INSPIRATION. 115 



^5*0-5- of ^an inch (10 to 12'5 /,), in diameter, which are applied directly to the 

 walls of the blood-vessels. The epithelium here does not seem to be regular- 

 ly desquamated as in other situations. Examination of injected specimens 

 shows that the blood-vessels are so situated between the cells, that the blood 

 in the greater part of their circumference is exposed to the action of the air. 

 The entire mass of venous blood is distributed in the lungs by the pul- 

 monary artery. Arterial blood is conveyed to these organs by the bronchial 

 arteries, which ramify and subdivide on the bronchial tubes and follow their 

 course into the lungs, for the nourishment of these parts. It is possible that 

 the tissue of the lungs may receive some nourishment from the blood of 

 the pulmonary artery ; but as this vessel does not send any branches to the 



a 

 a, 'y 



FIG. 43. Section of the parenchyma of the human lung, injected through the pulmonary 



artery (Schulze). 

 a, a, c, c, walls of the air-cells ; 6, small arterial branch. 



bronchial tubes, the bronchial arteries supply the matters for their nutrition 

 and for the secretion of the mucous glands. 



The foregoing anatomical sketch shows the adaptation of the trachea and 

 bronchial tubes to the passage of the air by inspiration to the deep portions 

 of the lungs, and the favorable conditions which it there meets with for an 

 interchange of gases. It is also evident, from the great number of air-cells, 

 that the respiratory surface must be very large, although it is impossible to 

 form an accurate estimate of its extent. 



MOVEMENTS OF EESPIBATION. 



In man and in the warm-blooded animals generally, inspiration takes 

 place as a consequence of enlargement of the thoracic cavity and the en- 

 trance of a quantity of air through the respiratory passages, corresponding 



