262 SPECIAL PHYSIOLOGY. 



the upper or proximal end be irritated, evidences of pain ensue ; where 

 as, if the distal portion of the divided anterior root be irritated, move- 

 ments occur in the muscles below, without manifestations of sensation; 

 but if the proximal portion be irritated, no movements in those muscles 

 take place. Sometimes, however, irritation of the distal cut portion of 

 the anterior roots produces slight evidences of pain, formerly spoken of 

 as the result of recurrent sensibility, and referred to the existence 

 of a few recurrent afferent fibres which pass from the anterior root 

 upwards along the posterior root to the cord. The pain which follows 

 irritation of the distal cut end of the anterior root, has also been at- 

 tributed to the excited cramps or movements being themselves the 

 cause of pain, by inducing irritation in the sensory fibres of the mus- 

 cles. Certain slight movements produced by irritation of the posterior 

 roots are due to reflex action. 



Additional evidence of the properties of the two roots, is furnished 

 by an experiment in which the anterior roots of the three spinal nerves 

 which supply the hind leg of a frog, are divided on the left side ; whilst 

 the posterior roots of the corresponding nerves, are divided on the 

 right side. On then pinching or cutting the left leg, or even cutting 

 it through, evidence of pain is given by the frog, in energetic motions 

 of every part of the body, excepting those of the limb itself; whereas, 

 if the right leg be pinched or cut, or even cut through, no evidence 

 of pain follows, and no decided motion, excepting the twitching of 

 the muscles that happen to be divided. From these and the preced- 

 ing experiments, it becomes evident that the posterior roots of the 

 spinal nerves contain the afferent fibres, and convey the effects of 

 sensory impressions inwards to the cord ; whilst the anterior roots con- 

 tain efferent fibres, and convey the effects of motorial stimuli to the 

 muscles. In the trunks and principal branches of the nerves, both 

 sets of fibres are usually intermixed. Hence, if a mixed nerve be lig- 

 atured at two points, irritation between the ligatures produces no 

 effect ; but if the lower ligature be relaxed, such irritation produces 

 movements in the muscles to which the branches of the nerve are dis- 

 tributed ; whilst, if the upper ligature be loosened, pain ensues on like 

 irritation. In the purely sensory branches of the mixed cranial or 

 spinal nerves, as, for example, in those distributed to the mucous 

 membrane of the tongue, and to the skin of the tips of the fingers, only 

 afferent fibres exist ; whilst in the proper muscular branches, the fibres 

 are chiefly efferent, though doubtless a few afferent fibres are inter- 

 mingled, for the purpose of conveying upwards to the nervous centres, 

 the effects of the special sensory impressions produced by the condition 

 of the muscle itself. It is through the spinal nerves that the contrac- 

 tion of the muscles of the trunk and limbs is excited; by them also, 

 those muscles are endowed with their special, though slight, sensibility, 

 are able to produce the muscular sense, the feelings of fatigue or 

 cramp, and to transmit impressions by which we recognize resistance 

 or weight. The spinal nerves likewise are the channels of sensation 

 for the skin and other soft parts of the trunk, limbs, and back-part .of 

 the head. 



The functions of particular nerves are determined, partly by their 



