472 THE ANATOMY OF THE HORSF- 



than these, but also on the side of the trachea. At the upper and back 

 part of the trachea a layer of muscular fibre is found, connecting together 

 the posterior edges of the cartilages. The oflS.ce of these muscular bands is 

 to diminish the area of the trachea. 



The broxchi consist at first of the two tubes into which the trachea 

 divides, the right being the more capacious of the two. Afterwards they 

 subdivide, like the branches of a tree, into lesser tubes, still called bron- 

 chial, which finally open into the air-cells of the lungs. These tubes differ 

 from the trachea in that each ring of cartilage is made up of several distinct 

 pieces, which overlap each other, and thus allow of considerable dilatation 

 during forcible expiration. The rings are held together by an elastic cellu- 

 lar substance, and are lined first by a fibrous layer, with which it is supposed 

 that some muscular tissue is mixed up, as in the lai-ger bronchi, and 

 internally by fine mucous membrane. 



THE LUNGS 



The lungs consist of two conical spongy bodies, adapted to the shape 

 of the thorax, the left being the smaller of the two. Between these halves 

 of the lungs is a space called the mediastinum, already described, occupied 

 by the heart, great blood-vessels, nerves, and glands ; they are capable of 

 great dilatation by the act of inspiration, and of being again reduced in size 

 by expiration. In sti'ucture, they are made up of three distinct parts — (1) 

 an external or serous coat, called the pleura, described at page 456 ; (2) a 

 middle or true pulmonary tissue, consisting of the intercellular passages 

 and air-cells, of the arteries and veins, lymphatics and nerves, bound 

 together by an areolo-fibrous tissue, and called the. 'parenchyma ; (3) the ter- 

 minal branches of the bronchial tubes. The pleura is simply a layer of 

 serous membrane, liable to its peculiar accidents and diseases, hereafter to 

 be described. The p)arenchyma has a beautiful pale rose colour in the healthy 

 subject. Though very delicate, it strongly resists external violence, and is 

 not easily torn. It is divided into a vast number of little polyhedral 

 lobules, each of which receives one of the tei-minating branches of a bron- 

 chial tube, and is again broken up into a cluster of air-cells, on the 

 walls of which the capillary branches of the pulmonary arteries and veins 

 are thickly spread out. The extent of surface upon which these vessels 

 ramify is enormous, probably ten or twelve times that of the skin. The 

 parenchyma of the lungs appears to be entirely passive in respiration, being 

 filled with air by the expansion of the cavity in which it lies ; and that, 

 again, being due to the act of the inspiratory muscles. 



The bronchial tubes divide and subdivide until they diminish to a 

 diameter of -^^ of an inch, when they terminate in the intercellular passages, 

 by which they communicate with the air-cells. At their terminations, the 

 mucous membrane ceases abruptly, the fibrous envelope being alone con- 

 tinued, together with the vascular network common to both. Thus the 

 mucous membrane lining the bronchi, and the fibrous walls of the air-cells, 

 are quite distinct ; and this will account for each being often the seat of a 

 peculiar inflammation, without extending to the other. 



