190 THE HUMAN BODY. 



rest, can be demonstrated histologically. If the sensor} 1 

 nerves of one fore limb of an animal be left at rest and those 

 of the other simultaneously excited for several hours, it will 

 be found, at the end of that time, that the nuclei of many 

 cells of the spinal ganglia of the brachial nerves on the stim- 

 ulated side are shrunken and distorted when compared with 

 those of the other side. But if some hours be suffered to 

 elapse before the animal is killed and the ganglia examined, 

 the nuclei of the cells on both sides will be found equally 

 large and well rounded. In carrier-pigeons after a long flight 

 and in wild sparrows shot at the close of day, the nuclei of 

 the nerve-cells connected with the origin of motor nerve-fibres 

 are found to be shrivelled, and the whole cell frequently dimin- 

 ished in size when compared with specimens taken from birds 

 after a period of rest. In old age the nuclei of many nerve- 

 cells are small and distorted, even after prolonged rest. 



Nerve-trunks and the white portions of nerve-centres are 

 sparsely supplied with blood and undergo but small and slow 

 nutritive changes in health. Their activity appears to consist 

 in the transmission of some molecular motion not affecting 

 the nutrition and chemical composition of the fibre, and not 

 using up its material. 



Excitant and Inhibitory Nerves. The great majority 

 of the nerve-fibres of the Body when they convey^n^rvous 

 impulses to a part arouse it to activity; they are exciffrn^ 

 fibres. ' There is, however, in the Body another very impor- 

 tant set which arrest the activity of parts and which are 

 known as inhibitory nerve-fibres. Some of these check the 

 action of central nervous organs, and others the work of 

 peripheral parts. For instance, taking a pinch of snuff will 

 make most persons sneeze; it excites centrally acting fibres 

 in the nose, these excite a centre in the brain, and this in 

 turn sends out impulses by motor fibres which cause various 

 muscles to contract. But if the skin of the upper lip be 

 pinched immediately after taking the snuff, in most cases the 

 reflex act of sneezing, which the Will alone could not pre- 

 vent, will not take place. The afferent impulses conveyed 

 from the skin of the lip have " inhibited " what we may call 

 the "sneezing centre;" and afford us therefore an example 

 of inhibitory fibres checking a centre. On the other hand, 

 the heart is a muscular organ which goes on beating steadily 

 throughout life; but if certain branches of the pneumogastric 



