THE BLOOD. 409 
(14) and the heart (15), while the trachea (13 13) is seen entering 
through its anterior boundary, below the cesophagus (9 9 9), and the aorta 
passes close to the spine above the latter. 
~ As the walls of the thorax expand by the action of the muscles 
which move the ribs, as well as by the contraction of the diaphragm, 
rendering its thoracic surface less convex, the cavity is enlarged and air 
is drawn in through the trachea, constituting the act of inspiration. On 
the other hand the contraction of the walls, and the forcing upwards 
against the diaphragm of the stomach and liver, by the action of the 
abdominal muscles, reduces the size of the thorax, forces out the air, and 
induces expiration. The repetition of these two actions is known by the 
general term respiration. 
Before proceeding to describe the heart and lungs, it will be necessary 
to examine the blood, for transmitting which fluid to all parts of the 
body the heart and its vessels are formed ; while, for its proper aeration, 
the lungs, windpipe, and larynx, are intended by nature. 
THE BLOOD. 
THE BLOOD, supplied from the food by the digestive process hereafter 
to be described, furnishes all the tissues of the body with a constantly 
renewed stream of the materials which they severally require, whether for 
their nutrition or for the functions of secretion and excretion performed 
by the various organs devote to these purposes. It is necessary, there- 
fore, that this fluid should be composed of elementary matters capable of 
combining to form the materials required, or of those substances ready pre- 
pared. ‘Thus, the muscles demand for their proper action fibrine and 
oxygen, both of which are largely combined in arterial blood, while the 
nervous system cannot respond to the calls of its grand centre without 
having a due supply of fatty matter, also, in combination with the oxygen 
obtained by respiration, which, however, is not only intended to afford 
this gas, but also to remove the carbon that would otherwise accumulate 
to a prejudicial extent. For these several purposes the blood must be 
supplied with lquid elements by absorption from the digestive organs, 
and with its oxygen, by imbibition through the delicate membrane lining 
the lungs on which it is spread as it passes through the system of blood- 
vessels specially set apart for that purpose. When it is considered that 
the stomach, bowels, liver, pancreas, and spleen, are all occupied almost 
solely in supplying the fluid with its grosser materials, and that the heart, 
Jungs, kidneys, and skin, are constantly engaged in circulating it, sup- 
plying it with oxygen, and purifying it from noxious salts and gases, its 
importance in the animal economy may be estimated as it deserves. 
AS IT CIRCULATES in, or immediately after it is drawn from, its appro- 
priate vessels, the blood consists of an opaque, thickish fluid, composed of 
water, fibrine, albumen, and various salts, and called Liquor sanguinis, 
coloured red, by having suspended in it a quantity of corpuscles of a 
peculiar nature, some being without any colour. When drawn from an 
artery or vein, and allowed to remain at rest for a few minutes, a coagula- 
tion takes place, by which the blood is separated into the clot (coagulum) 
and the serum. The former is composed of fibrine, having entangled in 
its meshes the corpuscles ; and the latter is the hquor sanguinis, without 
its fibrine. The blood corpuscles of the horse measure about the five- 
hundredth part of a line in diameter, being considerably larger than those 
of man, whose diameter is only the four-hundred-and-thirtieth part of a 
