NATURE OF THE FUNCTION. 



[Fig. 99. 



395 



A view of the Bronchise and Blood- Vessels of the Lungs as shown by dissection, as well as the relative 

 position of the Lungs to the Heart; 1, end of the left auricle of the heart; 2, the right auricle ; 3, the left 

 ventricle with its vessels; 4, the right ventricle with its vessels; 5, the pulmonary artery; 6, arch of the 

 aorta; 7, superior vena cava; 8, arteria innominata; 9, left primitive carotid artery; 10, left sub-clavian 

 artery; 11, the trachea; 12, the larynx; 13, upper lobe of the right lung; 14, upper lobe of the left lung; 

 15, trunk of the right pulmonary artery ; 16, lower lobes of the lungs. The distribution of the bronchia and 

 of the arteries and veins, as well as some of the air-cells of the lungs, are also shown in this dissection.] 



rings of the trachea does not commence until after the twelfth day, when they 

 first appear as transverse striae on the median line of the front only ; they 

 gradually become solid, and extend themselves on either side, until they nearly 

 meet at last on the median line on the back or vertebral side of the tube. The 

 history of the process in the human embryo, appears to be very nearly the 

 same. The first appearance of the lungs takes place about the sixth week, 

 at which time they are simple vesicular prolongations of the oesophageal mem- 

 brane. Their surface, however, soon becomes studded with numerous little 

 wart-like projections, and these are caused by the formation of corresponding 

 enlargements of their cavity. These enlargements soon become prolonged, 

 and. develop corresponding bud-like enlargements from their sides ; and in 

 this manner the form of the organ is gradually changed, a progressive increase 

 in their bulk taking place from above downwards, in consequence of the exten- 

 sion of the bronchial ramifications from the single tube at the apex. At the 

 same time, however, a corresponding increase in the amount of the parenchy- 

 matous tissue of the lung is taking place ; for this is deposited in all the inter- 

 stices between the bronchial ramifications, and might be compared with the 

 soil filling up the spaces amongst the roots of a tree. It is in this parenchyma 

 that the pulmonary vessels are distributed, and the portion of it which extends 

 beyond the terminations of the bronchial tubes, seems to act as the nidus for 

 their further extension. It can be easily shown that, up to a late period of the 



