160 PHILOSOPHY OF ZOOLOGY. 



those nerves which, in ordinary cases, are not under its 

 controul. Substances irritating the muscles to which these 

 nerves belong, excite them to action, but the nerves them- 

 selves resist the stimulus of galvanism or electricity. 



Though they do not communicate these sensations to the 

 sensorium, they appear to be, in some degree, excited to ac- 

 tion by the contents of the vessels to which they are distri- 

 buted. Thus, the blood excites the nerves of the heart, 

 and the food those of the stomach and intestines. These 

 nerves, in their turn, act upon the muscles of these organs, 

 and enable them to execute the requisite movements. 



The nerves of involuntary motion, chiefly take their rise 

 from the ganglia. Hence we perceive some of the proper- 

 ties of these medullary knots, in the changes which they 

 have produced on the nervous filaments in their passage 

 through them. These nerves become incapable, in ordi- 

 nary cases, of conveying sensations to the brain, or of exe- 

 cuting the purposes of volition, and they have become less 

 capable of being excited to action by the electric fluid. But 

 the most remarkable change consists in their becoming capa- 

 ble of continued action through life, withoutbeing exhausted, 

 or exhibiting any symptoms of fatigue. These circumstan- 

 ces favour the supposition, that the ganglia are destined to 

 remove the nerves, to which they give rise, beyond the di- 

 rect controul of the will, to bestow upon them new ener- 

 gies, and to form individual systems, capable of exercising, 

 to a limited extent, independent powers *. 



But although the nerves appear thus to occupy different 

 situations, and to perform different functions, they are con- 

 nected, and form one system, in which the different parts 



* See " Essays on the use of the Ganglions of the Nerves," by JAMES 

 JOHKSTONE, M. D. Phil. Trans. 1764, p. 177, and 1767, p. 118, and 

 1770, p. 30. 



