392 PHYSIOLOGY CHAP. 



only of the C0 2 , but also of the 2 , and thus can have no appreciable 

 value as a factor in the pulmonary gas exchanges seeing that if 

 on the one hand it facilitates the elimination of C0 2 from the 

 blood, it checks the absorption of O 2 on the other. 



Another question indirectly connected with the nature of the 

 pulmonary gas exchanges, is that which refers to the influence 

 which is, or can be, exercised upon them by the nervous system. 



At the end of 1892, Henriques demonstrated experimentally 

 in rabbits and dogs that stimulation of both vagi might cause 

 variations in the respiratory gas exchanges of the lungs. These 

 experiments were taken up by Maar in 1902, and extended both 

 to cold-blooded animals (tortoises) and to the warm-blooded 

 (rabbit). He endeavoured to define the precise effect of section 

 and artificial stimulation of the vagus and sympathetic upon the 

 pulmonary gas exchanges. The experiments on cold-blooded 

 animals led to the conclusion that absorption of oxygen by the 

 lungs was under the influence of the nervous system, the vagus 

 containing both nerve fibres that increase the absorption of 

 oxygen, and also other fibres that dimmish it. The former run to 

 the lung of the opposite side, the latter to that of the same side. 

 It was found impossible to establish any direct influence of the 

 sympathetic on respiratory pulmonary exchanges, nor did the 

 experiments on warm-blooded animals lead to any definite or 

 concordant results. The demonstration of a direct influence of 

 the nervous system on the pulmonary gas exchanges tells in favour 

 of Bohr's theory. 



In speaking of recent work on this subject, mention must be 

 made of Krogh (1904), who studied the cutaneous and pulmonary 

 respiration of the frog, and came to the conclusion that cutaneous 

 respiration (which serves especially for elimination of carbonic 

 acid) is independent of the nervous system, and can be explained 

 on purely physical grounds (gaseous diffusion), while pulmonary 

 respiration (which particularly controls the absorption of oxygen) 

 is, on the contrary, effected principally by secretory processes of 

 the epithelium, and is regulated by the nervous system. 



On the other hand, Loewy and Zuntz in their latest publication 

 (1904) still contend that the laws of diffusion adequately 

 account for the gas exchanges between the alveolar air and the 

 blood. They determined the velocity with which carbonic acid 

 traverses an excised frog's lung, and from this, taking into account 

 the varying thickness of the pulmonary walls, deduced the con- 

 ditions of gas diffusion in the human lung. They conclude that 

 the conditions of diffusion for the passage of oxygen from the 

 pulmonary alveoli to the blood, and thence to the tissues, are so 

 favourable that they ensure more than sufficient absorption of 

 oxygen, even in the most extreme cases of rarefaction of air com- 

 patible with life. 



