lo6 AN INTRODUCTION TO SCIENCE 



We may select as illustrations of the methods of the 

 science two problems of different orders: (i) the cause 

 of the coagulation of the blood ; (2) the nature of the 

 control which the nervous system exerts over the body. 

 Contributions towards the solution of these problems have 

 been made by the naturalists of all ages, although it still 

 remains for the scientific workers of the future to discover 

 facts which must be added to the chain of evidence before 

 the final verdict is given. The history of these problems 

 illustrates in a striking way the natural growth of Science. 



That blood clots a few minutes after it is shed is an 

 observation which could not fail to attract the attention of 

 primitive man. The more primitive the man, the more 

 numerous were the opportunities which he enjoyed of ob_ 

 serving this phenomenon. 



Why does blood clot when out of the body, and why does 

 it not clot while it remains within the blood-vessels? 



Aristotle knew the immediate cause of coagulation ; that 

 it is due to the formation of fibrin (or fibres, as he called 

 them), and his explanation of why the fibres form was a 

 natural one, although the very reverse of the true explana- 

 tion, as we shall see. "Coagulation occurs in the earthy 

 part of the blood, that is, in the fibres, during the evaporation 

 of the moisture." " If the fibres are removed from the blood 

 of a bull" if it is whipped with a bundle of twigs so that 

 the fibres are collected on the twigs "the blood will not 

 clot." "If the fibres be left the fluid coagulates, as does 

 also mud, under the influence of cold. For when the heat 

 is expelled by the cold, the fluid, as has been already stated, 

 passes off with it by evaporation, and the residue is dried up 

 and solidified, not by heat but by cold. So long, however, 

 as the blood is in the body it is kept fluid by animal heat." * 

 To the idea of the escape of heat which was set forth in 



* "On the Parts of Animals," Book ii., Chap. iv. Dr. Ogle's trans- 

 lation. 



