2 2 THE CAUSATION OF DISEASE. 



plasma will be imperfectly formed, and thus every cell 

 in the body will suffer for the error in one — it may be 

 very limited — patch of tissue, for the fluid medium 

 which surrounds each individual cell — at all events, all those 

 which are engaged in active vital processes — will be defective 

 in composition. 



We thus see how potent is one tissue to modify the cell-E 

 of others. In respect of this power, there is, however, none 

 so far-reaching in its influence as that constituting the nervous 

 system. Every individual system of the body is controlled by 

 the nervous system : by it the most subtle changes in capillary 

 blood-pressure may be brought about, and the amount of 

 capillary blood-pressure doubtless influences the circulation of 

 the plasma environing the cells ; the nervous system again 

 exerts a most potent control over secretion, excretion, the pro- 

 duction of heat, and many other processes which it is needless 

 here to enumerate. When we reflect, indeed, that every active 

 cell in the body, such, for instance, as the gland and muscle cell, 

 is directly supplied with a nerve filament, it becomes at once 

 obvious what a dominant influence the nervous system wields 

 over the rest of the body. The nerve twigs are, as it were, 

 the reins with which the nervous system governs the body, now 

 curbing the cell activities, now allowing them full play — nay, it 

 may even be urging them on to some specific effort, as when 

 the muscle inogen is exploded, the invisible muscle motion 

 being converted into sensible motion ; or when in a gland-cell 

 the zymogen is converted into the zyme. 



There is, however, one method by which the nervous system 

 influences the cell-E of the tissues which needs a special men- 

 tion. I allude to the influence of the mind. The operations of 

 the mind, or, as some would prefer to say, " mind," may influ- 

 ence cell-E in two ways : (i) directly, (2) indirectly. (1) That 

 mental states are capable of directly influencing bodily states 

 there can be no doubt. This subject has been fully dealt with 

 by Dr. Tuke in his classical work, and consequently needs no 

 elucidation here. Although the direct influence of mind over 

 body deserves to be considered under a separate head, yet, 

 physiologically speaking, the process is in no way peculiar ; it 

 acts in the same manner as the non-mental nervous influences 



