890 THE CENTRAL NERVOUS SYSTEM 



pulses to which muscular tone is due, and therefore reflex, pr dif- 

 ferent in nature and automatically discharged. Now, degeneration 

 of a muscle is not usually caused, or at least not for a long time, by 

 interruption of its afferent nerve-fibres, as in locomotor ataxia, or 

 after section of the posterior nerve-roots (Mott and Sherrington). 

 We can hardly suppose that in any case the trophic influence of the 

 cells of the spinal or sympathetic ganglia to which all other reflex 

 powers have been denied, is of reflex nature. And there is, indeed, 

 more evidence in favour of trophic tone being an automatic action 

 of the cord than for any of the other tonic functions hitherto con- 

 sidered. 



The evidence for respiratory automatism upon which the spinal 

 cord has been chiefly credited with true automatic action has pre- 

 viously been given (p. 278). 



The * Centres ' of the Cord and Bulb. We have frequently used the 

 word ' centre ' in describing the functions of the spinal cord, but the 

 term, although a convenient one, is apt to convey the idea that our 

 knowledge is far more minute and precise than it really is. When we 

 say that a centre for a given physiological action exists in a definite 

 portion of the spinal cord, all that is meant is that the action can be 

 called out experimentally, or can normally go on, so long as this portion 

 of the cord and the nerves coming to it and leaving it are intact, and 

 that destruction of the ' centre ' abolishes the action. For example, a 

 part of the medulla oblongata on each side ci the middle line in the 

 floor of the fourth ventricle above the calamus scriptorius is so related 

 to the function of respiration that when it is destroyed the animal ceases 

 to breathe. But this respiratory centre the ' nceud vital ' of Flourens 

 does not correspond in position with any definite collection of grey 

 matter, although it includes the nuclei of origin of several cranial nerves, 

 and forms an important point of departure for efferent, and of arrival 

 for afferent, fibres connected with the respiratory act. Its destruction 

 involves the cutting off of the impulses constantly travelling up the 

 vagus to modify the respiratory rhythm, and of the impulses constantly 

 passing down the cord to the phrenics and the intercostal nerves. And 

 just as the traffic of a wide region can be paralyzed at a single blow by 

 severing the lines in the neighbourhood of a great railway junction, or 

 more laboriously, though not less effectually, by separate section of the 

 same tracks at a radius of a hundred miles, so destruction of the 

 respiratory centre accomplishes by a single puncture what can be also 

 performed by section of all the respiratory nerves at a distance from 

 the medulla oblongata. But while nobody speaks of the destruction of 

 a ' centre ' when a reflex action is abolished by division of the peripheral 

 nerves concerned in it, there is a tendency, when the same effect is 

 brought about by a lesion in the brain or cord, to invoke that mysterious 

 name, and to forget that the cerebro-spinal axis is at least as much a 

 stretch of conducting paths as a collection of discharging nervous 

 mechanisms. 



It is, perhaps, a profitless task to enumerate all the so-called centres 

 in the bulb and cord. In addition to the great vaso-motor, respiratory, 

 cardio-inhibitory and cardio-augmentor centres in the bulb, which, 

 perhaps, have more right than the rest to be regarded ^as distinct 

 physiological mechanisms, if not as definitely bounded anatomical 

 areas, there have been distinguished ano-spinal, vesico-spinal, and 



