108 RESPIRATION 



permeable epithelium each y^V 7 mm - thick, with a thin layer of lymph 

 between them, might well serve as an organ with great capabilities ! So 

 closely are the alveoli packed together in the lung that besides them in 

 these large organs there is little else except the capillaries. 



THE CAPILLARIES. From Fig. 57, representing the histology of the 

 lung, the relation of the capillaries to the alveoli is obvious. The wall 

 of these tubelets (the ultimate bloodvessels, and those which alone 

 immediately supply the tissues) is simply one layer of flat epithelial cells, 

 here called endothelium, cemented edge to edge so as to constitute a 

 tube. Sihler claims that a plexus of fine nerve-fibrils surrounds the 

 capillaries in all parts of the body. This layer of protoplasm is often 

 less than one micron in thickness, and it practically lies in apposition to 

 the alveolar wall. So crowded are these capillaries, however, all over 

 the periphery of the alveoli that it is functionally almost a continuous 



FIG. 60 



^^ 



-a 



The pulmonary epithelium of a young dove: a, capillary; b, cell-groups in the capillary 

 flexus; c, contours of the larger epithelial cells; d, a capillary mesh-opening with two cell-islands; 

 /, capillary opening without any cell-groups. (Elenz.) 



surface. This endothelial layer, together with the alveolar surface 

 (in contact with each other save for the universal moistening lymph), 

 constitutes the osmotic animal membrane through which the processes of 

 external respiration take place. This is the living protoplasmic layer to 

 which the respiratory tubes conduct air and from which they lead it away 

 into the atmosphere. We will next inquire as to the motor power of this 

 ceaseless ebb and flow of waste and life-giving gases. 



THE THORAX. This is a muscular, bony, and cartilaginous box 

 adapted as a motor organ of respiration. The sides and bottom 

 of this flattened and conical bellows are all more or less movable by 

 means of muscles, of which the diaphragm is, functionally, the most 

 important. The ribs are so shaped that when they are raised, largely 

 by the external intercostal muscles, the capacity of the chest is increased 

 laterally, antero-posteriorly, and to a slight extent upward. The fibers of 



