792 



ORGANIC ANALYSIS. 



consentaneous action between the branches of 

 the third nerve supplied to the internal recti 

 muscles, and it is only by a considerable effort 

 that they can be made to act together ; whilst 

 the branches of the same nerve supplied to the 

 levatores palpebrarum, which for the most part 

 act consentaneously, may by a very slight effort 

 of the will be made to act separately ; but few 

 persons experience any difficulty in opening 

 one eye while the other is closed. Again, the 

 tendency to consentaneous action between the 

 internal rectus supplied by the third nerve and 

 the external rectus supplied by the sixth nerve, 

 as well as between the inferior oblique supplied 

 by the third nerve and the superior oblique by the 

 fourth, is as irresistible as that between the supe- 

 rior and inferior recti of the two eyes. We see 

 then that all the muscles supplied by corres- 

 ponding branches of the third nerve have not 

 this tendency to consensual action, and two 

 muscles supplied by the third nerve act con- 

 sentaneously with two other muscles supplied 

 by the fourth and the sixth nerves respectively. 



For the maintenance of the parallelism 

 between the axes of the two eyes, it is evidently 

 necessary there should be a consentaneous 

 action of the superior recti as well as of the in* 

 ferior recti ; it is also necessary that the internal 

 rectus and the inferior oblique of one eye should 

 act with the external rectus and the superior 

 oblique of the other; but it is by no means 

 evident why it is necessary, in order to effect 

 this, that the external rectus and the superior 

 oblique should each have a nerve specially 

 provided for them. We must not suppose we 

 are explaining the necessity for this arrange- 

 ment by asserting that " if in place of the sixth 

 nerves, the external recti muscles had received 

 each a branch of the third nerve, it would have 

 been impossible to make one of these muscles 

 act without the other," because, as we have 

 seen, there is no such irresistible innate ten- 

 dency in all the corresponding branches of the 

 third nerve to consentaneous action. Assuming 

 that the use of the oblique muscles is such as 

 we have mentioned, it is certainly curious to 

 observe that when corresponding muscles of the 

 two eyes are intended to act together, as the 

 superior rectus of one eye with the superior 

 rectus of the other, and the same with the infe- 

 rior recti, both muscles are supplied by the 

 third nerve, but the external rectus which acts 

 consentaneously with the internal rectus of the 

 opposite eye has a separate nerve, the sixth, and 

 the superior oblique, which acts with the infe- 

 rior of the opposite eye, has the fourth nerve 

 entirely devoted to it. 



There is one other phenomenon to which we 

 may briefly allude, namely, the adaptation of the 

 (ye. to distances. This will be found fully dis- 

 cussed under the articleVisiON ; but it deserves 

 a passing notice in this place, since one hypo- 

 thesis by which an attempt has been made to 

 explain it is that a change is effected in the form 

 of the eye by the action of its external muscles, 

 some writers attributing the influence in ques- 

 tion to the recti muscles, and others to the 

 obliqui. It seems by no means improbable 

 that the action of both sets of muscles at the 



same time might have the effect of increasing 

 the antero-posterior diameter of the eye-ball, 

 and thereby of adapting it for vision at small 

 distances. The following experiment seems to 

 favour the notion that there is some muscular 

 action in the adaptation of the eye to vision at 

 very small distances. 



Place a printed page closer to the eyes than 

 the natural focal distance, and merely look 

 upon the letters without making any effort to 

 read them : they appear confused and indis- 

 tinct; then by a considerable voluntary effort 

 which cannot be long sustained, and which is 

 attended with some pain, we may so adjust the 

 eyes that the letters appear perfectly distinct 

 and legible. This subject is attended with 

 great difficulties, indeed it is scarcely possible 

 to determine the precise mode in which adap- 

 tation is effected ; the hypothesis we have now 

 mentioned appears at least as probable as any 

 other; it is, however, opeirto objections. The 

 state of adaptation of the eye is often entirely 

 changed in a very short time by the local action 

 of narcotics, which at the same time dilate the 

 pupil, and this change is effected without any 

 apparent influence over the voluntary contrac- 

 tions of the muscles. Most observers state that 

 the eye is rendered long-sighted (presbyopic), 

 while others have experienced an opposite 

 result. Muller has recorded the results of the 

 application of belladonna to his own eye; the 

 general effect was to render the eye presbyopic, 

 but the capability of adaptation was not de- 

 stroyed by it. When the solution of belladonna 

 was applied to one eye both eyes were affected, 

 but in different degrees, so that both eyes could 

 not be adapted to distinct vision of the same 

 object at the same moment. These effects were 

 attended with great dilatation of the pupil, and 

 inasmuch as they were probably in some way 

 dependent on this, they are opposed to the hypo- 

 thesis that adaptation is effected by any action 

 of the external muscles. 



(G. Johnson.) 



ORGANIC ANALYSIS. Under the term 

 organic analysis are included the various me- 

 thods of discovering the constituent parts of 

 substances, formed either directly by the vital 

 actions of organized beings, or indirectly by 

 subjecting the products of such actions to the 

 further operation of re-agents. To treat the 

 subject in its full extent would, however, be 

 foreign to the purpose of the present article, 

 in which I shall confine myself to the analysis 

 of the products of animal life, and particularly 

 of those combinations liable to be met with in 

 the human frame. 



In this analysis two distinct objects present 

 themselves : the first consists in the determin- 

 ation of the proximate principles which enter 

 into the constitution of the substance to be 

 analyzed, and the second in the discovery of 

 the elementary composition of the different 

 proximate principles. 



I shall therefore in the first place describe 

 the various means by which we recognize the 

 occurrence of the more important proximate 

 animal principles, and determine as fur as we 



