1128 PHYSIOLOGY 



in the spinal cord. There is no doubt that, if the cord be cut across in the upper cervical 

 region and artificial respiration maintained for some time, cessation of the respiration 

 may be followed by rhythmic contractions of the respiratory muscles. These are 

 especially marked in young animals and if the activity of the cord has been heightened 

 by the injection of small doses of strychnine. Careful observation of the movements 

 shows however that they cannot be spoken of as respiratory, since although rhythmic, 

 they are not co-ordinate. The diaphragm may contract either simultaneously or in 

 alternation with the intercostals, and muscles which are essentially expiratory at the 

 same time as those which we are wont to regard as inspiratory. These experiments 

 show merely that the motor centres of the cord can enter into rhythmic activity under 

 the influence of asphyxial conditions. The movements affect the muscles of the limbs 

 as well as those essentially respiratory in function. 



THE AUTOMATICITY OF THE RESPIRATORY CENTRE 



We have now to inquire what it is that keeps the respiratory centre in 

 activity. Is the rhythmic discharge of inspiratory impulses from the centre 

 due to rhythmic or continuous stimulation of afferent nerves, or is the centre 

 so constructed that under the normal conditions of its environment the 

 metabolic activity of its constituent parts tends, like that of the heart cells, 

 to assume a rhythmic character? In other words, is the activity of the 

 centre reflex or automatic ? It has been found by Rosenthal that rhythmic 

 respiratory movements are maintained even after complete section of the 

 brain stem at the level of the superior corpora quadrigemina, section of the 

 cord at the level of the seventh cervical nerve, and division of both vagi and 

 of the posterior roots of all the cervical spinal nerves. It is true that if the 

 sections of the brain stem be placed as low as the strios acousticce, the re- 

 spiratory movements are profoundly modified and give place to a series of 

 inspiratory spasms. We might argue from this that the centre was capable 

 of a very imperfect degree of automatic action, but needed the stimulus of 

 afferent impulses from the vagi or from the higher parts of the brain to 

 render these actions adequate for the respiratory needs of the organism. 



In the above experiment the centre cannot be regarded as free from all afferent 

 stimuli, since the mere closure of the demarcation current in the cut ends of the nerves 

 would cause a certain amount of excitation, and the animal does not survive sufficiently 

 long to allow this condition to pass off. Hering has shown that in the ' spinal cord 

 frog ' (i. e. one in which the brain has been destroyed) section of all the posterior roots 

 absolutely abolishes all mobility, the injection of strychnine being without effect. A 

 typical spasm however can be at once produced by exposing and stimulating the 

 stump of one of the cut posterior roots. We might suppose that the respiratory centre 

 would be similarly devoid of automatism if absolutely free from afferent stimuli. It 

 must be mentioned however that, according to Sherrington, it is possible to excite 

 strychnine or asphyxial spasms in a dog or cat with isolated spinal cord, in which all 

 the afferent roots below the transection have been divided six or seven hours previously. 

 He therefore is of opinion that in the mammal the motor nervous mechanism can be set 

 into activity apart from the incidence of afferent impressions. The respiratory centre 

 tends to respond to all stimuli, continuous or rhythmic, by means of rhythmic discharges, 

 and there can be no doubt that, if we take the medulla in connection with the rest of 

 the hind- and mid-brain, we are justified in regarding its activity as automatic. 



The automatic activity of the heart is intimately dependent on the 

 saline constituents of the blood. It may be abolished or diminished by 



