Resp iration 167 



Countless numbers of these lobules, bound together by 

 connective tissue, are grouped after the same fashion to 

 form by their aggregation the lobes of the lung. The right 

 lung has three such lobes, and the left, two. 



The walls of the air sacs are of extreme thinness, con- 

 sisting of delicate elastic and connective tissue, and lined 

 inside by a single layer of thin epithelial cells. In the con- 

 nective tissue run capillary vessels belonging to the pul- 

 monary artery and veins. Now these delicate vessels are 



FIG. 88. 



A, diagrammatic representation of the ending of a bronchial tube in air sacs, or alveoli ; 



B, termination of two bronchial tubes in enlargement beset with air sacs (Huxley); 



C, diagrammatic view of an air sac. A lies within sac and points to epithelium 

 lining wall; B, partition between two adjacent sacs, in which run capillaries; 

 c, elastic connective tissue (Huxley). 



surrounded on all sides by air sacs. It is evident, then, 

 that the blood flowing through these capillaries is sepa- 

 rated from the air within the sacs only by the thin walls of 

 the vessels and the delicate tissues of the air sacs (Fig. 90). 



This arrangement is perfectly adapted for an interchange 

 between the blood in the capillaries and the air in the air 

 sacs. This will be more fully explained in sec. 260. 



253. Inspiration and Expiration. The act of breathing 

 consists of a series of rhythmical movements, succeeding 

 one another in regular order. In the first movement the 



