34 ANIMALIA VERTEBRATA. 
the sides of the cavity of the trunk, they are articulated at 
one extremity with the vertebra, and most generally at the 
other with the sternum; sometimes, however, they do not 
encircle the trunk, and there are genera in which they are 
hardly visible. 
There are never more than two pairs of hichibess but some- 
times one or the other is wanting, or even both. ‘Their forms 
vary according to the movements they have to execute. The 
superior members are converted into hands, feet, wings or 
fins, and the inferior into feet or fins. 
The blood is always red, and appears to be so composed as 
to sustain a peculiar energy of sentiment and muscular strength, 
but in various degrees, corresponding to their quality of re- 
spiration : from which originates the subdivision of the verte- 
brata into four classes. 
The external senses are always five in number, and reside 
in two eyes, two ears, two nostrils, the teguments of the 
tongue, and those of the body, generally. In some species, 
however, the eyes are obliterated. 
The nerves reach the medulla through the foramina of the 
vertebre or those of the cranium; they all seem to unite with 
this medulla, which, after crossing its filaments, spreads out 
to form the various lobes of which the brain is composed, and 
terminates in the two medullary arches called hemispheres, 
whose volume is m proportion to the extent of the intelligence. 
There are always two jaws, the greatest motion is in the 
lower one, which rises and falls; the upper jaw is sometimes 
immovable. Both of these are almost always armed with 
teeth, excrescences of a peculiar nature, which in their chemi- 
cal composition are very similar to that of bone, but which 
" grow by layers and transudations one whole class, however, 
that of birds, has the jaws invested with horn, and the genus 
Testudo, in the class of reptiles, is in the same case, % 
_ The intestinal canal traverses the body from the mouth to 
the anus, experiencing various enlargements and contractions 
having appendages and receiving solvent fluids, one.of which,” 
the saliva, is discharged into the mouth. The others, which 
are poured into the intestine only, have various names: the 
