THE LIVING SUBSTANCE 17 



gunpowder oxidise, i.e. unite with the oxygen of the air so as 

 to produce great quantities of highly elastic gas. Now we 

 find that if we remove all oxygen from the neighbourhood of 

 the contracting parts of an animal, they very soon cease to 

 be capable of contraction. They are paralysed ; and it seems a 

 plausible suggestion that the chemical decomposition connected 

 with contraction is of the nature of an oxidation. 



Besides its manifestation in the form of movement, life gives 

 evidence of its presence in secretion. By secretion is meant 

 the continuous production of substances, not themselves living, 

 which subserve some secondary use for the animal. Thus the 

 hairs which clothe our bodies are composed of dead dry horny 

 substance, but they are of inestimable value in protecting our 

 bodies against undue loss of heat. But secretions may be 

 fluid : the digestive juice which reduces our solid food to the 

 liquid form, and thus renders it capable of mingling with our 

 living substance and contributing to its " growth by intus-sus- 

 ception," is such a secretion. The point which we wish to 

 emphasize at this juncture is that whenever we penetrate to 

 the point where secretion is being actively carried on we find 

 masses of clear semi-fluid substance essentially similar to 

 that which we found in contractile fibres, and that it is by the 

 continual decomposition of this substance that the secretion is 

 produced. Similarly, if we investigate any quickly growing 

 part we find the same clear semi-fluid substance in a word, 

 all manifestations of life are associated with this substance, 

 or, putting it differently, this substance is the living substance, 

 and the body of an animal consists essentially of this substance 

 and of its secretions. 



This marvellous " living substance " is termed protoplasm, 

 and since in it is hidden the secret of life we may look at it 

 a little more closely. It is not a simple uniform substance, 

 but a mixture of substances. In every mass of living proto- 

 plasm we may detect a nucleus or several nuclei. The nucleus 

 is usually a rather compact body of an oval or spherical shape, 

 but in exceptional cases it may take the form of a rod, or even 

 of a spiral thread or it may be represented by a cloud of 

 granules. In the vast majority of cases, however, it is an 

 oval or spherical body. Its distinguishing feature is that it 

 contains a substance called chromatin, which has a remarkable 



