566 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



more pronounced. Again, animals in which respiration has been artificially 

 maintained for a long time may, after section of the cord at the junction with 

 the bulb, exhibit respiratory movements after artificial respiration has been 

 suspended. The respiratory movements under these circumstances are, how- 

 ever, of a spasmodic character, and distinctly unlike the co-ordinated rhythmi- 

 cal movements observed in normal animals ; the movements are rather of the 

 nature of spasms simulating normal respirations. 



The Rhythmic Activity of the Respiratory Centre. The rhythmic sequence 

 of the respiratory movements is due to periodic discharges from the respiratory 

 centre. The cause of this periodicity is still obscure, but the fact that the 

 rhythm continues after the combined section of the vagi and the glosso-pharyn- 

 geal nerves, of the spinal cord in the lower cervical region, of the posterior 

 roots of the cervical spinal nerves, and of the spinal bulb from the parts 

 above, indicates that the rhythm is inherent in the nerve-cells, and is not 

 caused by external stimuli carried to the centre through afferent nerve-fibres. 

 Loewy l has shown that under the above circumstances, when the centre is iso- 

 lated from afferent nerve-impulses, the rhythmical activity of the centre is due to 

 the blood, which, while acting as a continuous excitant, causes discontinuous or 

 periodic discharges, so that, although we usually speak of the activity of the 

 respiratory centre as being automatic that is, not immediately dependent upon 

 external stimuli yet as a matter of fact the apparently automatic discharges 

 are in reality due to the stimulation by the blood ; the centre is therefore auto- 

 matic only with reference to external nerve-stimulation. 



The rhythm as well as the rate, force, and other characters of the discharges 

 may be affected materially by the will and emotions ; by the composition, 

 supply, and temperature of the blood ; and especially by certain afferent im- 

 pulses, pre-eminently those originating in the pneumogastric nerves. As to 

 the influence of the will and emotions, we are able, as is well known, to modify 

 voluntarily to a certain extent the rhythm and other characters of the respira- 

 tions, while the striking effect of emotions upon respiratory movements is a 

 matter of almost daily observation. The importance of the composition of 

 the blood is manifested by the marked effect upon the respirations when the 

 blood is deficient in O, when it contains an excess of CO 2 , and during muscu- 

 lar activity, when in the blood there is a relative abundance of certain products 

 resulting from muscular metabolism. If the blood-supply to the centre is 

 diminished, as after severe hemorrhage or after clamping the aorta so as to 

 interfere with the cerebral circulation, the respirations are less frequent and 

 the rhythm is affected, the form of breathing having a Cheyne-Stokes char- 

 acter (p. 532) ; conversely, an increase in the blood-supply causes an increase 

 in the rate. An increase or decrease in the temperature of the blood induces 

 corresponding changes in the rate ; thus, in fever the frequency of the move- 

 ments increases almost pari passu with the augmentation of temperature, while 

 if the temperature of the blood be reduced by applying ice to the carotids, the 

 rate is lessened. 



1 Pflilger>s Archivf. Physiologic, 1889, vol. xlii. pp. 245-281. 



