28 ANIMALIA VERTEBRATA. 



There are never more than two pair of members, but sometimes one 

 or the other is wanting, or even both. Their forms vary according to 

 the movements they have to execute. The superior members are con- 

 verted 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 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 obli- 

 terated. 



The nerves reach the medulla through the foramina of the vertebrae 

 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 medul- 

 lary arches called hemispheres, whose volume is in 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 immoveable. Both 

 of these are almost always armed with teeth, excrescences of a peculiar 

 nature, 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, which is also 

 the case in the genus Testudo, in the class of Reptiles. 



The intestinal canal traverses the body, experiencing various enlarge- 

 ments 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 

 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 par- 

 ticular vessels styled lacteals, and carried into the veins ; the residue 

 of the nourishment of the parts is also carried into the veins by vessels 

 analogous to these lacteals, and forming with them one same system 

 called the lymphatic 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 



