364 
DR. PHILIP ON THE POWERS OF LIFE. 
Sleep published in the Philosophical Transactions for 1833., are duly considered, it 
will appear that a principal cause of difference in the laws of these systems depends 
on the difference of the laws of excitability in the organs of their leading powers. In 
those of the leading power of the sensitive system, all degrees of excitement are fol- 
lowed by a rapid proportional exhaustion of excitability ; so that the effect of the usual 
stimulants of life for a few hours, renders a state of inactivity essential to the main- 
tenance of their health : while the exhaustion of the excitability of the organs of the 
leading power in the vital system by those stimulants, is the operation of many times 
as many years, the one determining the recurrence of sleep, the other the natural 
duration of life. 
Thus it is that those, in whom, from habits of dissipation, extreme, labour, or other 
causes, the excitability of the vital system is to a certain degree exhausted, but who as 
they approach middle life cease to be exposed to such causes, and during that portion 
of life, that is from thirty to fifty or fifty-five, feel little inconvenience from the effects 
of their early habits, there still being in the vital system sufficient excitability for the 
usual functions of life ; after this period, when the defect of excitability begins to be 
felt sooner or later by all, feel the effects of its expenditure which had been so profuse 
in early life : many striking instances of which I have witnessed. Similar observa- 
tions apply to long-protracted illness, severe misfortunes, or any other cause which at 
any period of life in a great degree, and for a considerable length of time, tend to 
exhaust the excitability of the vital system, although for a certain time the individual 
may enjoy his usual health after such causes have ceased to operate *. 
The organs of the leading power in the vital system, as appears from the facts stated 
in my paper on the Nature of Death, possess at birth a high degree of excitability, a 
degree beyond that proportion which constitutes the firmest state of health — the cause, 
as there pointed out, of many of the most fatal diseases of infancy — which is by the 
operation of the usual stimulants of life gradually reduced till it bears a due propor- 
tion to those stimulants, by which the powers of the constitution are confirmed. At 
length from their continued operation the fault is a defect not a redundancy of ex- 
citability, to which every day necessarily adds, till they can no longer excite the 
organs on which that power depends ; for in every instance the immmediate cause of 
absolute death, which is very different from what we call death, is the failure of that 
power -j~. And here as there are no means in the constitution, as in the case of the 
organs of the leading power in the sensitive system, of restoring the excitability of 
its organs, they at length finally cease to be excited. Thus it is that in almost all 
cases of great longevity we find that there has been little exposure during life to 
powerful causes of exhaustion of either body or mind, for we have seen that the ner- 
vous is immediately under the influence of the sensorial power ; and that such in- 
stances are most frequent in the colder of the temperate climates, heat, on the one 
* See what is said of the excitability of the two systems in my papers on the Nature of Sleep and Death. 
f My paper on the Nature of Death. Philosophical Transactions for 1834. 
