26 



THE CELL AND MAMMALIAN 



HISTOLOGY 



We have from time to time in this book used the word ' cell ', 

 but have not defined it ; nor shall we attempt now a formal 

 definition which would satisfy a philosopher, for the concept of 

 which it is the name is one of those, common in biology, where the 

 general meaning is clear but the limits impossible to lay down. 

 When an animal is closly examined there is an obvious 

 distinction between parts which are living and those which 

 are not ; the sentence, ' I have cut my finger to the quick ', 

 where ' quick ' is used in the Bibhcal sense of ' alive ', shows 

 that long before zoology was a science the popular mind 

 made a distinction between the living part of the body and the 

 dead or formed parts such as hair or nails or the outermost 

 layers of the skin. More detailed investigations confirm this 

 distinction, and to the material of which the hving parts of the 

 body are made the name protoplasm is applied. This term had 

 previously been proposed in 1848 by Hugo von Mohl for the 

 living part of the plant cell — that portion which lies between 

 the watery sap and the wall. It is now used generally for all the 

 living material of both plants and animals. Microscopic investi- 

 gation shows that most often this protoplasm is not in one con- 

 tinuous mass, but is separated into a number of separate units 

 by some sort of wall. These units are called cells, and the simplest 

 definition of a cell is ' a quantity of protoplasm surrounded by 

 some sort of wall '. The word ' cell ' means a small room, and 

 in prisons it still has that sense. It was used by Robert Hooke in 

 1665 for the small cavities and their walls which can be seen in 

 cork, and which are in fact dead plant cells, and from that 

 starting-point its meaning has slowly changed. It was early 

 known that many cells contain a specially dark part, and Robert 

 Brown in the early part of the nineteenth century recognised 

 that this was a normal feature of plant cells and called it the 

 nucleus, although he did not invent this use of the word. We now 

 generally deny the name cell to anything which does not contain 

 a nucleus, and define a cell as ' a quantity of protoplasm contain- 

 ing a nucleus and surrounded by some sort of wall '. There are, 



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