THE NERVOUS SYSTEM 223 



nerve, has been measured by experiment upon the lower ani- 

 mals, and estimated in the case of man at about two hundred 

 feet per second. As compared with that of electricity, this is 

 a very slow rate, but, in respect to the size of the human 

 body, it is practically instantaneous. (Read Note 5.) 



28. The Functions of the Spinal Cord. As the anterior and 

 posterior roots of the spinal nerves have separate functions, so 

 the anterior and posterior columns of the cord are distinct 

 in function. The former are concerned in the production of 

 motion, the latter in sensation. If the cord be divided, as 

 before in the case of the nerve, it is found that the parts 

 below the point of injury are deprived of sensation and of 

 the power of voluntary motion on both sides of the body a 

 form of paralysis which is called paraplegia 



29. This form of disease paraplegia is sometimes seen 

 among men, generally as the result of a fall, or some other 

 severe accident, by- which the bones of the spine are broken, 

 and the cord is crushed, or pierced by fragments of bone. 

 The parts which are supplied by nerves from the cord above 

 the point of injury are as sensitive and mobile as before. The 

 results are similar, whether the division happens at a higher 

 or lower portion of the spinal cord; but the danger to 

 life increases proportionately as the injury approaches the 

 brain. When it occurs in the neck, the muscles of inspira- 



5. The Speed of Sensation and Thought. "The rate of nervous 

 and mental action is not the same in all individuals. In comparing the 

 records made by astronomers, it has been found that an appreciable dif- 

 ference exists in the rapidity with which the same occurrence may be 

 noted by different observers. This is known as the * personal equation,' 

 and is allowed for with the greatest nicety in the making up of astro- 

 nomical reports. With very delicate apparatus for marking time, the 

 various nervous impulses have been observed; from forty to a hun- 

 dred feet in a second are the estimates of different experiments as to the 

 speed of sensation ; or, as it has been expressed, it would take a full- 

 grown whale a second to feel the stroke of a harpoon in his tail." 



28. Functions of the anterior and posterior columns of the cord? If the cord be 

 divided ? 



29. Paraplegia ? Result and danger to life ? When the injury occurs in the neck ? 



