BATE OF CONDUCTION IN NERVES. 223 



are separated at the roots of the nerves, which are always double, and 

 spring from different points of the spinal cord. 



The white nerve-fibres are excitable by artificial stimuli, and con- 

 duct the effects produced by these at and along all parts of their 

 course; and the gray matter likewise has the same properties. During 

 life, however, it is the peripheral extremities of the sensory and affer- 

 ent reflex white fibres, which are chiefly excitable, whilst in their 

 course up to the sensorium, they usually act as conductors only ; and, 

 again, it is the central ends of the motor fibres, which are chiefly 

 excitable, whilst in the rest of their course to the muscles, they usually 

 act as conductors only. But during life, the white fibres have no 

 power, so far as we are aware, of completing a sensation; nor do they 

 originate, or form the source of. a motorial stimulus ; these two special 

 properties or forms of nervous excitability, are limited to the gray 

 matter. Hence the gray matter is said to be more highly endowed, 

 and to constitute what are called active nervous centres. Besides 

 being concentrated in masses, the gray matter has a more complex 

 microscopical structure, and is more vascular than the white matter. 



The conducting power of motor nerve-fibres is such, that it takes a 

 certain appreciable time for the effects of a stimulus to travel along 

 them. The rate of conduction in the frog has been determined by the 

 following interesting experiment performed by Helmholz. An upright 

 blackened cylinder, made to revolve so many times in a second by 

 clockwork, named a Kymographion, has two pins brought in contact 

 with its surface ; an upper one, attached to a galvanic apparatus, serves 

 to record the moment of entrance of an exciting current into the upper 

 end of a long nerve; whilst a lower one, attached to the muscle sup- 

 plied by the nerve, records the moment and duration of the contraction 

 of the muscle, by rising as the muscle contracts, and so describing a 

 curved line on the cylinder. The circumference of the cylinder, the 

 number of its rotations per second, the length of the line described by 

 the lower pin, before it begins to ascend, and finally the length of the 

 excited nerve, furnish data for the calculation of the rapidity with 

 which the excitability of the nerve is brought into play along it; in 

 other words, the rate of movement of the nerve-change through the 

 nerve. Quite recently. Helmholz has devised another, much more 

 complicated, but more delicate apparatus, for determining this rate. 

 In the motor nerves of the frog, at a temperature between 52 and 

 70, the rate of conduction was found to vary from 81 to 126 feet per 

 second. In warm-blooded animals^ and in man, it has been esti- 

 mated to be rather more than 200 feet per second. The rate of motion 

 of an electrical current travelling along a metallic wire, has been 

 shown to be 462,000,000 feet per second. Light travels about 40,000 

 miles in the same period. The rate of conduction of impressions in 

 sensory nerves, has been calculated by Hirsch, at about 110 feet per 

 second. The same observer states that the rate of propagation differs 

 in regard to the nerves of touch, hearing, and sight; but the numerical 

 results obtained by him are variable. Some difference, however, may 

 exist in different nerves, for contraction of the iris in rabbits occurs 

 quickly on irritation of the third cranial nerve, but more slowly after 



