72 PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 



produced by the movements excited, sensations thereby felt, and mental deter- 

 minations consequent on these, by which the successive volitions are guided. But 

 it is admitted that, in all sciences, " Reason can sometimes go farther than Ima- 

 gination can venture to follow ;" and in no department of science can we more 

 reasonably expect to meet with such examples than in tracing the actions of that 

 exquisite mechanism, by which the sensations and powers of living animals are 

 placed in connection with the world which is given them to inhabit. 



But we may go a step farther, and understand more distinctly the mode in 

 which sensations continually regulate and guide muscular actions, if we reflect 

 on the phenomena to which MULLER has very properly directed the attention of 

 physiologists under the name of Consentient motions, and of which the study of the 

 eye furnishes us with some of the most instructive examples. 



I need hardly say that this term is applied in cases where different nerves, 

 and thereby muscles, are excited to action simultaneously, and where it is difficult 

 or impossible to separate the combination. Such cases occur very frequently, 

 both as to the strictly voluntary and the sympathetic or reflex movements, but 

 especially as to the latter ; and the following are the facts most important to be 

 observed in regard to them. 



1. The strictly voluntary motions thus simultaneously performed, are chiefly 

 where the action that is willed requires considerable exertion, and is performed 

 with difficulty. " Thus when we wish to contract the muscles of the external 

 ear, we induce contraction of the occipito-frontalis muscle at the same time, with- 

 out wishing it. During the most violent muscular action, many muscles act by 

 association, although their action serves no apparent purpose. Thus a man mak- 

 ing much exertion moves the muscles of his face, as if they aided him in lifting a 

 load," &c. 



2. In regard to most of the cerebral motor nerves, and nerves moving the 

 trunk of the body, particularly when these act in obedience to sensation or emo- 

 tion, the most important fact regarding their consentient action is, that this 

 tendency is observed especially in the opposite nerves of the same pairs. Thus 

 in the latter description of movements performed by the irides of the eyes, by the 

 muscles of the face, by the pharynx, diaphragm, intercostal muscles, abdominal, 

 lumbar, and perineal muscles, in the actions of winking from bright light, of 

 deglutition, breathing, coughing, sneezing, vomiting, laughing, sighing, weeping, 

 straining for evacuation of any of the viscera of the abdomen or pelvis, it is 

 certain, and is essential to the due performance of each action, that the corre- 

 sponding portions of the nerves of the same pair, on each side of the body, should 

 be affected, and should act on the muscles, exactly alike ; and this is observed, 

 even when the sensation exciting the movement is felt only through one nerve, 

 and on one side of the body ; as in the contraction of both pupils from bright 

 light acting on one eye, or in the simultaneous and successive contractions of all 



