2 INTRODUCTION 



became evident why the liver secreted and the biceps con- 

 tracted : 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 generalisa- 

 tion, 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, sur- 

 rounded 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. 



