PULMONARY GLANDS. 423 
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 34. 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 continued, 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 extend- 
ing to the other. 
PULMONARY GLANDS, &c. 
IN CONNEXION WITH THE LUNGS are three bodies, the uses of two ot 
which are not very clearly made out. These are the thyroid body, just 
below the larynx; the thymus gland, chiefly developed in the fcetus ; 
and the bronchial glands, which are merely lymphatic glands of the usual 
character, situated around the principal divisions of the bronchi. 
THE THYROID Bopy is not very fully developed in the horse, and has 
little interest. connected with it, seldom being enlarged, as in the dog and 
in the human species. It consists of two oval masses, about the size of an 
egg, lying on each side of the trachea, just beneath the larynx, and con- 
nected by a band or middle lobe. The use of the thyroid body is not 
ascertained. 
JUST WITHIN THE THORAX, and in close contact with the trachea, a 
somewhat similar body to the preceding is met with in the foetus and 
young foal ; but it soon wastes away as the young animal grows up. This 
is the thymus gland (known to cooks as the sweetbread), resembling in 
shape the thyroid body, but of a paler colour. Like it, the use of this 
gland is not fully known; but in structure it is more like the con- 
glomerate glands; and Sir Astley Cooper, who examined it most minutely, 
supposed that it is intended “to prepare a fluid, well fitted for the foetai 
erowth and nourishment, from the blood of the mother, before the birth 
of the foetus.” 
THE BRONCHIAL GLANDS are merely lymphatic glands, similar to those 
in other parts of the body, and grouped around the large bronchial tubes. 
They are of a greyish colour, stained with black in patches. 
