CHAP. IL] 



RESPIRATION. 



593 



shewn by applying what may be considered as natural stimuli to 

 the endings of the nerve in the lungs ; and the results so obtained 

 have an especial value since the artificial 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 cannot 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 perhaps 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 experi- 

 ment on a rabbit a curve be taken by means of the isolated slip of 

 the diaphragm, 328, it will be seen (Fig. 82 A) that the slip 

 elongates somewhat ; that is to say, previously in a state of slight 

 tonic contraction, it changes in the direction of expiration. If on 



FIG. 82. EFFECTS OF DISTENSION AND COLLAPSE OF LUNG. (Head.) 



Both curves are described by a lever attached, as stated in 328, 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. 



F. 



38 



