458 THE ANATOMY OF THE HORSE 



As tlie walls of the tliorax expand by the action of tlic 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, b}' 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 

 I'enewed stream of the materials which they severallj' require, whether for 

 their nutrition or for the functions of secretion and excretion performed by 

 the various organs devoted to these purposes. It is necessar}'-, therefore, 

 that this fluid should be composed of elementary matters capable of com- 

 bining to form the materioJs required, or of those substances ready prepared. 

 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 fatt}' matter, also, in combination with the oxygen obtained by respira- 

 tion, which, however, is not only intended to afibrd 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 liquid 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 especially set apart for that 

 purpose. When it is considered that the stomacli, bowels, liver, panci-eas, 

 and spleen are all occupied almost solely in supplying the fluid with its 

 grosser materials, and that the heart, lungs, kidneys, and skin are constantly 

 engaged in circulating it, supplying 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 red corpuscles of a 

 peculiar nature, being white. When drawn from an artery or vein, and allowed 

 to remain at rest for a few minutes, a coagulation 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 liquor sanguinis, without its fibrine. As in all of the mammalia 

 but the camels, these bodies are circular flattened discs, and are of the same 

 size (nearly) in all animals of the same species, whatever may be the age or 



