554 HANDBOOK OF PHYSIOLOGY. 



Their adaptation to the purpose of simple conduction is, perhaps, due 

 to the contents of each fibre being completely isolated from those of ad- 

 jacent fibres by the myelin sheath in which each is inclosed, and which 

 acts, it may be supposed, just as silk, or other non-conductors of elec- 

 tricity do, which, when- covering a wire, prevent the electric condition 

 of the wire from being conducted into the surrounding medium. 



Velocity of a Nervous Impulse. The change which a stimulus sets up in 

 a nerve, of the exact nature of which we are unacquainted, appears to travel 

 along a nerve-fibre in both directions with considerable velocity in the 

 form of a wave. Helmholtz and Baxt have estimated the average rate 

 of conduction in human motor nerves at 111 feet (nearly 29 metres) per 

 second; this result agreeing very closely with that previously obtained. 

 It is probably rather under than over the average velocity. Rutherford's 

 observations agree with those of Von Wittich, that the rate of transmis- 

 sion in sensory nerves is about 140 feet (42 metres) per second. The 

 velocity of the nerve impulse in motor nerves has been calculated by notic- 

 ing the duration of the interval between two contractions of the same 

 muscle when stimulated by means of two pairs of electrodes, one placed 

 behind the nerve close to the muscle, and the second placed at a known 

 distance further away from the muscle. The contraction ensues when 

 the stimulus is applied further from the muscle later than the other 

 case, and the interval between the two contractions is occupied by the 

 passage of the impulse down the nerve. With these data it is concluded 

 that the velocity of the passage of the nerve impulse in a frog's motor 

 nerve is 28 to 30 metres per second. In the human motor nerve, cal- 

 culated by applying the stimulus through the skin instead of directly 

 to the nerve, the velocity is greater, viz., about 33 to 50 metres per 

 second. In sensory nerves the velocity is said to be about 30 to 33 

 metres per second. Various conditions modify the rate of transmission, 

 of which temperature is one of the most important, a very low or a very 

 high temperature diminishing it; fatigue of the nerve acting in the 

 same direction, but increase of the stimulus up to a certain point increas- 

 ing it, as does also the Jcatelectrotonic condition of the nerve. 



The Gerebro- Spinal Nervous System. The parts of which this sys- 

 tem is composed are the following : (a) the spinal cord and its nerves ; 

 (b) the brain made up of cerebrum, crura cerebri and the ganglia in con- 

 nection with them, pons varolii, cerebellum, and the medulla oblongata 

 or bulb which connects the upper parts of the system with the spinal 

 cord, or medulla spinalis. 



All of these parts of the nervous system are nerve-centres, in contra- 

 distinction to nerve-trunks, and differ from the nerves in being made up 

 of nerve-cells and their branchings as well as of nerve-fibres. As now 



