- DEVELOPMENT OF THE RESPIRATORY APPARATUS. 941 
ciliated, but it has become flat and pavement-like. Two kinds of epithelial cells may be 
recognised—(1) a few small granular polygonal cells, arranged singly or in groups of two 
or three ; (2) more numerous thin cells of large size, and somewhat irregular in outline. 
Outside the epithelium is a delicate layer of faintly- fibrillated connective tissue. This is 
strengthened by a network of elastic fibres which is specially well marked around the 
mouths of the alveoli, and is also to some extent carried over the walls of the air-cells. 
Muscular fibres are likewise present on the walls of the infundibula, but it is questionable 
if any are prolonged over the air-cells. 
Pulmonary Vessels.—The pulmonary artery, as it traverses the lung, divides with 
the bronchi, and closely accompanies these tubes. The resultant branches do not anasto- 
mose, and for the most part they lie above and behind the corresponding bronchi. The 
fine terminal divisions of the artery join a dense capillary plexus which is spread over the 
alveoli or air-cells. This vascular network is so close that the meshes are barely wider 
than the capillaries which form them. In the partitions between adjacent alveoli there 
is only one layer of the capillary network, and thus the blood flowing through these 
vessels is exposed on both aspects to the action of the air in the air-cells. The radicles 
of the pulmonary vein arise in, and carry the blood from, the pulmonary capillary plexus. 
Each afferent arteriole supplies the blood which flows through the capillaries spread over 
a number of neighbouring alveoli, and in like manner each efferent venous radicle drains 
an area corresponding to several adjoining air-cells. At first the veins run apart from 
the arteries, but after they have attained a certain size they join them and the bronchi. 
As a rule the pulmonary veins are placed on the lower and front aspect of the corre- 
sponding bronchi. 
DEVELOPMENT OF THE RESPIRATORY APPARATUS. 
The larynx, the trachea, the bronchi, and the lungs arise as an outgrowth from the 
ventral aspect of the foregut. The first indication of a respiratory tract occurs in the 
human embryo when it has attained a length of 3-2 mm., on or about the fifteenth day 
of development. A median longitudinal groove makes its appearance within the foregut 
on its ventral wall. This extends from the pharynx in front to the region of the stomach 
behind, and it gradually deepens as it passes backwards. The hinder or gastric end of 
this groove ends in a blind diverticulum or pocket, which freely communicates with the 
cavity of the foregut, and forms a hollow median protrusion on the ventral aspect of 
this portion of the primitive alimentary canal. Further, it is lined with entoderm or 
hypoblast continuous with the entodermal lining of the foregut. 
Trachea and Larynx.—The groove on the ventral aspect of the foregut becomes 
first partially and then completely separated from the part of the foregut which lies on 
its dorsal aspect by two lateral ridges which grow inwards and finally meet. Two tubes 
are thus formed—viz. one behind, the cesophagus, and the other in front, the trachea and 
larynx. At their cephalic ends a communication between the two tubes is preserved as 
the permanent communication between the larynx and pharynx. 
The cephalic end of the air-tube, which is thus separated off from the foregut, becomes 
enlarged to form the larynx, whilst the remainder is developed into the trachea. The 
cartilages of the larynx do not make their appearance until the eighth or ninth week. 
The thy roid cartilage i is believed to be formed out of the ventral portions of the cartilages 
which support the 4th and 5th visceral arches of the two sides united in the median plane. 
The epiglottis takes form in the upper or front part of the furcula (see chapter on Embryo- 
logy, p. 33), whilst the arytenoids are developed in its lower or back part. The cricoid 
cartilage and the tracheal rings are formed in the mesoderm of the air-tube. 
Lungs and Bronchi.—The entodermic diverticulum, or pocket in which the gastric 
end of the primitive respiratory groove terminates, very early bifurcates into two vesi- 
cular portions, which represent the primitive right and left bronchi and lungs. From the 
first the right pulmonary vesicle is slightly the larger of the two. Both elongate, and 
almost immediately each part undergoes a subdivision—the right into three vesicles, and 
the left into two vesicles—thus early indicating the three lobes of the right lung and the 
two lobes of the left lung. The hypoblastic or entodermal subdivisions thus formed are 
surrounded by mesoderm. The main subdivisions continue to branch and rebranch, 
pushing their way into the pulmonary mesoblast, until the complete bronchial tree is 
formed. The method of subdivision is very characteristic, and from the first the various 
branches are bulbous or flask-shaped at their extremities. These bifurcate, and although 
at first the two subdivisions in each case appear of equal impor tance, one grows out as the 
continuation of the main bronchial stem, whilst the other remains as a lateral branch. 
