272 RESPIRATION 



Theories of Interchange of Gases in the Lungs and in the Tissues. 



The above discussion is on the basis of the mechanical interpretation of the 

 transfer of gases in the lungs and in the tissues. By this theory it is assumed 

 that the oxygen passes from the air in the lungs through the moist pulmonary 

 membrane of the alveoli to the capillary walls into the blood-plasma, obeying 

 the physical laws of gas diffusion. Likewise in the tissues this theory pre- 

 supposes the difference in the mechanical tension in the capillary blood- 

 plasma, the lymph, and the living tissue will lead to diffusion of the oxygen 

 in the direction of lowest pressure. 



Some facts have indicated that we cannot account for the transference 

 of oxygen by the purely mechanical theory. The idea has been advanced 

 that the living epithelial wall of the lung, as well as that of the capillary, exerts 

 a distinct influence on the passage of oxygen of such nature as might be re- 

 garded as a secretion of this gas. This theory finds some additional support 

 in the fact that in the air bladders of certain fishes a distinct secretion of oxygen 

 has been proven. 



THE NERVOUS REGULATION OF THE RESPIRATORY 

 APPARATUS. 



Like all other functions of the body the discharge of which is necessary 

 to life, the respiratory movement is essentially an involuntary act. Unless 

 this were the case, life would be in constant danger, and would cease on the 

 loss of consciousness for a few moments, as in sleep. It is, however, of ad- 

 vantage to the body that respiration should be to some extent under the 

 control of the will. For, were it not so, it would be impossible to perform 

 those respiratory acts such as speaking, singing, and the like. 



The Respiratory Nerve Center. It has been known for centuries 

 that there exists a region of the central nervous system on the destruction of 

 which both respiration and life cease. Flourens, 1842, after many series 

 of experiments as to the exact position of what he called the "knot of life" 

 (nceud vital], placed it in the fourth ventricle, at the point of the V in the 

 gray matter at the lower end of the calamus scriptorius; a district of consider- 

 able size, 5 mm., on both sides of the middle line. Observers subsequent to 

 Flourens have attempted to show that the chief respiratory center, on the 

 one hand, is situated higher up in the nervous system, in the floor of the third 

 ventricle (Christiani), or in the corpora quadrigemina (Martin and Booker, 

 Christiani, and Stanier), or lower down in the spinal cord. The balance 

 of experimental evidence, however, is to prove that the sole centers for respira- 

 tion are in a limited district in the medulla oblongata in close connection with 

 the vagus nucleus on each side, with which they are probably identical. 

 The destruction of this region stops respiration. If the center be left in 

 connection with the muscles of respiration by their nerves, although the 



