308 THE CAUSATION OF DISEASE. 



is so or not clearly depends upon the meaning we set upon the 

 word function. 



The term Function is familiarly used to designate some 

 obvious purpose which a structure fulfils. Thus, we say the 

 function of a muscle is to contract ; of a gland, to secrete ; of a 

 nerve-centre, to start or modify nervous impulses ; of connective- 

 tissue, to bind together the elements of the tissues ; of a blood- 

 vessel, to conduct blood. Now, using the term in this sense, 

 it is obvious that we must make a distinction between the 

 active functional state of which the first three instances afford 

 examples, and the mere passivity of which the last two are 

 examples. It will be quite unnecessary to say anything further 

 of this latter kind of function ; but it is necessary to say some- 

 thing of the former kind. Still using the word function in 

 the sense indicated — i.e., to designate some obvious purpose 

 which a tissue fulfils — we may define the active function of a 

 tissue as that part of its activity which is not necessary to its 

 own vitality, but is displayed for the benefit of the organism 

 at large. Thus we may regard the secretion of bile by the 

 liver- cells as apart from and independent of the nutrition or 

 vital process of the liver-cell : according to this view, the 

 liver-cell is a machine for the elaboration of bile out of certain 

 crude materials brought to it, and the process of bile manu- 

 facture is something quite distinct from the vital process of 

 the machine or cell itself. In a similar way we may regard 

 a muscle-cell as a machine for getting force out of the food 

 materials supplied to it, the process by which the machine is 

 kept alive constituting its nutrition, that by which it converts 

 insensible into sensible motion constituting its function. 



How far we are justified in thus sharply separating nutri- 

 tion and function it is difficult to decide. According to the 

 older views, such functional exercise was the outcome of actual 

 protoplasmic break-down — e.g., the force spent in muscular 

 contraction was, it was thought, the result of an equivalent 

 break-down or destruction of muscle-tissue, and the secretion 

 of a gland-cell was supposed to take place at the expeuse of 

 actual protoplasmic destruction. Now, if this view be correct, 

 it follows that "function" in the sense here used is part and 

 parcel of the nutrition of the cell, or, at all events, it cannot 



