306 



the extent to which the contractions shall be carried ; while ex- 

 perience in some cases, and instinct in others, teaches what mo- 

 tor nerves must act, in order to excite the commencement of these 

 sensations. 



2. The consentienee of different nerves, and thereby different 

 muscles, in the performance of complex actions, — for example, 

 the exact consentienee of the nerves of the same pair on each side 

 of the body in the acts of deglutition, respiration, coughing, sneez- 

 ing, vomiting, straining, &c., is partly to be ascribed to the 

 cause assigned by Miiller, — the " conducting power of the me- 

 dullary substance at the roots of the nerves, whereby those which 

 lie contiguous to each other are apt to be affected simultaneously ;" 

 and this is probably the reason why the action of rolling the one 

 eye outwards, being always intended to be performed simultane- 

 ously with the rolling inwards of the other, is excited by distinct 

 nerves (viz., the 4th and 6th) from those which excite this latter 

 movement; whereas the motion upwards and downwards, being 

 intended to be performed simultaneously by the two eyes, is ex- 

 cited through one pair of nerves only, viz. the 3d. But it appears, 

 from attending to the actions of the 4th and 6th nerves, — those 

 which roll the eye outwards, — that this proximity of origin can- 

 not be the main cause of consentienee of action in motor nerves, 

 the opposite nerves of these pairs never acting together in the na- 

 tural state, but each always acting with a portion of the 3d nerve 

 on the opposite side. And it thus appears, that the truly efficient 

 reason of the consentienee of motor nerves in all cases, and the 

 only one which can be assigned in this case, is the identity of the 

 resulting and guiding sensation. 



3. The remarkable combination of movement observed in the 

 actions of the 4th and 6th nerves on one side of the body and of 

 a part of the 3d on the other, is conclusive proof that neither con- 

 nection of nerves at their root, nor union in their course, is con- 

 cerned in producing such combinations, and that the plexuses of 

 nerves cannot be instrumental in producing any of the combina- 

 tions observed among the muscles of the limbs. 



But the use of the plexuses as to muscular motion becomes suf- 

 ficiently obvious when we observe, that by this contrivance the 

 sensitive nerves of every muscle in the extremities are placed in 

 connection with a large surface of the spinal cord, and, therefore, 

 probably the guiding sensations resulting from their action are evi- 

 dently more distinct; and again, that the motor nerves of every 

 muscle are connected with a larger surface of the spinal cord, and 



