PHYSIOLOGY. 117 



habits are once established, any deviation from them is readily 

 hurtful to the system-^-^-They are often rigidly fixed, have a 

 considerable influence on the action of the brain, and govern 

 the revolutions of the animal system. Thus, any causes pro- 

 ducing a deviation from the usual degree of force and velocity 

 (No. 4.) are apt to destroy the measure of it altogether. " I 

 take as an example of this, the heart. In a man in health, and 

 at rest, the motion of the heart is performed with wonderful re- 

 gularity in point of size, strength, and quickness. Muscular 

 motion exercised for any time, accelerates the motion of the 

 blood towards the heart ; and as the blood is propelled with 

 greater velocity, it will accelerate the motion of the heart, and 

 increase the frequency, fullness, and force of its contractions to 

 a certain extent. Every person is accustomed to bear a certain 

 degree of muscular exercise without much disturbance in the 

 regularity of the heart's motion. But if a man runs fast, not 

 being accustomed to do it, he will occasion a considerable irre- 

 gularity in the contractions of the heart, which become convul- 

 sive, spasmodic, and are sometimes interrupted altogether." 

 And, in like manner, causes producing a deviation from the 

 usual order and velocity in the succession of motions (No. 5.) 

 are apt to destroy the power of the mind in following that or- 

 der, or in giving any measure to the several motions which 

 should be performed ; and perhaps, in this way, the effects of 

 debility, of several passions, and of surprise, are, in some mea- 

 sure, explained. 



CXXIV. The brain seems, by its constitution, to be dis- 

 posed to the alternate states of rest and activity, as appears in 

 the alternate states of sleep and waking, which constantly take 

 place in every animal ; but wherein this constitution consists it 

 is difficult to discover. " In every animal, is perhaps too 

 generally said : but in the first three classes, the Mammalia^ 

 Aves, and Amphibia, we are certain that sleep is universal. 

 Every animal of these classes naturally has certain vicissi- 

 tudes of sleep and waking, and, when in the state of health, 

 once in twenty-four hours. Occasional deviations from this do 

 not prevent our supposing a law of nature ; the induction is very 

 general. I need not add that these are states of rest and ac- 

 tivity with regard to most parts of the system. The animal 



