38 ANIMALIA VERTEBRATA. 



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 members, but sometimes 

 one or the other is wanting, or even both. Their forms vary ac- 

 cording to the movements they have to execute. The superior 

 members are converted into hands, feet, wings or fins, and the infe- 

 rior 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 

 obliterated. 



The nerves reach the medulla through the foramina of the ver- 

 tebrae 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 in pro- 

 portion 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 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, 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 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 



