2. An anastomosis, or junction with the ophthalmic branch of the fifth, is described by most anatomists, and may be readily verified in the recent subject One or more branches, having very much the same direction and ap pearance with the preceding to the sympa thetic, pass backwards from the sixth nerve, in the anterior part of the sinus ; leaving it at a very acute angle, inclining outwards as they go, and finally, entering the wall of the sinus to join -the ophthalmic branch, not far from the Gasserian ganglion. These branches also exist in the sheep, and some other of the lower animals.* 3. A very fine filament from the sixth nerve to the ciliary or lenticular ganglion has been described by several authors.
Subsequently to the cavernous sinus, the course of the nerve is but short. Arriving at the posterior extremity, or apex of the orbit, the nerve lying to the outer side of that part of the third which supplies the inferior rectus and oblique muscles, runs slightly upwards, and turning outwards, continues for a very short distance along the inner surface of the external rectus. It finally breaks up into numerous minute filaments, which enter the ocular surface of this muscle to be distributed to it.
Physiology of the sixth nerve.—The function of the nerve is, perhaps, sufficiently indicated by the preceding details. Since anatomy shows that its terminal distribution is exclusively to a muscular surface, we should on this ground alone be tolerably entitled to predicate its motor function.
The little that is known of its comparative anatomy confirms the inference. In all the higher vertebrata it is distributed to the ex ternal rectus. In some, however, it experi ences an enlargement, and a further distribu tion. The muscle which sweeps the broad nictitating membrane over the bird's eye, and the funnel-shaped, or choanoid muscle N,vhich surrounds the optic nerve and eyeball of many mammalia, are both supplied from this nerve.
So also one or two cases are recorded, in which an injury of this nerve from disease in the neighbourhood has produced paralysis of the external rectus, and an inward squint. While, vice versii, the experiment of galvanising the nerve has been accompanied by violent contractions of the muscle, and an external strabismus.
The insensibility of the nerve is, perhaps, less certain than might at first appear, though Longet* distinctly states that pinching the nerve at its origin is unattended by signs of pain. The branch of junction with the oph thalmic nerve seems to be, from its direction and appearances, much more like a filament from the sensitive to the motor nerve, than from the latter to the former. If this be the case, they would seem to be somewhat ana logous to the junction of the numerous branches of the fifth with the portio dura on the face. And in the absence of direct ex periment upon the nerve beyond the seat of this union, one might conjecture it as possible, that the sixth nerve was possessed of a slight sensibility similar to that of this portion of the seventh. Concerning the import of the junc tion with the sympathetic, little can here be said ; for although, as compared with the size of the communicating nerves, this union is larger than most others, yet there does not seem any sufficient reason for supposing other differences.
The distribution of a branch from the sixth to the ciliary ganglion has been thought by Longet and others to explain the persistence of movements of the iris after paralysis of the third nerve. But besides that the constant existence of this filament seems hardly veri fied ; perhaps the interposition of a ganglion between the paralysed nerve and the ciliary filaments might alone be thought a sufficient explanation of the inconstancy or imperfec tion of the result, without requiring the ex istence of another and an uninjured channel as the cause;