6 FIRST PRINCIPLES. 



CONSTITUENTS OF PLANTS CONSUMED BY HORSES. 



Water. Woody fibre. 



Nitrogenous matter. Vegetable acids. 



Crude fat.- Mineral matter. 

 Starch and sugar. 



The chemical composition of the constituents of food eaten 

 by horses, will be discussed on page 62 et seq. 



CIRCULATION OF BLOOD AND RESPIRATION. 



The circulation of blood is carried out by the heart, which 

 is a hollow muscle that acts like a force pump, and by tubes 

 (blood-vessels) which are respectively classed as arteries, 

 capillaries, and veins. The arteries convey blood from the 

 heart ; the veins take blood to the heart ; and the capillaries, 

 which are of very minute calibre, form a connection between 

 the arteries and veins. There are two systems in the dis- 

 tribution of blood, namely, the general circulation, and the 

 lung (pulmonary) circulation. In the general circulation, the 

 nutritive matter is carried by the arteries to the capillaries, 

 through the extremely thin walls of which it passes to nourish 

 the various tissues ; and the capillaries take up from the 

 tissues waste material, which is brought by the veins to the 

 heart, and which consists chiefly of carbonic acid, water, and 

 broken-up nitrogenous matter. Having completed the round 

 of the general circulation, the blood, which is now loaded 

 with impurities, is carried by the pulmonary arteries to the 

 air-cell capillaries of the lungs, where most of its carbonic 

 acid escapes into the air cells, and is conveyed from them 

 into the outer air by the bronchial tubes, windpipe and air 

 passages of the head. While passing through the capillaries 

 of the air cells, the blood absorbs oxygen from the air that 

 is in these minute cavities, and carries it through the heart to 

 the capillaries of the general circulation, which give it out to 

 a greater or less extent to the tissues, where it is used for 

 purposes of combustion (oxidation). Haemoglobin, which 



