282 HANDBOOK OF PHYSIOLOGY. 



It has been known for centuries that there exists a district of the 

 central nervous system on the destruction of which both respiration 

 and life cease. All attempts to localize this district, however, before 

 those of Flourens were unsuccessful. Flourens, after many series of 

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

 life" (noeud 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 dis- 

 trict of considerable size, viz., 5 mm., on both sides of the middle line. 

 Observers subsequent to Flourens have attempted to show that the chief 

 respiratory centre on the one hand is situated higher up in the nervous 

 system, e.g., in the floor of the third ventricle (Christiani), or in the 

 corpora quadrigemina (Martin and Booker, Christiani, and Stanier), or 

 on the other hand, lower down in the spinal cord, and that the medullary 

 centres, if they exist, are either accessory or subservient to such centres. 

 The balance of experimental evidence, however, is to prove that the sole 

 centres for respiration is 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 district stops respira- 

 tion forever; whereas, if it be left in connection with the muscles of 

 respiration by their nerves, although the remainder of the central nervous 

 system be separated from it, respiration continues. It may be considered 

 almost certain that the medullary centre is the only true respiratory 

 centre, and that the observations of Langendorff, that in newly-born 

 animals in which the medulla has been cut immediately or a few milli- 

 metres below the point of the calamus scriptorius respiration continues 

 for some time as in normal animals cannot be received. We are indebted 

 to Marckwald for much information on this subject, and he has come to 

 the conclusion that normal respiration does not occur after division of 

 the bulb from the cord, and that the so-called respiratory movements 

 noticed by Langendorff are merely tetanic contractions of the respirax 

 tory muscles with which often enough other muscles take part. 



The action of the medullary centre is to send out impulses during 

 inspiration, which cause respiratory movements of the 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) of the intercostal and other muscles which 

 produce raising of the ribs, chiefly through the intercostal nerves, and 

 (d) of the diaphragm through the phrenic nerves. 



If any one of these sets of nerves be divided, respiratory movements 

 of the corresponding part cease. 



Similarly it may be supposed that the centre sends out impulses dur- 

 ing expiration to certain other muscles. It has been suggested, however, 

 that the centre consists of two parts, or is double, and that it is made 

 up of an inspiratory centre, which is constantly in action, and of an ex- 



