Of the Action of the Recti Muscles.

307

have been occasionally referred to: we may now examine the contrivances provided for this purpose, and the nature of the external parts intended for the protection of the organ of vision.

The optic nerve, of which about an inch is interposed between the foramen opticum and the eyeball, contributes to hold the eye forward towards the front of the orbit. The intervals between the different parts contained in the orbit are filled with adipose substance.

Six muscles are inserted into the sclerotic coat of the eye, of which four are termed recti, and two obliqui.

The recti are thin flat muscles which rise from the margin of the foramen opticum, and extend, one over the upper part, one upon the outside, a third upon the inside of the eyeball, and a fourth below it, to be inserted each by a thin tendon into the sclerotic at about five lines from the edge of the cornea.

The four recti are distinguished individually by the names of, superior, inferior, internus, and externus, with which the terms attollens, deprimens, adducens and abducens, are used synonymously. By careful dissection a layer of membrane may be separated from the part of the sclerotic between the insertion of the recti and the cornea. This membrane is termed the tunica albuginea, and is considered to be the aponeurosis of the recti muscles.

It is easy to understand that these muscles acting singly would direct the eye to four equidistant points in a circle, and acting in concert might turn the axis of the eye towards all the intermediate points: and it is equally obvious that they must exert a constant effort to retract the eye, against which the elasticity of the optic nerve,

308

Of the Action of the Obliqui.

and the adipose substance in the orbit, would make very inadequate resistance.

The two remaining muscles appear intended to counteract the effect last alluded to.

The obliquus superior or trochlearis rises from the upper and inner part of the margin of the foramen opticum, and advances obliquely forwards and inwards towards the margin of the orbit, where a loop of membrane is attached, through which its tendon passes, and is subsequently reflected downward, backward and outward, to be inserted into the upper part of the eyeball behind its vertical axis.

The obliquus inferior oculi rises from the nasal process of the superior maxillary bone, and passes obliquely outwards and backwards below the eyeball to be inserted into the sclerotic within the rectus externus, and behind the transverse axis of the eye.

The action of the obliqui is involved in some obscurity: there can indeed be no doubt respecting their principal use; by drawing the eye forward they prevent that constant retraction which would otherwise be produced by the recti. But individually they are calculated to give each its specific direction to the eye: the obliquus superior points the optic axis downwards and outwards; the obliquus inferior, on the other hand, directs the eye upwards and outwards.

What renders this question still more intricate, is that three nerves are employed to supply the six muscles that have been described.. The fourth nerve supplies the obliquus superior, the sixth supplies the rectus externus, and the third supplies the remaining muscles. ́

It is remarkable again, that of the six muscles of the

Intricacy of the Nerves of the Orbit considered. 309

eyeball, three turn the optic axis directly or obliquely outward, and that each of these three muscles is supplied from a separate nerve; two indeed having one nerve exclusively distributed to each.

The intricacy of the muscular nerves of the eye admits, however, of a conjectural explanation. We may remark that their distribution is not such as to allow of our opposing the recti to the obliqui: in following this indication we are stopped by the fact that the third nerve supplies half or the greater part of each class. But from the close anatomical relation between the origins of the third nerve and of the fourth, we may conclude their function to be not materially different; whereas the sixth nerve rising from a remote point, seems distinguished essentially from both the others.

It appears to be a principle in the construction of the nervous system, that nerves of motion rise near the origin of those sentient nerves, through which the actions they controul are habitually excited.

This principle is remarkably exemplified in all the spinal nerves, and in the distribution of the fifth and seventh cerebral nerves; and the origin of the third and fourth nerves is sufficiently near that of the optic nerve to bring them under the same law. Now when we examine the connexions of the sixth nerve, we find it passing to the back part of the medulla oblongata, and rising near the fifth and the seventh; that is to say, it rises near those nerves which comprehend within their functions the sensibility of the surface of the eye, an influence over the secretion of the lachrymal gland, and the sense of hearing. When again we examine the distribution of the sixth nerve, we find it forming the sole supply of

a muscle which has a remarkable consent with the three offices alluded to. The rectus externus or abducens oculi, which it supplies, directs the axis of the eye outwards. And we may remark, 1. that when the optic axis is directed outwards, the surface of the eye is carried towards the orifices of the ducts of the lachrymal gland: 2. that the reversion of the eye for vision is commonly suggested by impressions upon the organ of hearing: and 3. as an instance of the consent between the common feeling of the eye and the action of the abductor, that when an ass is destroyed by pithing, if while imperfect life yet remains in the head the eyelids be rendered incapable of closing by the division of the portio dura, and the surface of the eye be then touched, the motion of the eye to avoid the offending substance is in a direction outwards.

When the eyelids are kept shut, the eyes are commonly in motion: during sleep, as Mr. Bell remarked, the eyes are frequently turned upwards; but sometimes they are directed straight forwards. Whenever I close either eye singly, it is immediately drawn outwards, and upon raising the eyelid, is seen in the act of returning to its place.

Squinting consists in a want of consent between the muscles of the two eyes, through which defect the optic axes are habitually directed towards different points. The inclination of one eye inwards may be so great as to exclude it from the vision of objects towards which the other is turned, or may be so slight as to allow of the distorted eye taking in part of the same field of vision with its fellow. In either case it appears that those who squint, habitually neglect the impressions upon the distorted eye, and see with but one.

Of the Eyelids and Tunica Conjunctiva.

311

The cause of squinting is obscure: for though it frequently happens that the eye which squints has an imperfect vision, so as to favour the supposition that it is instinctively averted in order to prevent the perception of objects becoming confused; yet in other cases, vision with either eye is equally good, and the patient can at will employ either singly, but cannot prevent the other from turning away from the object of vision.

Perhaps in cases of the latter description the original adjustment of the two eyes is not true; so that if both were directed towards the same object, it might necessarily appear double, upon the same principle as in the remarkable case transcribed in a note to page 305.

The parts employed for the protection of the eye are, the eyelids with their muscles, the tunica conjunctiva, and the lacrymal gland.

The eyelids are two folds of skin to which shape and firmness is given by two slips of cartilage termed the tarsi. Upon the surface at which the eyelids meet, the skin is gradually transmuted into a mucous mem'brane termed the conjunctiva which lines the tarsal cartilages, and is reflected from the inner surface of the eyelids upon the sclerotic coat to cover the front of the eye, the tunica albuginea, and the cornea. The tarsal cartilages have a membranous joint at either corner, from which a ligament extends to the adjoining bone: the ligament on the inside is well defined, and of a bright silvery colour, and is called the tendo oculi; it extends to the nasal process of the superior maxillary bone: the external ligament is broader and of a membranous character; it extends to the frontal process of the malar bone.

The opposite edges of the tarsal cartilages are so