4.32 INTRODIICTOKY. 



motion ; but from it are transmitted to the mind, the impres- 

 sions of the senses ; and from it are conveyed the impulses of 

 the will to the different muscles of the body. 



The brain appears to be principally composed of the medul- 

 lary substance : the spinal marrow is the great stem leading 

 I'rom it, which conducts the numej-ous ramifications of the 

 nerves through every part of the body. The muscular fibre 

 consists of many series of filaments, whose distinctive property 

 consists in contraction, arising, either from the contact of an 

 external body, or the agency of the will carried along the 

 nerves. 



The whole elements of the body are derived from the nutri- 

 tive or vegetative system. Substances taken into the stomach 

 are converted into fluids, of which the blood is the prin- 

 cipal, and contains the general elements of the animal sys- 

 tem. It is composed of carbon, hydrogen, oxygen, and azote ; 

 in combination with fibrin, gelatine; and the compound sub- 

 stance called albumen, forming a constituent part of the animal 

 solids and fluids ; phosphorus and lime, from which the bones 

 are principally formed ; iron, and oily or fatty matter. 



Hence, the nutritive fluid sustains, and supplies the expen- 

 diture of all the solid parts of the body, which are subject to 

 perpetual change, from respiration, perspiration, &c. 



The intestinal canal extends from the mouth, to the opening 

 of the rectum ; and in its length, consists of various degrees of 

 expansion, which anatomists have designated by different names. 

 The food, in its passage through this alimentary canal, is con- 

 verted into chyle, the fluid of nutrition, and is absorbed by ves- 

 sels called thd lacteals, and conducted into the veins, by a sepa- 

 rate set of vessels ; which form what is termed the lymphatic 

 system. 



The blood, after having served the purposes of nutrition, is 

 returned to the heart by the veins. This blood, for the purpose 

 of restoring its arterial character, must pass, either wholly or in 

 part, through the lungs, or organ of respiration, before it is con- 

 veyed by the arteries to the different parts of the body. In the 

 three first classes of animals with vertebrae, the lungs consist of 

 a number of small cells, through which the external air passes 

 on nihalation. 



The quantity of respiration depends on two causes: first, 



