The Second or Optic Nerve

The optic nerve (N. opticus) is composed of fibers which are the central processes of the ganglion cells of the retina. The fibers converge within the eyeball to the optic papilla, where they are collected into a round trunk, the optic nerve. The nerve thus formed pierces the chorioid and sclera, emerges from the posterior part of the eyeball, and passes backward and inward to the optic foramen. After traversing the latter it decussates with its fellow of the opposite side to form the optic chiasma or commissure. In the orbit the nerve is slightly flexuous and is embedded in the fat behind the eyeball and surrounded by the retractor oculi muscle. Its intraosseous part is an inch or more (ca. 3 cm.) long. The sheath of the nerve is formed by prolongations of the membranes of the brain, and includes continuations of the subdural and subarachnoid spaces.

The greater part of the fibers of the optic nerve cross in the chiasma to the tract of the opposite side. In the tract the fibers proceed to (1) the internal geniculate body, (2) the posterior part of the thalamus, and (3) the anterior quadrigeminal body (indirectly). The fibers which go to the internal geniculate body appear to belong to Gudden’s commissure and to be nonvisual in function. The visual fibers, which come from the outer part of the retina of the same side and the inner part of the retina of the opposite side, terminate about cells in the anterior quadrigeminal body and the part of the thalamus which corresponds to the pulvinar and external geniculate body of man. From the cells of the former fibers pass to the nuclei of the motor nerves of the eyeball, and complete the reflex arc. Fibers proceed from the cells of the thalamus to the visual area of the cortex in the occipital part of the hemisphere.