426 TEXT-BOOK OF PHYSIOLOGY 



of the inspiratory center is automatic or autochthonic (Gad) in character, 

 expressive of the idea that the rhythmic discharge of nerve impulses is due 

 to some stimulating agent generated in the nerve-cells of the center itself, the 

 activity of which is conditioned by the gaseous condition of the blood; 

 or whether it is reflex in character, that is, due to the action of nerve im- 

 pulses transmitted from different regions of the body through afferent nerves. 

 The solution of this problem has apparently been settled by experiments 

 the object of which was the division of all afferent nerve-paths that might 

 have central connections with the center. The results of experiments of 

 this character are somewhat as follows: When the vagus nerves are divided 

 the respiratory movements at once diminish in number per minute but at 

 the same time increase in depth and amplitude. The number of respiratory 

 movements under these circumstances varies in different animals from four 

 to eight per minute, a rate which continues practically constant so long as the 

 animal lives, which may be a period varying from a few days to several 

 weeks. The relative duration of the respiratory phases also undergoes a 

 change, inspiration becoming longer than expiration and at the same time 

 becoming more or less spasmodic in character. 



Inasmuch as it is a familiar observation that the normal rate of the respira- 

 tory movement is frequently disturbed by nerve impulses transmitted to the 

 center, through afferent nerves other than the vagi, as well as from higher 

 centers in the brain, section of the vagi has been supplemented by a trans- 

 verse section of the spinal cord at the level of the first dorsal nerve, by section 

 of the dorsal roots of the cervical nerves and by a transverse section of the 

 region of the brain just posterior to the corpora quadrigemina, a series of pro- 

 cedures which practically isolates the center from all transmitted impulses. 

 Nevertheless, the inspiratory center still continues to discharge nerve im- 

 pulses to the respiratory muscles at a rate not differing much from that 

 witnessed after section of the vagi. At most the diminution in the rate 

 will not be more than two or three more per minute. The results of these 

 experimental procedures would seem to indicate that the fundamental rate 

 of discharge of nerve impulses is approximately from four to six per minute. 

 This conclusion has been strengthened by the results of experiments designed 

 to suspend the activity for some minutes by the withdrawal of the blood 

 by temporarily occluding the blood-vessels passing to the head. With the 

 resuscitation of the center, after the release of the blood-stream and at a 

 time when there are reasons for believing that the afferent paths are still 

 incapable of conduction, the initial rate of discharge was practically constant, 

 about four per minute in the cat. The same result was observed in some 

 instances in cats when in addition to producing anemia the vagi as well 

 as the region posterior to the corpora quadrigemina were divided (Stewart). 



It may therefore be assumed that the respiratory center possesses an in- 

 dependent automatic rhythm which is, however, much slower than that 

 characteristic of it when all afferent paths leading to it are intact. 



Accepting the statement that the fundamental rhythm of the inspiratory 

 center is automatic that is, due to a stimulus generated within itself 

 the question at once arises as to the nature of the stimulating agent. By 

 some investigators it has been assumed that the stimulus is connected with 

 the content or pressure of oxygen, by others with the content or pressure of 

 carbon dioxid, and that the variations in the respiratory rhythm are depend- 



