28 VETERINARY PHYSIOLOGY 



2. Pyrimidin Bases, such as thymin, which contain the 



asymmetric ring 



— N— C = 



I I 

 = C C— 



I II 



3. Hexoses (CeHiA)' (P- 285). 



4. Metaphosphoric Acid (HPO3). 



(3) Functions. — The part taken by the nucleus iu the 

 general life of the cell is not 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. 

 In certain cells, e.g. cells of the nervous system, it has been 

 found to shrink and to become displaced from its central 

 position as a result of continued activity. Important inter- 

 changes of material ^o on between the nucleus and the 

 protoplasm. 



2nd. It is the great reproductive organ of the cell play- 

 ing an important part in transmitting inherited characters 

 (p. 617). 



C. Reproduction of Cells. 



Cells do not go on growing indefinitely. When they 

 reach a certain size they generally either divide to form 

 two new cells, or die and undergo degenerative changes. 

 The reason of this is possibly to be found in the well-known 

 ph3^sical fact, that, as a sphere increases in size, the mass 

 increases more rapidl}^ than the superficies. Hence, as a 

 cell becomes larger and larger, the surface for nourishment 

 becomes smaller and smaller in relation to the mass of 

 material to be nourished. Probably the altered metabolism 

 so produced sets up the changes which lead to the division 

 of the cell. These changes have now been very carefully 

 studied in a large number of cells, and it has been shown 

 that the nucleus generally takes a most important part in 

 division. 



