This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Six muscles are connected with the eyeball, four straight and two oblique. One muscle, the levator palpebrae, goes to the lid. The four recti muscles, superior, inferior, external, and internal, arise from a common tendinous origin, forming a ring or tube called the ligament of Zinn. This ligament or tube surrounds the optic foramen and is attached to the opposite side of the sphenoidal fissure. Through it run the optic nerve and ophthalmic artery, the third, fourth, and the nasal branch of the ophthalmic (fifth) nerve.
The levator palpebrae and superior oblique arise to the inner side and above the optic foramen close to the origin of the other muscles. The superior oblique, after passing through its trochlea or pulley at the inner upper angle of the orbit, continues downward, backward, and outward between the superior rectus and the eye, to be inserted above the extremity of the inferior oblique.
Fig. 96. - The roof of the orbit has been removed, showing the contents.
1 he inferior oblique arises from the anterior edge of the orbit just to the outer side of the lachrymal groove. It passes outward, upward, and backward, over the external surface of the inferior rectus, to be inserted beneath the external rectus.
The recti muscles insert into the sclera 5 to 7 mm. back from the cornea. In the operation for internal squint or strabismus, the internal rectus muscle is cut. It possesses the longest tendon of insertion, while the external possesses the shortest. The recti muscles pull the eyes toward their respective sides. The superior oblique turns the cornea down and out and rotates it inwardly. The inferior oblique turns the cornea up and slightly out and rotates the eye outward. A disarrangement of any of these muscles produces diplopia or double vision.
The arteries of the orbit are derived from the ophthalmic artery, which breaks up into its various branches soon after it passes through the optic foramen. In enucleation of the eye there is practically no bleeding, because the arteria centralis is the only one divided, and it is small. In evisceration, or cleaning out of the contents of the orbit, the main trunk of the ophthalmic will not be cut unless the very apex is invaded. Hemorrhage is readily controlled by packing gauze into the orbital cavity.
The veins of the orbit are the superior and inferior ophthalmic. The former is much the larger and more important. It not only drains the upper portion of the orbit, but communicates directly with the angular branch of the facial, at the inner canthus of the eye. The infection of erysipelas sometimes travels along these veins directly from the nose, face, and scalp without, to the cavernous sinus and meninges 6 within, causing thrombosis and death. The inferior ophthalmic usually empties into the superior; its anastomoses at the anterior portion of the orbit with the veins of the face are much smaller and, therefore, not nearly so dangerous.