836 PULMONARY MUCOUS TISSUE. 



in the blood an accumulation of carbonic acid which directly 

 excites the respiratory nervous centre. 1 For the most part, 

 however, respiration is caused by a simple reflex act, of which 

 this gray matter forms the centre ; the consideration of which 

 leads us to consider the centripetal and the centrifugal 

 nerves. 



2. The Centripetal Paths. The centripetal nerves of res- 

 piration are first the pneumo-gastric, leading to the medulla 

 oblongata at the vital point / to these, however, must be 

 udded the greater number of the sensory nerves of the skin. 



The pneumo-gastric nerves transmit to the nervous centre 

 the vague sensory impressions made upon the pulmonary 

 surface, which impressions constitute the desire to breathe. 

 If the pneumo-gastric nerve be cut off above the root of the 

 lung, and its central extremity excited, the respiratory move- 

 ments are seen to become more forcible and rapid, while if 

 the excitation be very great, the contraction of the diaphragm 

 is changed into actual tetanus, so that animals die by arrest 

 of the respiration while in a state of tetanic inspiration. 

 One of the fibres of the pneumo-gastric nerve appears 

 to have a special influence over the respiratory reflex act : 2 

 this is the upper laryngeal, which appears especially to give 

 rise, in opposition to the pneumo-gastric trunk, to phenomena 

 of expiration : if this nerve be cut, and its upper (central) 

 extremity excited, expiration takes place with great force, and 

 if the excitation be very forcible, the animal falls into a state 

 of tetanus of the expiratory muscles. A similar phenomenon 

 takes place in the complaint known as whooping-cough, 

 which is only an affection of the superior laryngeal nerve ; 

 inasmuch as it excites this nerve, and increases the move- 

 ments of expiration to an extraordinary degree. As, during 

 expiration, the diaphragm is passive, so, when centripetal 



1 It must not, however, he supposed that the carbonic acid alone 

 causes respiration: we know that the elements of the nervous cen- 

 tres consume oxygen, just as do the other elements of the other 

 tissues when at work. The presence of a large quantity of car- 

 bonic acid in the blood will produce no respiratory movement if the 

 irritability of the gray matter of the fourth ventricle has ceased, 

 on account of the want of oxygen, as in cases of asphyxia. 



2 Spasm of the diaphragm, so closely associated with intestinal 

 irritation and pleuritis, may be caused by irritation of the pneumo- 

 gastric nerve conveyed to the respiratory centre. I have observed 

 that administration of ipecacuanha tends to the increase of the 

 spasm. (Am. ed.) 



