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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 700/* 3 , in the fetus of four weeks, is taken from His, and the figures represent 

 multiples of that volume. 



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 developmental 

 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 physiologi- 

 cally 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 devel- 

 oped cells become greater. 



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 num- 

 ber 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 few observations on the increase in the number of 

 functional nerve-cells with age. Kaiser, 2 as is shown in the accompanying 

 table, found in man increasing numbers of large nerve-cells in the ventral 

 horns of the spinal cord at the ages named : 



Number of Developed Cells in the Cervical Enlargement of Man at 

 Different Ages (Kaiser'). 



Age. Number of Nerve-cells. 



Fetus, 16 weeks 50,500 



" 32 118,330 



New-born child . 104,270 



Boy, fifteen years 211,800 



Male, adult 221,200 



1 Schiller : Comptes rendus de I' Academic des Sciences, Paris, 1889. 



2 Die Functionen der Ganglienzellen des Halsmarkes, Haag, 1891. 



