NERVE-FIBRES AND NERVE-CELLS 125 



"explosive" metabolite if they are contractile, or of a 

 secernible metabolite if they are glandular, under the direct 

 supervision of nerves specially set apart for their work. 

 Physiologists spoke of motor and inhibitory, vaso-constrictor 

 and vaso-dilator, calorific and frigorific, trophic nerves, 

 glandular nerves, etc., but the search for so many distinct 

 varieties has not been very successful. In the case of one 

 organ, and perhaps of one only, do we see distinct evidence 

 of its activity being influenced by two antagonistic nerves. 

 The heart is a most conscientious slave. Seventy times a 

 minute or thereabout it beats, as long as life lasts, without 

 any command from the Will. Its fault lies in a slight incli- 

 nation towards an excess of zeal. It is apt to work too 

 hard, producing a pressure in the blood-vessels which is not 

 good for the system, and leads to strain and consequent 

 dilatation of the heart itself. Therefore we find that while it 

 is stimulated to work by slow-acting and apparently not 

 very forcible sympathetic nerves, it is restrained when nec- 

 essary by a most peremptory "vagus." If the sympathetic 

 is stimulated, the result is more work with a diminution of 

 the heart's nutritive balance ?, e., exhaustion. If the 

 vagus act, it does less work and its condition of nutrition 

 improves. These two nerves have been taken as types of 

 two classes of nerves, the one katabolic, breaking down ; 

 the other anabolic, building up the one leading to the 

 diminution of the reserve of foodstuffs in the tissues, the 

 other to their accumulation. There is some evidence, which 

 we have not space to detail, of the existence of these two 

 sets of nerves in connection with other organs ; but com- 

 paratively few instances of excitement or restraint can be 

 pointed out which are necessarily the direct results of 

 specific nerves and not the indirect results of the regulation 

 of the blood-supply by vaso-motor action. As Dr. Langley 

 pointed out in his address as President cf the Physiological 

 Section of the British Association at Dover, evolution is still 

 proceeding, the nervous system is making experiments ; all 



