THK NERVOUS SYSTEM. 581. 



This kind of tone must be distinguished from that mere firmness and 



tension which it is customary to ascribe, under the name of tone, to 

 all tissues that feel robust and not flabby, as well as to muscles. The 

 tone peculiar to muscles has in it a degree of vital contraction: that of 

 other tissues is only due to their being well nourished, and therefore com- 

 pact and tense. 



All the foregoing examples illustrate the fact that the spinal cord is 

 a collection of reflex centres, upon which the higher centres act by send- 

 ing down impulses to set in motion, modify or control them. The 

 movements or other phenomena of reflex action are, as it were, the func- 

 tion of the ganglion cells to which an afferent impression is conveyed by 

 the posterior nerve- trunks in connection with them. The extent of the 

 movement depends upon the strength of the stimulus, the position in 

 which it is applied as well as the condition of the nerve-cells; the con- 

 nection between the cells being so intimate that a series of co-ordinated 

 movements may result from a single stimulation. Whether the cells 

 possess as well the power of originating impulses (automatism) is doubt- 

 ful, but this is possible in the case of (h) vaso-motor centres which are 

 situated in the cord (p. 246), and of (i) sweating centres which must be 

 closely related to them, and possibly in the case of (;') the centres for 

 maintaining the tone of muscles. 



The Nutrition (a) of the muscles appears to be under the control of 

 the spinal cord. When the nerve-cells of the anterior cornu are diseased 

 the muscles atrophy. In the same way (b) the bones and (c) joints are 

 seriously affected when the cord is diseased. The former when the 

 anterior nerve-cells are implicated, do not grow, and the latter are dis- 

 organized in some cases when the posterior columns are affected, (d) 

 The skin, too, is evidently only maintained in a healthy condition as 

 long as the cord and its nerves are intact. No doubt part of this influ- 

 ence which the cord exercises over nutrition is due to the relationship 

 which it bears to the vaso-motor nerves. 



Within the cord are contained, for some distance, fibres (a) which 

 regulate the dilatation of the pupil, (b) which have to do with the glyco- 

 genic function of the liver, (c) which control the nerve-supply of the 

 vessels of the face and head, (d) which produce acceleration of the 

 heart's action, and, (e) have a termotaxic action on the muscles, etc. 



THE RELATIONS OF THE DIFFERENT PARTS OF THE BRAIN. 



Before considering the parts of the brain separately, it will be best for 

 the comprehension of the plan of its construction to take a general survey 

 of the whole. The brain on superficial examination presents four 

 distinct parts, viz. (a.) The large and prominent masses of nervous 



