1 8 HUMAN PHYSIOLOGY 



complicated network (Fig. 1), and it is these fibres which 

 have a special affinity for basic stains. Between these fibres 

 is a more fluid material which may be called the nuclear 

 plasma. Digestion in the stomach removes the nuclear 

 plasma, but leaves the network unacted upon. 



The nuclear fibres are made up of two substances, a ground 

 substance or matrix, which has no affinity for colouring matter, 

 and which is hence called achromatin substance ; and a sub- 

 stance with a marked affinity for various dyes, set in the 

 former in a series of particles variously arranged, and dis- 

 tinguished as chromatin. It is the staining of this material 

 which colours the nucleus so deeply. 



The chromatin substance contains a large amount of 

 nucleic acid, and its richness in phosphorus has been demon- 

 strated by treating the cells with ammonium molybdate and 

 pyrogallol, which colours parts rich in phosphorus of a brown 

 or black tint. 



The nuclear fibres vary in their arrangement in different 

 cells. Usually they form a network, but occasionally they 

 are disposed as a continuous skein. In nuclei, with the 

 former arrangement of fibres, swellings may be observed 

 where the fibres unite with one another the nodal swellings. 

 The nucleoli are probably not all of the same nature, and 

 some may be simply specially well-marked nodal swellings. 

 The resting nucleus appears to be surrounded by a distinct 

 nuclear membrane, which is, however, probably really a 

 basket-like interlacement of the fibres at the periphery. 



(2) Functions. The part taken by the nucleus in the 

 general life of the cell is not yet fully understood. 1st, It 

 exercises an influence on the nutritive processes, since it 

 has been observed in certain of the large cells in lower 

 organisms that a piece of the protoplasm detached from the 

 nucleus ceases to grow, and, after a time, dies. Important 

 interchanges of material go on between the nucleus and the 

 protoplasm. 2nd, It is the great reproductive organ of the 

 cell, probably playing an important part in transmitting in- 

 herited characters. 



Reproduction of Cells. Cells do not go on growing in- 

 definitely. When they reach a certain size they generally 



