Chap. XIX] THE NERVOUS SYSTEM 373 



Speed of nerve-impulses. — The speed at which an impulse 

 travels along an afferent nerve-fibre is found to be about 140 

 feet (42.6 m.) per second. The efferent impulses travel some- 

 what slower, 110 feet (33.5 m.) per second. 



It may be interesting to note how very slow a nerve-impulse 

 is when compared with light which travels at the rate of about 

 186,000 miles per second, and sound which travels about 1100 

 feet per second. 



Reaction of nerve-endings. — A study of the previous classifica- 

 tion shows that the sensory nerve-endings are not all affected by 

 the same stimulus, nor do they react in the same way. Thus 

 some of the sensory nerve-endings are affected by pressure, and 

 others by temperature. The endings of the auditory nerve in the 

 ear are affected only by sound, and the endings of the optic nerve 

 in the eye are affected by ligh.:, though a similar effect may be 

 produced by a blow on the head, or an accident which jars the 

 spinal column. 



Gray matter. — The cell-bodies, dendrites, commencement of 

 the axis-cylinder processes, and their collaterals are not scattered 

 promiscuously throughout the body, but are gathered together in 

 certain definite regions or groups. These form the gray matter 

 of the brain, spinal cord, and ganglia. 



White matter. — The white matter consists of medullated nerve- 

 fibres and is found in the brain, spinal cord, ganglia, and also 

 in the nerve trunks distributed to all parts of the body. 



Neuroglia. — Neuroglia is not nervous tissue, but is a special 

 kind of tissue found in the brain and spinal cord, and serves 

 the same purpose as connective tissue in other parts of the body. 

 It consists of cells that give off many fine processes which extend 

 in every direction and form a supporting and connecting network 

 among the nerve-cells, nerve-fibres, and blood-vessels. 



Formation of nerve-trunks. — The nerve-fibre of each neurone 

 is, as has been described, of microscopic diameter, but when 

 a number of these nerve-fibres are bound together in a bundle we 

 have the plainly visible nerve-trunks, or nerves, such as are seen in 

 dissections of the body. 



Nerves are whitish cords which extend between cells situated 

 in different parts of the brain, spinal cord, and ganglia, also between 

 these centres and all parts of the body. They thus afford a means 



