VARYING ACTIVITY OF THE NUTRITIVE PROCESSES. 345 



2. Varying Activity of the Nutritive Processes. 



616. The nutritive operations go on with very great variations in 

 their relative activity, under different circumstances. As a general 

 rule it may be stated that, the greater the demand for the functional 



, activity of the organ or tissue, the more energetic is its nutrition ; and 

 vice versd. Now this is readily understood, when it is considered, that 

 the active state of many structures essentially consists in an act of 

 nutrition ; thus the energy of the secreting processes is really de- 

 pendent, as we have seen, upon the growth of the secreting cells, which 

 make up the essential part of the gland, and the energy of the absorb- 

 ing and assimilating processes is dependent upon the development of 

 the cells, which select and elaborate the nutrient matter. This growth 

 is regulated mainly by the supply of blood ; being increased by the 

 afflux of blood towards the part, in consequence of the influence of the 

 nerves upon the vessels, or through any other change in the current of 

 the circulation. Thus the secretions are increased in amount, by emo- 

 tions of the mind, that act (probably through the sympathetic nerve) in 

 regulating the calibre of the arteries supplying their respective glands*: 

 or the interruption of the function of one gland shall occasion an 

 increased . nutrition, and consequently an augmented secretion, in its 

 fellow, as when one of the kidneys is hypertrophied, through a dis- 

 ease in the other, that renders it incapable of performing its office. 

 Still it would appear, that there may be variations in the activity of 

 these organs, resulting from causes inherent in themselves (of the 

 nature of which we know little or nothing) ; and that here,, as else- 

 where, active nutritive operations will promote the circulation of blood 

 through the parts, whilst a languid state of the function will retard it. 



617. In certain other tissues, however, the functional activity would 

 seem to consist rather in a waste Or decay of structures previously 

 developed ; this is the case especially in Nerve and Muscle, which are 

 found to undergo disintegration, in exact proportion to the degree in 

 which they are exercised ; whilst the degree in which this waste is 

 repaired, depends upon the supply of nutritive material, the quiescent 

 state of the part, and other circumstances. But even here we find 

 that functional activity occasions increased nutrition; in the same 

 manner as burning a lamp with a high flame .increases the amount of 

 fluid drawn up by the wick. For neither the nerves nor the muscles 

 can act with energy, without a large supply of arterial blood ; and this 

 is drawn to them on the principles already mentioned ( 600) as in- 

 creasing the energy of the local circulation. The determination of 

 blood to the parts, thus established, favours their increased nutrition ; 

 and thus we find that, under favourable circumstances, any set of 

 Muscles, which is habitually exercised, undergoes a great increase of 

 development ; whilst, in like manner the Nervous centres, if too great a 

 demand be made upon their activity, are liable to become hypertrophied 

 (especially in young persons), and may thus become subject to disorders, 

 which temporarily or permanently destroy their powers. In these cases, 

 then, the functional activity determines the increased supply of blood, 

 and occasions the augmented growth ; and increased nutrition will 



