16 PHYSIOLOGY 



We have defined a cell as a small mass of protoplasm containing a 

 nucleus. Since we shall have to use the term ' protoplasm ' on many 

 occasions in the course of this work, we must have a definite concep- 

 tion of what we mean by it. The term is often used by histologists as 

 implying a substance of certain definite chemical and staining characters. 

 When employed by physiologists it generally implies any material 

 which we can, on a study of its behaviour to changes in its environ- 

 ment, regard as endowed with life. Huxley has defined it as " the 

 physical basis of life." Though it may be convenient to have a word 

 such as protoplasm signifying simply ' living material,' it is important 

 to remember that there is no such thing as a single substance proto- 

 plasm. The reactions of every cell as well as its organisation are the 

 resultant of the molecular structure of the matter of which it is built 

 up. The gross methods of the chemist show him that the composition 

 of the ' protoplasm ' of the muscle cell is entirely different from that 

 of a leucocyte or white blood corpuscle. The finer methods of the 

 physiologist show him that every sort of cell in the body has its own 

 manner of life, its own peculiarities of reaction to uniform changes in 

 its surroundings. No individual will react in exactly the same manner 

 as another individual, even of the same species, and the reactions of 

 the whole organism are but the sum of the reactions of its constituent 

 cells. There is not one protoplasm therefore, but an infinity of proto- 

 plasms, and the use of the term can be justified only if we keep this 

 fact in mind and use the word merely as a convenient abbreviation 

 for any material endowed with life. Even in a single cell there is more 

 than one kind of protoplasm. In its chemical characters, in its mode 

 of life, and in its reactions, the nucleus differs widely from the cyto- 

 plasm. Both are necessary for the life of the cell and both must be 

 regarded, according to our present ideas, as ' living.' In the cytoplasm 

 itself we find structures or substances which we must regard as on 

 their way to protoplasm or as products of the breakdown of proto- 

 plasm ; but in many cases it is impossible to say whether a given 

 material is to be regarded as lifeless or as reactive living matter. 

 Even in a single cell we may have differentiation among its different 

 parts, one part serving for the process of digestion while other parts 

 are employed for the purpose of locomotion. Here again there must 

 be chemical differences, and therefore different protoplasms. In 

 this work, therefore, protoplasm will be used in its broadest sense, 

 namely, as the physical basis of living organisms. 



STRUCTURE OF THE CELL. In every cell can be distinguished 

 the two parts nucleus and cytoplasm. The nucleus is generally an 

 oval or spherical body lying near the centre of the cell and bounded 

 by a definite contour or nuclear membrane. In its interior it contains 

 masses or filaments of a material known as chromatin, which are 



