CENTRAL NERVOUS SYSTEM. 



281 



root-fibres it was also determined that the cells of origin in the ventral horns 

 of the spinal cord increased at a corresponding rate Here is exemplified an 

 instance of long-continued enlargement of the nervous system by the regular 

 development of immature cells, a method of growth most marked probably 

 in those animals which increase in size so long as they live. 



Volumes of the Largest Cell-bodies in the Ventral Horn of the Cervical Cord of 



Man (based on Kaiser's records of the mean diameters). 



The volume TOO," 3 , in the fetus of four weeks, is taken from His, and the figures represent 

 multiples of that volume. 



Subject. 



Fetus 



Child at birth . . 

 Boy at fifteen years 

 Man, adult . . 



4 weeks 

 20 " 

 24 " 

 28 " 

 36 " 



Proportional 



volume of 



the cell-bodies 



1=70(V 3 . 



n 



17 I 



31 \ 



67 | 



81 j 



124} 



124 f 



160 



Time interval. 



36 weeks. 



15 years. 

 15 " 



It is believed that in this case the new cells and new fibres are not, 

 strictly speaking, new morphological elements, but are the result of devel- 

 opmental changes taking place in the cells present in the system from an 

 early period. 



A distinction is thus to be made between cell-elements which, because 

 they are not developed, are therefore not a part of the system already physio- 

 logically active, and those cells already organized together and which are 

 fully functional. When, therefore, it is said that the cells of origin for the 

 ventral root-fibres increase in number, the increase refers to the latter group, 

 and not to the total number of elements of both kinds present in the cord. 

 In other words, the number of cells appears to increase because the number 

 of developed cells becomes greater. 



In support of this suggestion the observations on the growth of the fibres 

 in the roots of the frog's spinal nerves maybe cited. 1 Hardesty found the 

 greatest number of medullated fibres in the ventral roots, nearest the cord, 

 and in the dorsal roots, nearest the spinal ganglion. inns in each the 

 greatest number was nearest the cells of origin, an arrangement which is 

 most readily explained by assuming that some of the fibres had grown but a 

 short distance from their cells of origin at the time the frog was killed. 



On the other hand, Schiller-' counted the number of nerve-fibres in the 

 oculo-motor nerves of cats, and found but a very slight difference in this 

 number between birth and maturity. So far, then, as this nerve is concerned, 

 it is found in the cat to be nearly complete at the time of birth. 



In man there are very lew observations on the increase in the number of 



'Hardesty: Journal of Comparative Neurology, 1899, vol. ix. 



2 Schiller: Comples rendus de VAcadtm.it des Sciences, I'uris. L889. 



