PHYSIOLOGY. 93 



respiration, accordingly, continues as long as that of the heart. It 

 is true that it is not so frequent as that of the heart ; it is but as 

 one to three or four; but it is prodigiously frequent; and certainly 

 the same question occurs : Why does the ordinary action of res- 

 piration go on greatly longer than any other muscles of the body 

 continue to perform their functions ? It is because the respira- 

 tion, as much as the heart, is without the concurrence of occasional 

 animal power ; which appears from hence : Let the respiration 

 be with unusual force for some time, as speaking above an 

 hour together, or blowing upon a wind instrument, or being 

 exercised in actions which require stronger inspirations, and 

 we will feel the effects of lassitude and debility ; only habit and 

 custom vary these. The lassitude or debility accordingly, may 

 in some measure, depend upon the state of the muscular fibre 

 itself, but it depends more upon the animal power ; and this has 

 this singular constitution, that it necessarily requires the alter- 

 nate state of rest; and I shall use the circumstance of sleep and 

 watching, which are affections of the animal power, as an argu- 

 ment of its being peculiar to that power. 



" As it was proper to consider the action of muscular fibres 

 in the two views, of the Vigour and the Mobility of contrac- 

 tion in the state of health, it is necessary to consider the morbid 

 state of the muscular fibre also, as it respects the vigour or the 

 mobility ; and it will preserve the clearness of our ideas, if we 

 fix particular terms to the excess and defect of each. The ex- 

 cess of vigour, therefore, I name THE INCREASED TONE ; and 

 the defect of vigour I call, with every body else indeed, DEBILI- 

 TY ; on the other hand, the excess of mobility I name IRRI- 

 TABILITY, and the defect TORPOR. 



" I begin with the state of vigour ; and I may first inquire if 

 this can be morbid in excess. The occasion of this will appear 

 from a passage of Dr. Whytt, who says, (Works, p. 527,) 

 ' A greater degree of that power in the nerves which is neces- 

 sary for motion, can only give more force and steadiness to the 

 muscles when they all possess it in an equal degree : the in- 

 crease, therefore, of this power, is hardly to be accounted a dis- 

 temper ; it is never exerted, except in consequence of an effort 

 of the will, of some affection of the mind, or of the action of 



