1202 PHYSIOLOGY 



nerve be divided, after hemisection on the right side, the left half of 

 the**diaphragm at once commences to contract rhythmically with each 

 respiration (Porter). It is evident that the cessation of respiration 

 after section of the cord is not due to a condition of shock of the lower 

 spinal centres, since it is possible for impulses to pass down the cord 

 and to cross over to the contra-lateral diaphragm nucleus immediately 

 after hemisection of the cord on the side of the nucleus. 



THE QUESTION OF SPINAL RESPIRATORY CENTRES. Several physio- 

 logists, e.g. Brown Sequard, Langendorff, and Wertheimer, have described 

 respiratory centres 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 con- 

 tractions 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, how- 

 ever, 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 strice acousticce, 

 the respiratory 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 



