[176] The two branches of the ramus ophthalmicus superficialis were spoken of as the ram. opth. superficialis and ram. opth. profundus in my Monograph on Elasmobranch Fishes. The nomenclature in the text is Schwalbe’s, which is probably more correct than mine.
[177] Marshall thinks that this nerve may be the remains of the commissure originally connecting the roots of the third and fifth nerves. This suggestion can only be tested by further observations.
[178] Schneider holds that anterior roots are present in Amphioxus, but I have been unable to satisfy myself of their presence.
[179] These non-gangliated roots of the fifth nerve are not to be confounded with the motor root of the fifth nerve in higher types. They appear to form the anterior root of the adult which gives origin to the ramus ophthalmicus.
[180] If Marshall’s view about the ramus ophthalmicus profundus (p. [461]) is correct, the third must still be, as it no doubt was primitively, a mixed motor and sensory nerve.
[181] In the higher types, as is well known, the fifth nerve has its roots formed on the same type as a spinal nerve. The fact that this is not the case in the lower types, either in the embryo or the adult, is a clear indication, to my mind, that the mammalian arrangement of the roots of the fifth nerve has been secondarily acquired, a fact which is a most striking confirmation of my views as to the differences between the cranial and spinal nerves.
CHAPTER XVI.
ORGANS OF VISION.
In the lowest forms of animal life the whole surface is sensitive to light, and organs of vision have no doubt arisen in the first instance from limited areas becoming especially sensitive to light in conjunction with a deposit of pigment. Lens-like structures, formed either as a thickening of the cuticle, or as a mass of cells, were subsequently formed; but their function was not, in the first instance, to throw an image of external objects on the perceptive part of the eye, but to concentrate the light on it. From such a simple form of visual organ it is easy to pass by a series of steps to an eye capable of true vision.