320 THE BODY AT WORK 



which the deposit of subchromate of silver extends, when the 

 chrome-silver method of displaying neurones has been used. 

 Sensation-impulses enter neuronic chains. The condition 

 which, when it affects the seat of consciousness, is known as 

 pain, progresses up the vertebrate neuropil. 



Energy is developed within the nervous system. The 

 force of impulses is adjusted to the resistance which they 

 have to overcome. Stimulation of the millions of twigs of 

 the vagus nerve in the lungs brings about the gentle move- 

 ments of ribs and diaphragm which constitute peaceful respira- 

 tion. A crumb of bread touching the mucous membrane of the 

 larynx stimulates a few of the endings of the same vagus 

 nerve. Like an avalanche, the impulses gather head as they 

 advance, causing, not the diaphragm and intercostal muscles 

 alone to do their utmost, but calling into action half a dozen 

 accessory muscles of respiration. It is difficult to account for 

 this reverberation of the messages which clamour for the ejec- 

 tion of the crumb of bread without figuring them as spreading 

 from neurone to neurone, urging each in turn to deliver its 

 maximal discharge. 



Neurones are provided with material which serves as a store 

 of energy. In their cell-bodies, including their dendrites, are 

 to be seen coarse granules of nucleo-protein, which, being 

 fitted in between groups of neuro-fibrillae, assume an angular 

 form. They are known as Nissl's corpuscles, or are termed 

 " tigroids," owing to the spotted appearance which they give 

 to the substance of a cell. If the nerve-cells of birds be ex- 

 amined just after they have alighted from a migratory flight, 

 the granules are found to be few and small. In a bee re- 

 turning to the hive at evening with its last load of pollen, they 

 are smaller than they were when it commenced its morning's 

 work. They disappear in certain pathological conditions, and 

 under the influence of various drugs ; and since their presence 

 is revealed by staining, their disappearance is spoken of as 

 " chromatolysis." 



The wasting of tigroids during functional activity proves 

 clearly that nerve-cells do work, in the physical sense. Energy 

 is expended in transmitting messages from receptor to effector, 

 from sensory cell to muscles, from recipient nerve-ending to 

 glands. Have nerve-cells any privileges or duties ? Their 



