MOVEMENTS OF RESPIRATION. 



121 



ment of these parts. It is possible that the tissue of the lungs may receive some nourish- 

 ment from the blood conveyed there by the pulmonary artery ; but, as this vessel does 

 not send any branches to the bronchial tubes, it is undoubtedly the bronchial arteries 

 which supply the material for their nutrition and for the secretion of the mucous glands. 

 This is one of the anatomical reasons why inflammatory conditions of the bronchial tubes- 

 do not extend to the parenchyma of the lungs, and vice versa. 



FIG. 39. Section of the parenchyma of the human lung, injected through the pulmonary artery. (Schulze.) 

 a, a, c, c, wall of the air-eel i ; 6, small arterial branch. 



The foregoing anatomical sketch shows the admirable 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 the elements 

 of the air and blood. It is also evident, from the enormous number of air-cells, that the 

 respiratory surface must be immense. 1 



Movements of Respiration. 



In man and in the warm-blooded animals generally, inspiration takes place as a con- 

 sequence of enlargement of the thoracic cavity and the entrance of a quantity of air 

 through the respiratory passages corresponding to the increased capacity of the lungs, 

 In the mammalia, the chest is enlarged by the action of muscles ; and, in ordinary respi- 

 ration, inspiration is an active process, while expiration is comparatively passive. 



A glance at the physiological anatomy of the thorax in the human subject makes it 

 evident that the action of certain muscles will considerably increase its capacity. In the 

 first place, the diaphragm mounts up into its cavity in the form of a vaulted arch. By 

 contraction of its fibres, it is brought nearer a plane, and thus the vertical diameter of 

 the thorax is increased. The walls of the thorax are formed by the dorsal vertebrae and 



1 Hales estimated the combined surface of the air-cells at 289 square feet; Keill, at about 152 square feet; and 1 

 Lieberkuhn, at 1,500 square feet. There are not sufficient data on this point for us to form any thing like a reliable 

 estimate. It is simply evident that the extent of surface must be very great. In passing from the lower to the 

 higher orders of animals, it is seen that Nature provides for the necessity of an increase in the activity of the respira- 

 tory process, by a diminished size and a multiplication of the air-cells. 



