16 
INTRODUCTORY. 
that its volume is proportioned to the intellectual energy. The 
true nature of the medullary substance has not yet been ascer¬ 
tained by anatomists. When minutely examined, it appears a 
soft matter, of a cream-coloured white; and seems to consist 
of infinitely small globules. It appears itself to be devoid of 
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 
from it, which conducts the numerous 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 the 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- 
