130 Ctwierian ^stem of Zoology, 



similar to bone in their chemical composition, but which 

 increase by layers {couches) and by transudation. But one 

 entire class, namely, birds, have their jaws covered with 

 horny beaks, and this is the case with the genus tortoise in 

 the class of reptiles. The above characters, which are chiefly 

 external, would enable any person unacquainted with physio- 

 logy to distinguish verlebrated animals from those in the other 

 divisions. Some orders of articulated animals approach in 

 form the nearest to vertebrated animals, but they have no 

 internal skeleton. 



The characters derived from the internal structure and orga- 

 nisation are striptly physiological, and the learner must refer 

 to what has been stated in the first and second essays respect- 

 ing them. 



Jn the division of vertebrated animals, the nerves unite with 

 the spinal jpord through passages in the vertebrae, or in the 

 skull s they appear all to unite into a double bundle {faisceau) 

 which forms this spinal cord, and which, after having crossed 

 its filaments, spreads and swells out to form the divers tuber- 

 dps of which the brain is composed, and terminates in two 

 medullary masses called hemispheres, the I'elative volume of 

 which corresponds with the extent of intelligence. 



The intestinal canal extends from the mouth to the anus, 

 undergoing, in its course, various expansions and contractions : 

 it has different appendages, and receives different dissolving 

 fluids. That which pours into the mouth is called the saliva; 

 other fluids which enter the intestines have received different 

 names; the two principal are the secretion from the gland 

 called the pancreas, and the bile which is secreted by another 

 very considerable gland called the liver. 



While the digested aliments traverse the 'alimentary canal, 

 the part proper for imtrition, called the*chyle, is absorbed by 

 particular vessels called lacteals, and is carried into the veins ; 

 the residue of the nutrition is also carried into the veins by 

 vessels analogous to the lacteals, forming with them one sys- 

 tem, called the system of the lymphatic vessels. 



The veins bring back to the heart the blood which has 

 served to nourish the different parts, and which has been 

 renewed by the chyle and the lymph ; but this blood is obliged 

 to pass wholly, or in part, into the organ of respiration, to 

 regain the nature of arterial blood by the absorption of oxygen, 

 and by exhalation, before it is carried back into the arteries. 

 In the first three classes of animals, the respiratory organs are 

 lungs, an assemblage of cells into which the atmospheric air 

 penetrates. In fishes the respiration is performed by gills, a 

 series of lammae between which the water passes. 



