298 THE HUMAN BODY. 



body of the cell arise several branches, the great majority 

 of which rapidly subdivide. One process of the cell (a), 

 although giving off several very fine branches, retains its 

 individuality, and is continued as the axis-cylinder of a 

 nerve-fibre in an anterior spinal root. At 2, in Fig. 87, is 

 represented a nerve- cell from the posterior part of the gray 

 matter of the spinal cord. It also has an axis-cylinder 

 process, differing somewhat in its way of branching from 1, 

 but probably ultimately giving rise to one or more cylinder- 

 axes of sensory nerve-fibres. Cells such as those represented 

 in Fig. 87 are found also in many parts of the brain. 



The Structure of Nerve-Centres. These consist of white 

 and gray nerve-fibres, of nerve-cells, and of connective tis- 

 sue and blood-vessels, arranged together in different ways 

 in the different centres. 



What arise from the protoplasm of a nerve-cell? What becomes 

 of most of the branches? How does one branch of some nerve- 

 cells of the spinal cord differ from the remainder? As regards this 

 branch, how do other nerve-cells differ from the above? 



Of what do nerve-centres consist? 



APPENDIX TO CHAPTER XIX. 



1. The co-operation of the parts of the body may be illustrated as 

 follows : 



a. Feign a blow at a person's eye ; the lids will close involuntarily, 

 even if he be told beforehand that he is not to be actually struck. 



b. Count a boy's pulse and breathing while he is sitting quietly, 

 then let him run a hundred yards at full speed, and immediately 

 afterwards again count pulse and breathing movements. Both will 

 be found accelerated; the breathing, to carry off from the blood the 

 carbon dioxide given it by the working muscles, and to bring in new 

 oxygen to replace the large amount used by the working muscles; 

 the heart-beat, to renew more rapidly the blood-flow through the 

 muscles. 



c. Tickle the inside of the nose with a feather. This, in itself. 



