28 ANIMALIA VERTEBRATA. 
peculiar energy of sentiment and muscular strength, but in various de- 
grees, corresponding to their quality of respiration: from which originates 
the subdivision of the Vertebrata 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, af- 
ter 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 in proportion to the extent of the intelli- 
gence. 
There are always two jaws, the greatest motion is in the lower one, 
which rises and falls; the upper jaw is sometimes immoveable. Both of 
these are almost always armed with teeth, excrescences of a peculiar na- 
ture, which in their chemical composition are very similar to that of bone, 
but which grow by layers and transudation; 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, ex- 
periencing 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 vari- 
ous names: the two principal ones are—the juices of the gland called the 
pancreas, and bile, a product of another very large gland named the liver. 
While the digested aliment is traversing its canal, that portion of it 
which is fitted for nutrition, called the chyle, is absorbed by particular ves- 
sels styled lacteals, and carried into the veins; the residue of the nourish- 
ment of the parts is also carried into the veins by vessels analogous to 
these lacteals, and forming with them one same system, called the ympha- 
tic system. 
The blood which has served to nourish the parts, and which has just 
been renewed by the chyle and lymph, is returned to the heart by the 
veins—but this blood is obliged, either wholly or in part, to pass into the 
organ of respiration, in order to regain its arterial nature, previous to being 
again sent through the system by the arteries. In the three first classes 
this respiratory organ consists of lungs, that is, a collection of cells into 
which air penetrates. In fish only, and in some reptiles, while young, it 
consists of branchie or a series of lamin, between which water passes. 
In all the Vertebrata, the blood which furnishes the liver with the mate- 
rials of the bile is venous blood, which has circulated partly in the parietes 
of the intestines, and partly in a peculiar body called the spleen, and 
