2 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



cells become elongated, and form muscular fibres ; in the connective 

 tissues we have a preponderating amount of intercellular material, 

 which may become permeated with fibres, or be the seat of the deposi- 

 tion of calcareous salts, as in bone. Instances of chemical changes in 

 the cells themselves are seen on the surface of the body, where the 

 superficial layers of the epidermis become horny {i.e. filled with the 

 chemical substance called keratin) ; in the mucous salivary glands, 

 where the cells become filled with mucin, which they subsequently 

 extrude; and in adipose tissue, where they become filled with 

 fat. 



In spite of these changes, the variety of which produces the great 

 complexity of the adult organism, there are many cells which still 

 retain their primitive structure ; notable among these are the white 

 corpuscles of the blood. 



A cell may be defined as a mass of living material containing in 

 its interior a more solid structure called the nucleus. The nucleus 

 exercises a controlling influence over the nutrition and subdivision 

 of the cell. 



The living substance is usually pervaded with granules ; one of 

 these minute particles called the centrosomc exercises an attractive 

 influence on the granules and fibrils of the protoplasm in its neighbour- 

 hood, and the appearance so produced is called the attraction sphere. 

 The attraction sphere becomes especially prominent, and divides into 

 two when the cell is about to divide ; this usually precedes the 

 division of the nucleus. 



Living material is called protoplasm, and protoplasm is charac- 

 terised by (1) its power of movement (seen in amoeboid movement, 

 ciliary movement, muscular movement) ; (2) its power of assimila- 

 tion, that is, it is able to convert into protoplasm the nutrient 

 material or food which is ingested ; (3) its power of growth — this 

 is a natural consequence of its power of assimilation ; (4) its power 

 of reproduction — this is a variety of growth ; and (5) its power to 

 excrete, to give out waste materials, the products of its other 

 activities. 



Of all the signs of life, those numbered 2 and 5 in tlie foregoing 

 list are the most essential. Living material is in a continual state of 

 unstable chemical equilibrium, building itself up on the one hand, 

 breaking down on the other ; the term used for the sum total of these 

 intra-molecular rearrangements is metabolism. The chemical sub- 

 stances in the protoplasm which are the most important from this 

 point of view are the complex nitrogenous compounds called Proteuls. 

 So far as is at present known, proteid material is never absent from 



