CHAP. ii. LUNGS AND HEART OF THE HORSE. 407 



in inspiration is nasal cavities, pharynx, glottis, trachea, bronchi, 

 bronchial tubes, air-cells of the lungs. 



The trachea, or windpipe, is a flexible elastic tube, formed of about 

 fifty rings of cartilage or gristle, which are incomplete behind, that 

 is, along the line where the oesophagus touches the trachea throughout 

 its length. The organ of voice, the larynx, is a cartilaginous box at 

 the top of the trachea. When food is swallowed, the act of deglutition 

 causes a little lid, the epiglottis, to close the entrance to the trachea, 

 over which therefore the food glides towards the oesophagus. 



The trachea breaks up primarily into two bronchi, each of which is 

 constructed like a small trachea. Each bronchus branches like a tree, 

 the ultimate ramifications with their air-cells constituting the spongy 

 texture of the lungs. 



The lungs are completely enveloped in a delicate, transparent, 

 glistening, serous membrane, called the pleura, which is reflected over 

 the internal walls of the thorax, so that in the movements incident to 

 breathing, the moist smooth surface of the pleura of the lung glides 

 upon the similar surface of the pleura of the wall of the thorax. 



THE ORGANS OF CIRCULATION. The apparatus of circulation con- 

 sists of the heart, a central organ whence the blood is propelled ; the 

 arteries, or tubes (vessels) which carry blood from the heart ; the veins, 

 or tubes (vessels) along which blood travels to the heart; blood capillaries, 

 very delicate tubes with permeable walls, forming the connection out- 

 side the heart between the arteries and the veins ; the lymphatics, a 

 system of spontaneously arising capillaries and veins which collect the 

 lymph, or overflow from the blood amongst the tissues, and return it 

 to the heart. 1 



The heart is a hollow muscle, divided lengthwise into two inde- 

 pendent chambers the right side on the track of the dark (or venous) 

 blood, the left side on the track of the scarlet (or arterial) blood. 

 Each chamber of the heart is again divided into two, the auricle above, 

 and the ventricle below. The whole organ is enclosed in a delicate 

 serous membrane like the pleura, called the pericardium. Internally 

 there is a similar lining membrane, the endocardium. The heart is 

 situated opposite the third, fourth, fifth, and sixth pairs of ribs. Its 

 base is forward, whilst its apex points downward in the direction of the 

 diaphragm. It is about 10 inches long, its antero-posterior diameter 



1 From the minute hair-like tubes (capillaries) in which arteries end, the portion of the 

 blood which contains all the nutritive elements is constantly leaking into the tissues, which 

 use what amount of it they require, while the rest is taken up into the absorbent vessels 

 (lymphatics), and again is poured into the blood on its return to the heart through the veins 

 by the thoracic duct. The blood is therefore always giving up something to the tissues, and 

 always getting something back again from the absorbents, the total result being that the 

 blood-vessels get rid of the whole quantity of blood which they contain, and get back a 

 similar bulk of new fluid about every twenty-four hours. But like the air in the lungs, 

 the fluid is never entirely good, nor altogether bad ; it is constantly giving up its best 

 materials, and as constantly getting back a mixture of good and bad : the good to be used 

 for the support of the structures, and the bad to be excreted from the skin, kidneys, 

 intestines, and other excretory organs, whose beneficent action is continuous, indeed cauuot 

 be interrupted for a short time even without damage to health, if not risk of life. Sir George 

 Brown, C.B., in Journal of the Royal Agricultural Society, vol. xxii., second series, 1886. 



