DEVELOPMENT OF THE KESPIKATOKY APPARATUS. 1099 



the wall. As the tubes diminish, the cartilaginous deposits show a corresponding reduc- 

 tion in size, until at last, in bronchi of 1 mm. diameter, they disappear altogether. The 

 glands in relation to the tubes for the most part cease to exist about the same point. 

 The muscular or middle coat, which in the trachea and primary bronchi is confined to 

 the dorsal wall of the tube, forms a continuous layer of circularly arranged bundles in the 

 bronchi as they ramify within the lung. Spasmodic contraction of the muscular coat gives 

 rise to the serious symptoms which accompany asthmatic affections. The muscular fibres 

 of the middle coat may be traced as far as the atria, on the walls of which they are 

 present in considerable numbers. The mucous lining of the tubes becomes greatly thinned 

 as it is followed into the smaller bronchioles. It contains a large number of longitudinally 

 arranged elastic fibres, and is disposed in longitudinal folds, so that when ^he tube is cut 

 across the lumen presents a stellate appearance. The mucous membrane is lined with 

 ciliated columnar epithelium. 



Structure of the Atria and Alveoli. The walls of the atria and alveoli 

 are exceedingly fine and delicate, but, nevertheless, constituents continuous with those 

 observed in the three coats of a bronchus are found entering into their construction. 

 The epithelium is reduced to a single layer. Further, it is no longer columnar and 

 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 also are present on the walls of the atria, 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 dorsal to 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 partition 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 afferent 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 inferior and ventral aspects 

 of the corresponding bronchi. 



DEVELOPMENT OF THE KESPIRATORY APPARATUS. 



The larynx, trachea, bronchi, and lungs all arise as an outgrowth from the ventral 

 aspect of the foregut. The first indication of a respiratory tract occurs in the human 

 embryo early in the third week, on or about the fifteenth day of development, and when 

 the embryo is but little more than 3 mm. in length. At that period a median longitudinal 

 groove makes its appearance in the ventral wall of the foregut, extending from the 

 primitive pharynx well towards the primitive stomach, and deepening gradually as it 

 passes caudal wards. 



The cranial end of the respiratory tube becomes enlarged and forms the larynx, 

 the intermediate portion forms the trachea, and the caudal end bifurcates in the 

 floor of the groove into two tubes the future bronchi are already indicated by slight 

 bulgings before the two tubes divide which grow caudalwards on either side of the heart, 

 into a mesodermic mass, from which the connective tissue of the future lungs is ultimately 

 developed. The respiratory tube is lined with entoderm continuous with the entodermal 

 lining of the foregut. 



The groove becomes deeper and constricted, its lateral margins approximate, and finally 

 meet dorsally, and the groove separates off from the foregut as a distinct tube. This 

 differentiation necessarily results in the production of two tubes or canals, a ventral one 

 forming the respiratory tube, and a dorsal one the oesophagus. The separation of the two 

 ubes commences at the caudal end and proceeds cranialwards towards the pharynx, into 

 which both the oesophagus and the respiratory tube open. 



