THE VICTORIAN NATUEALIST. 153 



thyroid gland, and until the last few years was looked upon as 

 having at the most something to do with the elaboration of the 

 blood ; but its function was not known, and it was regarded as 

 one of those parts, of which we still retain a few in our bodies, 

 the use or purpose of which is not very apparent. In fact, many 

 physiologists looked on it as a useless organ. Now we have 

 evidence, however, that it has a most important function in 

 preserving the nutrition of the nervous system. When it is lost 

 through disease or other cause the person becomes enfeebled in 

 his mind, and ultimately demented. At the same time evidence 

 of grave disease occurs in other parts of the body, and the case 

 runs slowly but inevitably to a fatal end. The treatment at 

 present adopted, with very considerable success, is to supply the 

 affected persons with the thyroids of other animals, chiefly the 

 sheep. Sometimes she is merely fed to a very large extent on 

 them. In other cases the fresh juice of the sheep's thyroid is 

 injected into the blood. But an equally successful, but at the 

 same time the most difficult, method of treatment is to transplant 

 a portion of a sheep's thyroid under the skin of the diseased 

 person. If this is successfully done the part so transplanted 

 grows, and performs the . functions of the diseased organ, the 

 course of the disease is stopped, and the affected person in a 

 number of instances recovers. 



ESSENTIAL CHARACTERS OF PROTOPLASM. 



Now let US see what are the chief of the characteristics common 

 to protoplasm, by virtue of which we can understand in some 

 degree how it forms a united whole of itself — sharply cut off 

 from all other substances with which we are acquainted. Proto- 

 plasm forms the whole of the simplest and least complicated 

 forms of living things, but in the course of growth and develop- 

 ment it becomes " differentiated" as it is called ; that is, various 

 parts originally the same become different, usually in order to 

 subserve different purposes in the economy of the one living 

 whole. But of simple undifferentiated protoplasm it may be said 

 that it has in the first place a fairly definite chemical composition. 

 It consists almost exclusively of a substance called albumin, a 

 substance which we may see in a state of nearly perfect purity in 

 the white of an egg. When dead and analyzed this is found to 

 consist of C, O, N, and H in fairly constant proportions. When 

 alive it always has also a quantity of water bound up with it in 

 the closest relationship, and there are traces of S, P, and Ca. and 

 other elements, apparently also in chemical combination with it ; 

 so that, although dead protoplasm may consist largely of albumin, 

 it is highly probable that living protoplasm is very much more 

 complex. We have reason to think that albumin is a chemical 

 substance that has at least a thousand atoms in each of its 



