FUNCTIONS OF THE SPINA^ CORD gig 



Trophic Tone. The degenerative changes that occur in muscles, 

 nerves, and other tissues when their connection with the central 

 nervous system is interrupted have been already referred to (p. 804). 

 It is possible to explain these changes in some cases without the 

 assumption that tonic impulses are constantly passing out from the 

 brain and cord to control the nutrition of the peripheral organs; 

 and we have seen that there is no real evidence of the existence of 

 specific trophic fibres. But the degeneration of muscles after sec- 

 tion of their motor nerves is difficult to understand, except on the 

 hypothesis that impulses from the cells of the anterior horn influence 

 their nutrition. The only question is whether these are the im- 

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

 ferent in nature and automatically discharged. Now, degeneration 

 of a muscle is not usually caused, or at least not lor 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. 284). 



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 of 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 



