26 VERTEBRATED ANIMALS. 
In the greater number, the upper jaw is completely fixed and motionless. 
Both are generally provided with teeth, excrescences of a peculiar nature, 
similar in chemical composition to bone, but which grow from the jaws 
by a process or secretion. The jaws of one entire class, however, (that 
of birds,) and tne genus Testudo, in that of reptiles, are invested with a 
horny substance. 
The intestinal canal extends from the mouth to the anus, in various 
degrees of expansion or contraction. It possesses certain appendices, and 
receives liquids of a solvent nature, viz: saliva, from the mouth, the secre- 
tion of the gland denominated pancreas, and the bile, which is produced by 
another large gland, the liver. In the passage of the food through the 
alimentary canal, the part of it adapted to the purposes of nutrition, and 
termed the chyle, is absorbed by the lacteal vessels, and conveyed into the 
pulmonary artery, where, in combination with the blood, it undergoes a 
certain change; and after each portion of the body has received its proper 
supply, the remainder is carried back into the veins, by a set of vessels 
analogous to the lacteals, and which, together, form what is usually called 
the lymphatic system. The veins carry back to the heart the blood which 
has served the purposes of nutrition. This blood, however, must pass either 
wholly or partially into the organ of respiration, for the purpose of resuming 
its arterial character, before it is carried back by the arteries, to the different 
parts of the body. In the three first classes of vertebrated animals, the organ 
of respiration consists of lungs, an assemblage of small cells, permeable 
by the external air. In fishes alone, respiration is performed by gills, or 
branchie—a series of lamine, between which the water passes. 
In all vertebrated animals, the blood which furnishes to the liver the 
materials of the bile, is supplied from the venous blood which has circulat- 
ed in the intestines, and which, after being reunited in a trunk called the 
vena porte, is again divided at the liver, and distributed in ramifications 
through its substance. 
The sexes in this division are always in separate individuals; but the 
mode in which fecundation is performed, is different in the various classes. 
Though, in all these points, the vertebrated animals have a general resem- 
blance, yet the various beings of which this division is composed, present 
peculiarities, which are the foundation of their arrangement into classes. 
These differences depend upon the nature and energy of their movements, 
which again are always proportioned to the quantum of respiration ; for upon 
the perfection of this function, in a great measure, depend the irritability of 
the muscular fibre, and the energy of the muscular action. The quantity 
of respiration depends upon the relative portion of blood, contained at 
every given instant of time, in the lungs, and the amount of oxygen which 
enters into the composition of the fluid. The quantity of blood is altogether 
determmed, by the peculiar disposition of the organs of respiration and 
circulation. 
