790 
on certain occasions, to cause a relaxation of the 
muscle to which it goes, the eye-bal! 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 dia t yen apron 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. f 
ORBIT. 
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. “ra ! 
The true use of these muscles we believe — 
to have been pointed out by John Hunter 
in a paper on the use of the — ~muscles, 
in his “ Observations on certain Parts of the — 
Animal Economy.” He first explains that fo 
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-ba ‘that 
the image of the object may be kept on the 
same point of the retina, and not be od 
to move over its surface. We have a fami 
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 obliqu 
muscles. “ To prevent any progressive motio 
of the object over the retina of the eye, eithe 
from the motion of the object itself, or of th 
head in some motions of that part, the straigh 
muscles are provided, as has been explai 
but the effects which would arise from 
other motion of the head, as from shoulder 
shoulder,* cannot be corrected by the acti 
of the straight muscles, therefore the oblic 
muscles ate provided. Thus, when we k 
at an object and at the same time move: 
head to either shoulder, it is moving in 
arch of acircle whose centre is the neck, 
of course, the eyes would have the same q 
tity of motion on this axis if the oblique 1 
cles did not fix them upon the object. W 
the head is moved towards the right shoul 
the superior oblique muscle of the right 
acts, and keeps the right eye fixed on 
object, and a similar effect is produced 
the left eye by the action of its inferior a 
muscle. When the head moves in a com 
direction, the other oblique muscles fp 
the same effect.” a 
If we again consider the direction and i 
tion of the oblique muscles, it is evider 
they are intended for the office which E 
has assigned them. Passing ou s 
above, the other below, in a direction aln 
right angles with the antero-posterior 
the eye-ball to their insertion near its 
their action must obviously be to ro 
eye upon that axis. The reason of th 
quity probably is that their direction bat 
as well as outwards, by enabling ther 
gonise the straight muscles, more cer 
* 
ca 
no 
* The motion here meant is that whic 
fected by flexion of the neck laterally, so @ 
proximate the ear to the shoulder, not 
ment which takes place between the 
cond cervical vertebra. 
