PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 78 



excited on both sides of the body ; and hence we instinctively carry the move* 

 ment to the same extent in both. 



It was a speculation of DARWIN, that the actions of inspiration and expira- 

 tion are originally determined by the uneasy sensation of anxiety in the chest 

 of the new-born child leading to irregular and convulsive movements, out of which 

 those are quickly selected, which are found by rapid experience to be effectual in 

 appeasing that uneasy feeling ; and although I do not agree to this statement, as 

 expressing the order of events at that early period of life, and can assign no cause 

 but Instinct for the original selection of the proper nerves and muscles for this 

 purpose, yet I believe that, at all periods of life, it is the sensation felt to result 

 from the action of inspiration already in progress, which determines the energy 

 Avith which it shall be performed, the extent to which it shall go, and even the 

 number of muscles that shall be excited to partake in it. 



And that this is the true account of the matter, we have farther and satisfac- 

 tory proof in the fact, that in various cases of disease, particularly in cases of Em- 

 pyema, the contractions of the muscles of inspiration on one side of the chest be^ 

 come ineffectual for inflating the lungs, and for appeasing the sense of anxiety in 

 the breast ; in which case their nerves are no longer excited, and those muscles 

 cease to act ; they remain flaccid, and even, according to the observation of Dr 

 STOKES, they gradually become paralytic from inaction ; a phenomenon, as I con- 

 ceive, almost exactly similar to the loss of power in some of the muscles of the 

 eyeball in cases of amaurosis affecting one eye. 



II. Again, another important application of the information acquired by 

 study of the nerves of the eyeball, is to explain the use of the Plexuses or ana- 

 logous contrivances, through which all the nerves, sensitive and motor, pass both 

 to the upper and lower extremities, very generally in the animal kingdom. 



In regard to the use of this very remarkable piece of structure, found in those 

 nerves, by which the most forcible and the most nicely regulated muscular move- 

 ments are effected, there have been various opinions. Several authors, among 

 others Sir CHARLES BELL, have supposed it to be intended to facilitate the com- 

 binations of different muscles for particular actions, proceeding on the plausible 

 supposition that, when the will acts simultaneously on several muscles, its influ- 

 ence proceeds from a single point, and is diffused from thence to those different 

 muscles. 



" The principal cause of the irregularity and seeming intricacy in the dis- 

 tribution of nerves, is the necessity of arranging and combining a great many 

 muscles in the different offices. Wherever we trace nerves of motion, we find 

 that before entering the muscles they interchange branches, and form an intricate 

 leash of nerves, or what is called a plexus. This plexus is intricate in propor- 

 tion to the number of muscles to be moved, and the variety of oombinations into 



VOL. XV. PART L X 



