$22 THE HORSE. 
THE TRACHEA AND BRONCHI. 
The TRACHEA is a flexible and elastic tube, formed of a series of incom- 
plete cartilaginous rings, about fifty in number, connected together by an 
elastic membrane, which also fills up the space left at the back of each 
ring. It passes down the lower margin of the neck, and, on arriving at 
the level of the base of the heart, it divides into two bronchi or lesser 
tubes, of somewhat the same character and structure as itself. In its 
course, it has the sterno-hyoideus and thyroideus in front, the cesophagus 
behind ; and the carotid artery, with the pneumogastric, recurrent, and 
sympathetic nerves on each side ; the jugular vein being more superficial 
than these, but also on the side of the trachea. At the upper and back 
part of the trachea a.layer of muscular fibres is found, connecting together 
the posterior edges of the cartilages. These are supposed by Mr. Percivall 
to have the power of dilating the trachea by their contraction. He 
explains this somewhat paradoxical action, by imagining that “in conse- 
quence of the passage being naturally elliptical, and the muscle being 
extended across its long diameter, the contraction of its sides will give the 
tube a circular figure, by increasing the curvature of the ring anteriorly, 
and therefore, in effect, will expand, and not contract, the calibre of the 
canal.” This theory is, however, now entirely abandoned, and it is 
generally admitted that the sole office of these muscular bands is to 
diminish the area of the trachea. 
THE BRONCHI 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 cellular substance, and are lined first by a fibrous layer, with which 
it is supposed that some muscular tissue is mixed up, as in the larger 
hronchi, 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 structure, they are made up of three 
distinct parts—(1) an external or serous coat, called the pleura, described 
at page 407 ; (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 terminal branches of the bronchial tubes. The 
pleura is simply a layer of serous membrane, liable to its peculiar acci- 
dents and diseases, hereafter to be described. The parenchyma 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 terminating branches of a bronchial tube, and is again broken up 
