xiv INTRODUCTION 



evident why the liver secreted and the biceps contracted: 

 the one is composed of secreting tissue and the other of 

 contracting tissue. 



Cells and Function. Physiologists and anatomists alike 

 devoted their energies to the study of these various tissues, 

 and, as the structure of the microscope improved, greater and 

 greater advances were made in their analysis, till at length 

 Schwann was enabled to make his world-famous generalisation, 

 that all the tissues are composed of certain similar elements 

 more or less modified, which he termed cells, and it became 

 manifest that the functions of the different tissues are due to 

 the activities of their cells. 



The original conception of the cell was very different from 

 that which we at present hold. By early observers it was 

 described as composed of a central body or nucleus, surrounded 

 by a granular cell substance with, outside all, a cell membrane. 

 As observations in the structure of the cell were extended, 

 it soon became obvious that the cell membrane was not an 

 essential part, and later, the discovery of cells without any 

 distinct nucleus rendered it clear that the essential part is 

 the cell substance, and this substance Von Mohl named 

 protoplasm, by which name it is since generally known. 



Protoplasm and Function. So far physiology had followed 

 in the tracks of anatomy, but now another science became 

 her guide. Chemistry, which during the last century has 

 advanced with enormous strides, and has thrown such im- 

 portant light upon the nature of organic substances, now lent 

 her aid to physiology ; and morphologists having shown that 

 the vital unit is essentially simply a mass of protoplasm, the 

 science of life bids fair to become the science of the chemistry 

 of protoplasm. 



The prosecution of physiology on these lines is still in its 

 infancy, but already it has ' changed the whole face of the 

 science. Physiology is no longer the follower of anatomy. 

 It is become its leader, and at the present time, as we shall 

 afterwards see, not only the various activities, but also the 

 various structural differences of the different tissues, are to be 

 explained in terms of variations in the chemical changes in 

 protoplasm. 



In the study of physiology this order of evolution must be 



