42 



PHYSIOLOGY. 



developing a cell, so that the parent cell becomes filled with one or 

 more generations of new cells, which may either disappear entirely, 

 as in the case of the ovum, or by the rupture of the original cells 

 the contents may be scattered and undergo an independent develop- 

 ment. Sometimes several nucleoli are seen within one nucleus ; 

 and sometimes several nuclei within one cell. 



Cells are concerned not only in the function of nutrition in the 

 development and restoration of parts, but also in several other organic 

 processes ; for instance in absorption and secretion, the elaborating 

 and selecting processes of which, we shall hereafter see, are per- 

 formed by their agency. 



Each cell is an independent organ; it lives for itself and by itself, 

 and is dependent upon nothing but a due supply of nutriment and of 

 the appropriate stimuli for the continuance of its growth and for the 

 due performance of its functions, until its term of life is expired. 

 The nucleus and cell-wall differ from each other in chemical compo- 

 sition, though both seem to be concerned in the development of 

 tissues. The cell-wall is a proteine compound. 



In the formation of tissues, the cells undergo changes which may 

 be described under two heads : first, Those affecting the cell-mem- 

 brane ; and secondly, Those in which the nucleus is concerned. In 

 the fibrous tissues, the cell-membranes become elongated, and so 

 folded and divided as to give the appearance of subdivision into 

 minute threads or fibres. In the compound tubular tissues, as muscle 

 and nerve, the cells are joined end to end, and the partitions at each 

 extremity being removed, a tube is formed, in which the proper 

 deposit of muscular or nervous matter takes place. 



In the simple tubular tissues, as in the capillaries, the tubes are 

 also formed by the coalescence of the walls of the cells at several 

 points owing to their elongation, here and there into pointed pro- 

 cesses, which unite and form the ramifications of the vessels. 



The metamorphoses of the nucleus, although equally important, 

 are much less numerous. In some cases it sends out radiating pro- 

 longations causing it to assume a stellate form, which is the case in 

 bone cells. In other cases it appears to resolve itself into a fasciculus 

 of fibres; and this, according to Henle, is the origin of yellow fibrous 

 tissue. The tubuli of the dental structure are formed by its separa- 

 tion into a number of distinct fibres, each composed of a linear 

 aggregation of granules. Lastly, according to Dr. Carpenter, it may 

 disperse itself still more completely into its component granules, by 

 whose reunion, certain peculiar vibrating filaments (the so-called 

 spermatozoa,) may be formed, possessing motor powers and destined 

 to perform most important offices in the function of reproduction. 



The development of cells goes on during the life of the organism 

 at every period of its existence. They are found floating in immense 

 numbers in the blood, chyle and lymph ; and even in diseased 



