ORGANS OF RESPIRATION AND VOICE. 179 



to the gaps in tlie 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 oesophagus, 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 glandulcc bronchiales. 



PL 129, ßg. 37, trachea and its branches from before : *, trachea ; '■ *, car- 

 tilaginous rings ; ', yellow elastic fibres; '", right, ", left bronchus, with their 

 ramifications. Fig. 38, 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 botryoiclal 

 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 



