352 The Philosophical Society of Glasgow. 



perhaps three or four times more numerous in the posterior than in the 

 anterior roots. 



As examples of the bearing of these measurements upon the struc- 

 tural and functional relations of the spinal cord and nerves, and of some 

 of the other nerves, the following may be extracted : — 



It was found that about 250 fibres (nerve-tubes) were to be observed 

 in TTTrth of an inch square of the cross section of the spinal marrow. This 

 would equal about 200,000 in f ths of an inch square — the area of the 

 spinal marrow in the dorsal region. The number might be 300,000 for 

 the upper part of the spinal cord . 



The fibres in the posterior or sensory roots alone surpass these in 

 number, and if we add those of the motor roots, it seems improbable 

 that all the fibres of the roots ascend through the spinal marrow. Al- 

 though the white substance does increase in thickness in the upper 

 parts, that increase does not appear to be commensurate with the size 

 of the roots of the nerves, and both the white and gray matter in the 

 lumbar and axillary enlargements are considerably greater than in the 

 parts above them. The conclusion seems irresistible, that a number of 

 fibres must remain within, originate in, or communicate with each other 

 in the cord, without reaching the brain. 



To take a rough estimate in another way. If we take the diameter 

 of the fibres at ^i\ and if, making allowance for the sheath-matter, and 

 the difierence between the circle and the square, we take the section of a 

 nerve-fibre as equal to an area of ^^th of an inch square. This would give 

 4,000,000 of fibres in a square inch, or nearly 500,000 in -12 of a 

 square inch, which is the average sectional area of all the posterior 

 roots in a grown person. 



If we take the external surface of the bodj'' as equal to fourteen feet, 

 and suppose all the sensitive nerves to reach the surface, and to be 

 equally distributed over it, this would give nearly 2i nerve-fibres for 

 each square tenth of an inch ; but making allowance for the difference 

 in the amount of sensory nerves in the more and less sensitive regions 

 of the skin, it seems probable that in the more sensitive there may be 

 a fibre for each oooth of an inch square, and for the least sensitive not 

 more than one fibre for each ^th of an inch square. 



The area of section of the optic nerve is nearly •O'i of a square inch. 

 The diameter of the nerve-fibres is probably from ^^ to ^^. If we 

 suppose one to occupy each square of —^ in section, we shall have 

 1,400,000, or nearly one million and a-half of fibres in each optic nerve. 

 But numerous as these fibres appear, their number is greatly inferior 

 to the peculiar bodies of the outer layer of the retina with which they 

 are connected, or in which they terminate, viz., the baccillary layer of 

 rods and cones. 



