; but are confined, with tlie heart, and freely 



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