PHYSIOLOGY. 



functions, at least those of sense and voluntary motion, are all at 

 rest in sleep, though there are considerable exceptions ; the 

 vital functions go on, yet even they have less activity in this 

 than in the other state. But it is to my purpose to notice that 

 it is chiefly confined to those animal functions which depend 

 upon the brain, and that it depends upon something fundamen- 

 tal in the constitution of the brain. The nature of this consti- 

 tution is a problem which we would earnestly wish to have 

 solved ; but we have little confidence in doing it. However, 

 according to a maxim of which I am very fond, initmrn sapi- 

 entice est stultitia caruisse, our first step is to get out of the 

 error with which mankind have been imbued. My first busi- 

 ness, therefore, is to consider the solutions of this problem offer- 

 ed by others." 



CXXV. The most common opinion is, that the brain is a 

 secretory organ, which secretes a fluid necessary to the func- 

 tions of the nervous system ; that this fluid is alternately ex- 

 hausted and recruited, and thereby gives occasion to the alter- 

 nate states of sleep and waking. But this supposition is attend- 

 ed with many difficulties. 1. It is probable, that the nervous 

 fluid existed in the animal embryo, before the action of the 

 heart, or any secretory function, could take place. 2. In ani- 

 mals, which, during the winter, suffer a temporary death, such 

 as bats, when, by heat, they are again restored to life, the vital 

 power of the solids is restored before the fluidity of the blood. 

 " Van Swieten tells us, that in the wing of a bat, expanded 

 so that it can be subjected to the microscope, a curious net- 

 work of red lines is seen, which, in the torpid state of the ani- 

 mal, appear perfectly at rest. When the animal, brought into 

 a chamber of a certain warmth, by degrees begins to shew the 

 powers of motion and of life, it can be perceived that a motion 

 arises in these vessels ; the red lines appear broken into diffe- 

 rent parts, having an oscillation which is confined to very small 

 bounds. By degrees the number of such moving pieces in- 

 creases, and the size of each constantly diminishes till at last 

 one entire fluid appears, moving with freedom before the eyes. 

 From this experiment it manifestly appears that an oscillatory 

 motion produced in the vessels restores the blood to its fluidity, 

 and sets the animal in motion. When the heat is applied, 





