THE CHEST, ITS CONTENTS AND CONNECTIONS. 49 



The Nostrils. — The cavity of the nostrils is very extensive in 

 the horse, affording a large entrance for the air in breathing, and 

 a considerable surface for the development of the organ of smell. 

 It is lined throughout by a mucous membrane called the Schneid- 

 erian or pituitary membrane, on which is spread out the nerve of 

 smell as well as a nerve of feeling. The cavity is divided by a 

 tliick cartilage, called the septem nasi, which is fixed in front to 

 the nasal, and behind to the maxillary, bones. This cartilage is 

 also covered by the membrane before spoken of; which, when 

 inflamed, as in colds, becomes very red, and in glanders is often 

 the seat of ulcers. 



The nostrils communicate with the sinuses of the head, and 

 above with the larynx. 



The entrance to the nostrils is formed by what is called the 

 false nostrils, which is formed by the skin, and possesses consider- 

 able flexibility, particularly in well-bred horses, in whom the 

 opening to the nostrils is much larger than in coarse-bred 

 animals. The false nostrils form a sort of cavity, which has no 

 opening at the upper part ; and this circumstance gives them 

 their name. 



The Heart 



Is a strong hollow muscle, of a conical shape, with its base 

 towards the spine, and its apex towards the left side, against 

 which it Is thrown at every contraction. It Is double, having a 

 right and a left side, the former containing black, and the latter 

 red, blood ; the right side Is the thinnest and weakest, being 

 devoted to the lesser office of the circulation of the lungs ; the 

 left, the stoutest, having to govern the general circulation of the 

 system. Each of these halves consists of two cavities, an auricle 

 and a ventricle : the former, which derives its name from its re- 

 semblance to a dog's ear. Is considerably thinner than the latter, 

 and is situated towards the base. The heart is formed prin- 

 cipally of fleshy fibres, connected together by cellular tissue, 

 whence It obtains its elasticity ; and Its surfaces, both internal 

 and external, arc lined by a fine transparent membrane. The 

 blood is prevented from moving In a retrograde course by 

 means of a number of valves : there are three In the left ven- 

 tricle, the edges of which are connected by tendinous cords 

 (cordaj tendlnere) to small fleshy eminences on the Inside 

 of the ventricle, called carneaj columns or fleshy columns. 

 These tendinous cords are more numerous In the valves of the 

 left ventricle than in the other parts, and being supposed with 

 the valves to resemble a mitre, are named mitral valves. There 

 are valves also In tlie right ventricle for a similar purpose, which 

 are named tricuspid, or three-pointed ; also in the great artery 

 or aorta, and in the pulmonary artery, where, having no cords. 



