310 RESPIRATION 



might be regarded as a secretion of this gas. This theory finds some 

 support in that a distinct secretion of oxygen in the air bladders of certain 

 fishes has been proven by Bohr. The theory apparently does not apply 

 to mammals. 



THE NERVOUS REGULATION OF THE RESPIRATORY 

 APPARATUS. 



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

 vantage to the body that co-ordination of respiratory movements should 

 be to some extent under the control of the will. For, were it not so, it 

 would be impossible to perform those voluntary 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" 

 (n&ud -vital) , placed it in the floor of the fourth ventricle, at the point of the V 

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

 considerable size, some 5 mm. in extent on each side 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 quad- 

 rigemina (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 respiration are in a limited district in the medulla 

 oblongata in close connection with the vagus nucleus on each side. They 

 are approximately identical in location. The destruction of this region stops 

 respiration. If the center be left in connection with the muscles of respira- 

 tion by their nerves, although the remainder of the central nervous system be 

 separated from it, respiration continues. It may be considered almost cer- 

 tain that the medullary center is the only true respiratory center. Langen- 

 dorff states that in newly born animals in which the medulla has been im- 

 mediately cut across at a level a few millimeters below the point of the 

 calamus scriptorius, respiration continues for some time, but this is ques- 

 tionable. Normal respiration does not occur after separation of the bulb 

 from the cord, and the so-called respiratory movements noticed by Langen- 

 dorff are merely tetanic contractions of the respiratory muscles in which often 

 enough other muscles take part. 



The action of the medullary center is to send out impulses during in- 

 spiration, which cause contractions of the inspiratory muscles a, of the 

 nostrils and jaws, through the facial and inferior division of the fifth nerves; 

 b, of the glottis, chiefly through the inferior laryngeal branches of the vagi; c 



