Prof. Allen Thomson on the Spinal Cord and Nerves. 353 



If we take the central spot of the retma or seat of perfect vision, 

 which is entirely occupied by cones, as equal to an area of roth of an inch 

 square, and the diameter of each cone as equal to a square of -s-iiTnTth of 

 an inch, there would be about 10,000 of the cones in the central spot. 



If the area of the whole retina be taken as equal to two square 

 inches, and we make a rough calculation of the relative proportion of 

 rods and cones over the whole retina, it seems probable that there may 

 be about five millions of cones and 160 millions of rods, and accordingly 

 many more of these bodies than of nerve-fibres. It seems probable, 

 therefore, that a subdivision of the nerve-fibres may take place before 

 they communicate through the nerve-cells of the retina with the cones, 

 and it is believed that in the granular layer of the retina a frequent 

 subdivision of the ultimate nerve-fibres occurs before they reach the 

 rods. The cones are probably the media through which separate im- 

 pressions of light are perceived, their distance from each other in the 

 central part of the retina corresponding nearly with that indicated by 

 the limits of most minute vision. 



With reference to the relation of the nerve-fibres to the muscles, it is 

 to be observed, that if the number of fibres in one square inch of the 

 cross section of a muscle be about 25,000, and if we take the cross 

 section of muscular substance in the human body as equal to 100 

 inches, we shall thus have not less than 2,500,000, or two and a-half 

 millions of muscular fibres in the body. But the motor roots are 

 scarcely a half of the size of the sensory roots, and the diameter of 

 their fibres is somewhat greater, so that there must be less than a half 

 of the number of fibres contained in them, that is, the half of 500,000, 

 or 250,000 fibres in all the motor roots of the spinal nerves, and there- 

 fore not more than one nerve-fibre for ten muscular fibi-es, and of these 

 fibres it is not certain that all pass up the spinal cord to the brain. 

 Farther, considering the obliquity of the direction of muscular fibres in 

 muscles, the above estimate of the number of fibres in all the muscles 

 is probably much too low, and consequently, the number of muscular 

 fibres in the body is greatly superior to that of the motor nerve-fibres, 

 which are the means of exciting muscular action. 



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