790 



ORBIT. 



on certain occasions, to cause a relaxation of the 

 muscle to which it goes, the eye-ball must be 

 then rolled upwards."* 



Sir C. Bell adduces no proof that the involun- 

 tary movements which he mentions are performed 

 by the oblique muscles; on the contrary they 

 may all be effected by the straight muscles. The 

 fact that these movements are involuntary is 

 no argument against their being produced by 

 muscles, which under ordinary circumstances 

 are strictly voluntary. Thus, Sir C. Bell says, 

 when the eye is exposed and irritated, the 

 cornea is directed upwards to a greater extent 

 than can be done by a voluntary effort. This 

 probably is the case, but we need not have 

 recourse to the oblique muscles in order to 

 explain it. Under the influence of the irri- 

 tation applied to the eye the superior rectus 

 contracts violently in order to elevate the cornea 

 beneath the upper lid, and thus to remove it 

 from danger; precisely in the same manner 

 under the irritating influence of strumous 

 ophthalmia the orbicularis muscle contracts 

 with a spasmodic force much exceeding that 

 of any voluntary contraction of that muscle. 

 Both oblique muscles have the striated struc- 

 ture of voluntary muscles, and the inferior 

 oblique receives a branch from the third nerve, 

 all the other muscles supplied by which are 

 known to be voluntary in their action. We 

 may, therefore, dismiss the idea that the ob- 

 lique muscles are specially concerned in pro- 

 ducing the involuntary movements of the eye. 



Further, it is our firm conviction that the 

 oblique muscles are in no way concerned in 

 circumduction of the eye-ball; that they nei- 

 ther abduct nor adduct, neither raise nor depress 

 the cornea, nor do they produce any of the in- 

 termediate movements. The following are the 

 circumstances which appear to us to favour 

 this conviction. 1st. Both oblique muscles 

 pass outwards almost at right angles with the 

 recti muscles, and are inserted close upon a 

 line intermediate hetween the anterior and pos- 

 terior half of the eye-ball : this direction and 

 insertion are evidently most unfavourable for 

 the production of any of the above-mentioned 

 movements. 2d. Those who assert that the 

 oblique muscles have the power of circum- 

 ducting the eye make the most contradictory 

 statements as to the direction which the eye 

 assumes under their influence. These opposite 

 statements are sufficiently accounted for when 

 we consider that they are founded on the re- 

 sults of traction on the oblique muscles after 

 death, when the fat and other parts in the orbit 

 have become firm and unyielding, and the 

 steadying influence arising from the antagonism 

 of the other muscles has ceased. 3d. The 

 recti muscles are of themselves capable of cir- 

 cumducting the cornea in all directions ; this 

 is evident from their direction and insertion, 

 and was proved by Sir C- Bell's experiments 



* See Sir C. Bell on the Nervous System, p. 177. 

 This supposition appears to us as unphilosophical 

 as the old theory of Phlogiston, which, in order to 

 explain the fact of a body becoming heavier when 

 deprived of this imaginary agent, attributed to 

 phlogiston a property of lightness. 



above-mentioned. 4th. There is an important 

 movement of the eye-ball which can be effected 

 by no other than the oblique muscles, and for 

 the production of which in all probability these 

 muscles are provided : the movement to which 

 we refer is rotation of the eye upon its antero- 

 posterior axis. 



The true use of these muscles we believe 

 to have been pointed out by John Hunter 

 in a paper on the use of the oblique muscles, 

 in his " Observations on certain Parts of the 

 Animal Economy," He first explains that for 

 perfect vision it is essential that when we are 

 examining an object, any motion of the object 

 or of our own bodies should be so counter- 

 acted by the movements of the eye-ball that 

 the image of the object may be kept on the 

 same point of the retina, and not be allowed 

 to move over its surface. We have a familiar 

 illustration of this when we keep our eyes 

 motionless and fixed on the ground, while 

 moving rapidly in a carriage ; the surface of 

 the road appears confused and the stones ar- 

 ranged in lines, as their images pass rapidly 

 over the retina : it is only when we allow the 

 eye to follow these objects that we have a dis- 

 tinct perception of any of them. Hunter then 

 goes on to explain the use of the oblique 

 muscles. " To prevent any progressive motion 

 of the object over the retina of the eye, either 

 from the motion of the object itself, or of the 

 head in some motions of that part, the straight 

 muscles are provided, as has been explained ; 

 but the effect? which would arise from some 

 other motion of the head, as from shoulder to 

 shoulder,* cannot be corrected by the action 

 of the straight muscles, therefore the oblique 

 muscles are provided. Thus, when we look 

 at an object and at the same time move our 

 head to either shoulder, it is moving in the 

 arch of a circle whose centre is the neck, and, 

 of course, the eyes would have the same quan- 

 tity of motion on this axis if the oblique mus- 

 cles did not fix them upon the object. When 

 the head is moved towards the right shoulder 

 the superior oblique muscle of the right side 

 acts, and keeps the right eye fixed on the 

 object, and a similar effect is produced upon 

 the left eye by the action of its inferior oblique 

 muscle. When the head moves in a contrary 

 direction, the other oblique muscles produce 

 the same effect." 



If we again consider the direction and inser- 

 tion of the oblique muscles, it is evident that 

 they are intended for the office which Hunter 

 has assigned them. Passing outwards, one 

 above, the other below, in a direction almost at 

 right angles with the antero-posterior axis of 

 the eye-ball to their insertion near its middle, 

 their action must obviously be to rotate the 

 eye upon that axis. The reason of their obli- 

 quity probably is that their direction backwards 

 as well as outwards, by enabling them to anta- 

 gonise the straight muscles, more certainly se- 



* The motion here meant is that which is ef- 

 fected by flexion of the neck laterally, so as to ap- 

 proximate the ear to the shoulder, not that move- 

 ment which lakes place between the first and se- 

 cond cervical vertebrae. 



