36 



VERTEBRATE ANIMALS. 



The blood is always red, and appears to have a composition proper for sustaining that 

 energy of sentiment and vigour of muscles, but in different degrees, which correspond 

 to the amount of respiration, from which originates the subdivision of the vertebrate 

 animals 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. Certain species, 

 however, have the eyes obliterated. 



The nerves reach the medulla through perforations of the vertebra?, or of the cra- 

 nium : they all seem to unite with this medulla, which, after crossing its filaments, 

 expands to form the various lobes of which the brain is composed, and terminates in 

 the two medullary arches (vuutes) termed hemispheres, the volume of which corre- 

 sponds to the amount of intelligence. 



There are always two jaws, the principal motion of which is in the lower one, 

 which rises and falls ; the upper is oftentimes entirely fixed : both of them are almost 

 always armed with teeth, excrescences of a peculiar nature, the chemical composition of 

 which is very similar to that of bone, but which grows by layers and transudations ; 

 one entire class, however, (that of birds,) has the jaws invested with horn*, and the 

 group of tortoises, in the class of reptiles, is in the same predicament. 



The intestinal canal is continued from the mouth to the anus, undergoing various 

 inflexions, and several enlargements and contractions ; having also appendages, and 

 receiving solvent fluids, one of which, the saliva, is discharged into the mouth : the 

 others, which flow into the intestine only, have various names ; the two principal are 

 the juices of the gland called the pancreas [or sioeet -bread'], and the bile [or gaW], 

 which is the product of another very large gland, named the liver. 



While the digested aliment is traversing its canal, that portion of it which is proper 

 for nutrition, and is termed the chyle, is absorbed by particular vessels, named lacteals, 

 and carried into the veins ; the residue of the nutriment of the parts is also carried into 

 the veins by vessels analogous to the lacteals, and forming with them one same system, 

 designated the lymphatic system.\ 



The veins return to the heart the blood which has served to nourish the parts, to- 

 gether with the chyle and lymph with which it has been renewed ; but this blood is 

 obliged to pass, either wholly or in part, into the organ of respiration, to regain its 

 arterial nature, previous to being again dispersed over the system by the arteries. In 

 the three first classes, this organ of respiration consists of lungs, that is, an assemblage 

 of cells into which air penetrates. In fishes only, and in some reptiles while young, it 

 consists of gills, or a series of laminae between which water passes. 



In all the vertebrate animals, the blood which furnishes the liver with the materials 

 of the bile is venous blood, which has circulated partly in the parietes of the intestines, 

 and partly in a peculiar body named the spleen, and which, after being united in a 

 trunk called the vena porta, is again subdivided at the liver. 



* M. Geoffroy St. Hilaire has described a structure in the bill of 

 birds which presents some approach to a dentary system. In a fcetus of 

 a Parroquet nearly ready for hatching* he found tliat tbe margins of the 

 bill were beset with tubercles arranged in a regular order, and having 

 all the exterior appearance of teeth ; these tubercles were not, indeed, 

 implanted in tbe jaw-bones, but formed part of the exterior sbeath of 

 he bill. Under each tubercle, however, there was a gelatinous pulp, 

 analogous to tbe pulps which secrete teeth, but resting on tbe ed^c of 

 the maxillary bones, and every pulp was supplied by vessels and nerves 

 traversing a canal in tbe substance of the bone. These tubercles form 

 the first margins of the mandibles, and their remains are indicated by 



canals in the horny sheath, subsequently formed, which contain a 

 softer material, and which commence from small foramina in the mar- 

 gin of the bone. In certain other birds (as the Mergansers) also, the 

 lateral edges of the bill are provided with horny processes or lamina? 

 secreted by distinct pulps, and analogous in this respect to the whale- 

 bone lamina? of the Whales, which are toothless Mammtllia, as are also 

 the ant eaters and Monotremtita : it is further remarkable that the 

 rudiments of dentition occur in the Jwtus of the toothless Whales. 

 — Kn. 



t The lymphatic vessels are also the media of cutaneous transuda- 

 tion. — Kn. 



