Chap, ii.] 



RESPIRATION. 



481 



stimulation of a nerve-fibre at a part of its course by means of 

 an electric current is at best a rough process, by which we can- 

 not hope to do more than approximate to the results actually 

 taking- place in the living body when the nerve is stimulated at 

 its endings by natural stimuli ; and the approximation is per- 

 haps less in the case of the exquisitely sensitive respiratory 

 centre than in many other cases. 



If in an animal in which a careful graphic record of the 

 respiratory movements is being taken, the trachea be suddenly 

 closed at the summit of an inspiration, the result is a pause 

 before the succeeding inspiration follows, that is to say, a 

 partial or temporary inhibition of inspiration ; and if during 

 such an experiment on a rabbit a curve be taken by means of 

 the isolated slip of the diaphragm, § 259, it will be seen (Fig. 

 96 A) that the slip elongates somewhat ; that is to say, previ- 

 ously in a state of slight tonic contraction, it changes in the 

 direction of expiration. If on the other hand the trachea be 

 suddenly closed at the end of an expiration (Fig. 96 B), when 



B 



Fig. 



Effects of Distension and Collapse of Lung. (Head.) 



Both curves are described by a lever attached, as stated in § 259, to a slip of 

 the diaphragm of a rabbit. A contraction of the diaphragm (inspiration) raises 

 the lever ; during relaxation of the diaphragm, the lever falls. 



In A, the trachea is closed at x, the height of inspiration ; a pause follows 

 during which the lever gradually sinks until an inspiration (a very powerful one) 

 sets in. 



In B, the trachea is closed at the end of expiration, x ; there follow powerful 

 inspirations. 



31 



