The Wonders of Microscopy 305 



mand the muscles to move. Our mouth waters at the sight of 

 palatable food: it is only by help of the microscope that the 

 physiologist is able to trace the message from eye to salivary 

 glands, and to show how in the cells or unit-corpuscles of these 

 glands there is a preparation of secretion which is discharged when 

 the trigger is pulled by a nervous command. The study of vital 

 activity requires experiment and chemical analysis, but it cannot 

 dispense with the microscope. So we venture to say that a third 

 wonder of microscopy is the revelation of the intricacy of minute 

 structure. 



The Stones and Mortar of the House of Life 



It is a long and tangled story which tells of the gradual dis- 

 covery of the cells or unit-areas of which all but the simplest living 

 creatures are built up, and of the living matter or protoplasm 

 which these cells contain or portion off. The genius of the short- 

 lived French anatomist Bichat had analysed a living body into a 

 web of tissues nervous, muscular, glandular, connective, and 

 epithelial. But to Schwann and Schleiden, Virchow and Goodsir, 

 is due the credit of a further advance the Cell-Theory cer- 

 tainly one of the triumphs of microscopes with brains behind 

 them. The Cell-Theory or Cell-Doctrine states three facts : ( 1 ) 

 that all plants and animals have a cellular structure ( being either 

 single cells or combinations of numerous cells) ; (2) that every 

 living creature, reproduced in the ordinary way, begins its life as 

 a single cell, and, if it does not remain at that humble level, pro- 

 ceeds to build up a body by the division and re-division of cells 

 which eventually form tissues and organs; and (3) that the activi- 

 ties of a many-celled organism are the co-ordinated summation of 

 the activities of the component cells. "Every animal," Virchow 

 said, "appears as a sum of vital units." Not that we are to think 

 of an ordinary animal as a colony of cells, as a mob is a collection 

 of angry men, or even as a battalion is a co-ordination of dis- 

 ciplined soldiers. It is nearer the truth to think of the fertilised 



