cures the delicate suspension of the eye-ball 
when the muscles are at rest, and a steady 
movement of it when any of them are thrown 
into action. We have the following experi- 
mental evidence to offer in support of the state- 
ment which Hunter has made. A dog was 
_ killed by the injection of air into a vein, and 
_ immediately the inferior oblique muscle was 
_ exposed by dissecting off the conjunctiva, with- 
Out in any way interfering with the surrounding 
parts; by means of two fine wires a slight 
electric current was then directed through the 
muscle. The effect was a rapid rotation of the 
eye upon its antero-posterior axis, so that a 
piece of paper placed at the outer margin of 
the cornea passed downwards and then inwards 
towards the nose. The superior oblique was 
then exposed at the back of the orbit, and was 
treated in the same manner. The rotatory 
movement produced was precisely the reverse 
of the former; the paper at the outer margin 
of the cornea passed upwards, and then in- 
wards towards the nose. In the case of the 
superior oblique the movement was less exten- 
_Sive, the irritability of the muscle being less, 
perhaps from the delay in exposing it, and from 
some slight injury inflicted on it in so doing. 
There could be no doubt as to the direction of 
the movement in both cases; there was not the 
slightest appearance of elevation, depression, 
abduction, or adduction of the cornea. This 
| experiment was witnessed by Dr. Todd and 
Mr. Bowman. The experiment was subse- 
_ quently repeated on another dog with precisely 
the same result. The superior oblique in the 
_ Second experiment did not contract so vigo- 
rously as the inferior, but the movement it pro- 
duced was the same as in the first experiment ; 
and when gentle traction was made in the pos- 
terior part of the muscle, the rotation of the eye 
_ was very decided, and in a direction the reverse 
of that in which it rotated under the influence 
_ of the inferior oblique; again there was not the 
_ slightest movement of circumduction. There 
can be no doubt that the function of these 
_ muscles is the same in all animals in which 
they exist; and any experiments to determine 
‘their use must be more satisfactory when per- 
formed on animals immediately after death 
‘than in the human subject at a considerable 
period after death, when the fat and muscles 
have become equally firm and _ unyielding. 
Under such circumstances it is evident that the 
results of traction upon the muscles cannot be 
relied upon as accurate. It is remarkable that 
this rotation of the eye should have excited so 
little attention; since, if we only recognise the 
_€xistence of such a movement, the use and 
“hecessity of the oblique muscles must be ac- 
d nowledged, it being evident, as we have pre- 
iously stated, that the straight muscles are 
‘Incapable of effecting it. 
__ Consensual movements of the two eyes.— 
‘Upon this subject we subjoin the following 
extract from Miiller: *#—‘ There is an innate 
tendency and irresistible impulse in the cor- 
‘tesponding branches of the third nerve to asso- 
A 
: 
J 
t 
* Physiology, by Dr. Baly, p. 99. } 
ORBIT. 
791 
ciate action; while\in the sixth nerve, not only 
is this tendency absent, but the strong action 
of one of these nerves is incompatible with 
the action of the other. These innate ten- 
dencies in the third and sixth nerves are ex- 
tremely important for the function of vision, °- 
for 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; one eye, for example, could not have 
been directed inwards while the other was di- 
rected outwards, so as to preserve the paral- 
lelism or convergence of their axes, but they 
would necessarily have diverged when one 
rectus externus had been made to act volunta- 
rily. To render possible the motion of one 
eye inwards while the other is directed out- 
wards, the external straight muscles have re- 
ceived nerves which have no tendency to con~ 
sensual action. In consequence, however, of 
the tendency in the two internal straight mus- 
cles to associate action, it is necessary, where 
one eye is directed inwards and the other out- 
wards, that the contraction of the rectus ex- 
ternus of the latter should be so strong as to 
overcome the associate action of the rectus inter- 
nus of the same eye; and in the effort to direct 
one eye completely outwards we actually feel 
this stronger contraction of the external rectus.” 
It is certainly contrary to our general notions 
of the skill and economy of nature to sup- 
pose that she would so clumsily construct and 
endow the muscles and nerves of the eye, that 
in order to direct one eye outwards the external 
rectus muscle must struggle with and overcome 
the internal rectus of the same eye in conse- 
quence of this “ irresistible impulse in the cor- 
responding branches of the third nerve to 
associate action.” Doubtless the generality of 
those who have no theory to support will acknow- 
ledge that in directing the eye outwards they are 
unconscious of any such struggle between 
opposing muscles as is here supposed, and that 
abduction'of the eye is attended with as little 
effort as either its elevation or depression. 
There is then no such irresistible tendency to 
associate action between the branches of the 
third nerve supplied to the internal recti muscles. 
Both internal recti muscles may be made to act 
at the same time, and thus to produce a con- 
vergence of the optic axes; and this being an 
unnatural position of the eyes is attended with 
a considerable and a painful effort, each inter- 
nal rectus having to overcome the external 
rectus of the same eye, which has a tendency to 
consentaneous action with the external rectus of 
the other eye. That the external rectus must 
have the advantage in any struggle between it 
and the internal rectus is evident from the 
greater thickness and consequent strength of the 
former muscle. The only muscles supplied by 
the third nerve in which this tendency to con- 
sentanecus action is irresistible, are the superior 
and inferior recti of both eyes; we cannot pos- 
sibly raise one eye without at the same time 
raising the other, nor can we depress one eye 
without a corresponding movement of the other. 
- Then, as we have seen, there is no tendency to 
