ORGANS OF RESPIRATION AND VOICE. 179 
to the gaps in the cartilaginous rings, is occupied by a layer of long yellow 
elastic fibre, a transverse muscular layer, and a mucous layer consisting of 
basement membrane and epithelium. ‘This posterior soft portion, in imme- 
diate contact with the anterior of the cesophagus, readily yields to the pres- 
sure of the food passing down the latter tube. The entire length of the 
trachea amounts to about four or five inches, and on the exterior circum- 
ference of the bronchi are seen numerous lymphatic glands filled with a 
black pigment, and called glandule bronchiales. 
Pl. 129, fig. 87, trachea and its branches from before: °, trachea; ”*, car- 
tilaginous rings; °, yellow elastic fibres; *, right, ", left bronchus, with their 
ramifications. Fig. 88, posterior view of the trachea: *, yellow elastic fibres, 
with their glandular granules; *, muscular layer, composed of transverse 
fibres; °’, soft elastic longitudinal fibres, strengthening the mucous mem- 
brane, °. 
3. THE LUNGS. 
The lungs, pulmones, constitute two conical spongy and elastic bags, 
occupying, with the heart between them, the cavity of the thorax. The 
color of the. lungs varies, in different parts and at different times, between 
bright red and dark purple. Their weight amounts in the male to about 
two and a half pounds, in the female to about two pounds. Each lung 
forms a cone, with the broad base resting on the diaphragm, the apex being’ 
directed upwards. On the inner opposed faces of each lung is a shallow 
depression into which the bronchi and vessels enter and emerge. ‘The 
right lung is divided by a fissure, nearly two inches deep, into three, and 
the left into two lobes. Each lobe exhibits externally a great number of 
small angular spaces, bounded by darker lines. Each bronchus, on reaching 
the lungs, divides into as many branches as there are lobes, and these, enter- 
ing the lobes, subdivide and bifurcate again and again, until the exceedingly 
minute ramifications end in small air cells, which in the adult probably 
communicate with each other. On inflating the lungs, these cells will 
become dilated, and project on the surface in small mammillary or botryoidal 
swellings. The bronchi, as they penetrate into the lungs, gradually lose 
their cartilaginous element, until finally they consist of a soft membranous 
tube, which ends in the cells above referred to, the number of which has 
been estimated at seventeen or eighteen hundred millions. 
The pulmonary artery, which conducts venous blood from the right 
ventricle of the heart, follows all the ramifications of the bronchi, and on 
the air-cells breaks up into a very delicate vascular plexus, from which the 
pulmonary veins take their origin. The venous blood circulating through 
this network of vessels absorbs oxygen from the air with which it is brought 
into contact by means of the air vessels. It then changes color, giving up 
a portion of carbonic acid and water. This change effected, it returns 
through four veins, two for each lung, to the left auricle. These vessels 
have. nothing to do with the nourishment of the lungs themselves, this office 
885 
