116 RESPIRATION 



tically plasma, outside the capillary; through the cell's wall, if it 

 has one; and then through a larger or smaller layer of protoplasm into 

 the interior of the cell. All these are either liquid or semiliquid pro- 

 toplasm, and hence each offers a minimum resistance to the progress of 

 the two respiratory gases. As is the case with the mechanism of external 

 respiration also, these tissues together constitute an animal membrane, 

 in reality complex both structurally and probably functionally. Yet 

 this may be considered as a simple membrane in studying the forces and 

 other conditions which determine the passage through it of the oxygen 

 and the carbon dioxide. What the different layers of this living partition 

 between the blood and the tissues have to do with this transit of the two 

 gases is at present undetermined. We can study it only as a whole, and 

 even then the precise functional conditions are none too certain. 



The Sequence and the Causes of the Respiratory Events. Let us now 

 systematically trace an imagined portion of oxygen from the atmosphere 

 inward to some tissue-cell and a portion of carbon dioxide outward from 

 a cell to the open air. We will note in the progress the process itself and 

 the causes which bring it about; we will see what actually happens to 

 and about the supposed molecules of oxygen and of carbon dioxide as 

 they pass in and out respectively. By "causes" we here may understand 

 the physical and chemical forces which combine to produce the orderly 

 sequence of respiration. This viewing of the events in order will serve 

 at once to make the actual process clear and to summarize the facts 

 already stated of its mechanism and chemistry. The description of these 

 events naturally divides into two parts, the course and the causes of the 

 movements of the two respiratory gases respectively. 



THE COURSE AND THE KINETICS OF THE OXYGEN INWARD. Pressing 

 in all directions at the nostrils is the atmosphere under a pressure of about 

 1032 grams on every square centimeter of surface. This is the weight of 

 a column of the atmosphere of that size many miles high above us, 

 Thus, gravity is the force which causes the air to pass inward in inspira- 

 tion. Before entrance can be made, however, space has to be provided 

 for it to move into. This space is furnished, as we have seen, at the 

 expense of crowding downward the abdominal viscera, the distending 

 of the abdominal walls, and by an enlargement of the thorax forward, 

 upward, and laterally, as well as downward. The mechanism of this 

 enlargement produced by muscular contraction has already been sug- 

 gested. 



As anyone at all familiar with the thoracic viscera realizes, however, 

 the matter is not in its physics so simple as this. The thorax is not a 

 space bounded by elastic walls which expand and so enlarge the "cavity 

 of the thorax." The only variable cavities in the thorax connected with 

 respiration are those minute spaces contained within the complicated 

 and highly elastic lungs namely, the alveoli and the terminations of the 

 numerous bronchi. Moreover, nearly surrounding the lungs are the 

 pleurae, and these are reflexed so as to line not only the outer surface of 

 the lungs, but also the inner surface of the thoracic wall proper. Thus, 



