814 A MANUAL OF PHYSIOLOGY 



the central nervous system is interrupted have been already 

 referred to (p. 699). 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 section 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 impulses 

 to which muscular tone is due, and therefore reflex, or different 

 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 considered. 



The evidence for respiratory automatism upon which the spinal 

 cord has been chiefly credited with true automatic action has 

 previously been given (p. 233). 



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 respira- 

 tion that when it is destroyed the animal ceases to breathe. But 

 this respiratory centre the ' noeud 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 destruc- 

 tion 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 inter- 

 costal 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 



