286 PULMONARY MUCOUS TISSUE. 



this question can be solved by physiological experiments. 

 This membrane is especially characterized by the large num- 

 ber of blood-vessels, consisting of a network of extremely 

 small capillaries, so small as to allow only of the passage of 

 a blood globule, and placed very close together, the meshes 

 which separate them being exceedingly fine. It is found, for 

 instance, that on a given surface of a pulmonary alveolus the 

 space occupied by the capillaries amounts to three-fourths 

 of the surface, and the intervals between them to only one- 

 fourth. As the entire surface occupied by the alveoli amounts 

 to two hundred square metres, it follows that the capillaries 

 form an area of 160 square metres. This network is exceed- 

 ingly fine and delicate, being only about the thickness of a 

 blood globule ; it nevertheless contains nearly two litres of 

 blood. It has also been calculated that in twenty-four hours 

 at least two thousand litres of blood pass through it ; this 

 network is thus continually renewed. These figures are 

 important, as enabling us to form some idea of the magni- 

 tude of the gaseous exchanges which, we shall see, take 

 place between the blood and the volume of air with which 

 it is brought nearly in contact, being separated only by the 

 thin wall of the capillaries and an extremely delicate epithe- 

 lium. 



We must, therefore, study the mechanism by means of 

 which the external air is brought in contact with the respir- 

 atory surface, and see how it is renewed after the diffusion 

 of gas between this surface and the blood has taken place. 



These phenomena in every way resemble those of the 

 digestion; but while the food received into the digestive 

 tube must, before it can be assimilated, undergo a number 

 of metamorphoses, the respiratory elements of the air are 

 assimilated at once. The air simply undergoes a slight 

 preparatory process, which brings it to the same state of 

 temperature and of humidity as the pulmonary surface with 

 which it is to come in contact. The origin of the pulmonary 

 tree is so arranged as to render it inevitable that the air 

 should undergo this slight modification : for the nasal 



form of flattened, circular groups ; these groups form a complete 

 layer. As they are also found in branches of a size from O m . 22 

 to O 31 . 18, they probably extend to the pulmonary lobules." (Kol- 

 liker, 1870). 



This opinion as to the presence of the muscular element in the 

 coat of the pulmonary vesicles was upheld by Moleschott, Fiso 

 Bonne, Hirschinann, and Chrzonszczewsky. 



