PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. gjj 



selected and regulated contractions of voluntary muscles are excited by the reflex 

 function of the cord, as, e. g. in the contraction of the orbicularis oculi and of this 

 muscle only, effected through the 7th nerve, on the same sensation being felt. 



As the 3d nerve appears to have roots in the posterior as well as anterior 

 portion of the crus cerebri, it is certainly quite possible that those of its fila- 

 ments which enter the lenticular ganglion are set on sensitive, not on motor por- 

 tions of the cerebro-spinal axis ; but if so, the observations already made shew 

 that they are capable of being excited by an influence acting downwards from the 

 strictly motor portions. 



The indirect and probably modified influence, resulting from volition, and 

 transmitted through the ganglia to the involuntary muscles, and of which we have 

 this unequivocal example in the eye, is in itself in all probability an important part 

 of the design of Nature in the construction of the sympathetic nerve and its gan- 

 glia. I perfectly agree with MULLER, that it is in this way only, that the effect 

 of muscular exercise on the action of the heart, and much of the beneficial 

 strengthening effect of exercise, is to be explained ; and this indirect influence of 

 voluntary muscular exertion on the heart is obviously important, as keeping its 

 actions in unison with any occasionally required increase of voluntary muscular 

 exertion ; and so enabling us to keep up exertions which must otherwise have 

 failed. And a slighter degree of the same indirect influence of exercise is seen in 

 the movements of the stomach and intestines, which become to a certain degree 

 torpid from inactivity of the voluntary muscles. For this slighter agency of vo- 

 luntary exertion on the moving organs supplied by the splanchnic nerves, there is 

 probably provision made, in these nerves passing through a greater number of 

 ganglia, before they reach the moving fibres, than the nerves of the heart, and 

 therefore having the indirect influence of the voluntary efforts transmitted through 

 them in a less degree of intensity. 



But it is very important, in reference to the use of the ganglionic nerves, to 

 observe, that the movement of the iris is capable of being affected, not only 

 through the 3d nerve, but likewise through the 5th nerve and the sympathetic, 

 i. e. by all the filaments which form part of the composition of the ciliary gan- 

 glion. I shall not enter on the observations which have been made on the differ- 

 ences observed in different muscles in this respect ; nor on the speculations of 

 some German physiologists as to the mode of action, particularly of the sympa- 

 thetic, on the iris ; but only observe that the effect chiefly observed from the section 

 of both these nerves on the iris, is a gradual and permanent contraction of the 

 pupil. The influence of both these nerves on the iris is therefore strictly analo- 

 gous to the kind of influence observed in experiments on animals, from injury of 

 different parts of the nervous system, or the sympathetic nerve, on other involun- 

 tary muscles, consisting, as MULLER states, " either in enduring contractions, or in 

 a long-continued modification of the ordinary rhythmic action of the organ ;" a 



VOL. XV. PART I. Z 



