36 VEllTEBRATE ANIMALS. 



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

 energjr of sentiment and vigour of muscles, but in different degrees, which corres])ond 

 to the amount of respiration, from wliieh 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 tlie cra- 

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

 expands to form the various lobes of which the brain is com])Osed, and terminates in 

 the two medullary arches (voutes) termed hemispheres, the volume of whicli corre- 

 sponds to the amount of intelligence. 



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

 whieli rises and tails ; the np|ier is ottentimes entirely fi.xed ; bi^th of them are almost 

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

 \Ahich is verj' 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 wdiieb, 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 sweet-bread^, and the bile [or gtiW], 

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



Whik' the digested aliment is traversing its canal, tliat portion of it which is proper 

 for nutrition, and is termed the chyle, is absorbed hy particular vessels, named laeteals, 

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

 the veins by vessels analogous to the laeteals, and forming ^^itli them one same system, 

 designated the hjmpliatic si/f!tein.\ 



The veins return to the heart the lilood 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 wdioUy or in part, into the organ of resj)iration, 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 wdiich air penetrates. In fishes only, and in some reptiles while young, it 

 consists of gills, or a series of lamin;!' between wliicb water jjasses. 



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

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

 and partly in a ])eculiar body named the spleen, and which, after being united in a 

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



s ill thL- liiirny sliciiih, ^ubKequ.-iitlr formed, which cnnt^in n 



» M. GLiiflroy St. Hilaire lias described ;i structure in lliu lull of 

 birds u liich fircseiits some aiiproaeli to a deiitarj- system. In a teetus ul 

 II Piirruifuet nearly ready for hatcliin|r, lie found that the niar|,'ius of the 

 hill «ere heset with tubercles arrHn^rell In a regular order, and having 

 all I lie e.\terlora|jj)earanee of teeth ; these (ulierelcs were not, indeed, 

 iiii|ilanted In the jaw-bones, but formed part of tlie e.vterior slieath of 

 he hill. Under each tubercle, however, there was a t'elatiiious pulf), 

 anilognus to the pulps which secrete teetli, but resting on the edije of 

 llie nia.\illary bones, and every pulp was supplied by vessels and nerves 

 iraversing a canal in the suhstaiiec of the hone. These tuhereles form 



gin of the bone. In certain other hlrds [as the iMergatisers) also, the 

 lateral edges of the hill are provided with horny processes or lamlnfe 

 secreted by illstlnct pulps, and analogous in this respect to the ^^ bale- 

 bone lamliue of the Whales, whicli are toothless Miimm/inn, as are also 

 the ant eaters and il/naufiewi/ta .- it is furtlier remarhahle that the 

 rudiments of dentition occur in the JiFlin of the toothless \\'liales. 

 — Ko. 



■1 'riic lymiihalic vessels arc also the media of cutaiicims transuda- 



