It would take us too far afield to describe the structure and mechanism of the organ of hearing. The manner in which the inner ear arises as a sac has been mentioned, but nothing was then said of the very important fact that in its beginning the inner ear was double in function. It not only served as an organ of hearing, but also as one of balancing. This double function it has retained in all vertebrates and a very beautiful mechanism —consisting of three semi-circular canals— is attached to the inner ear, which informs the brain of the movements and position of the head and body. When these canals are destroyed by disease we lose the power of balancing our bodies when we move.
At the end of the ear passage is set a membranous drum which vibrates with the waves of sound. The waves are conveyed to the inner ear by three small bones—the hammer, the anvil and the stirrup. The history of these bones is strange. The hammer was at an early stage of the evolution of mammals a part of the lower jaw; the anvil was the bone on the base of the skull with which it articulated. When mastication and molar teeth were evolved in the ancestry of mammals a new joint was formed in the lower jaw, and these two bones—the hammer and anvil—were taken into the service of the ear.
All these things sound too much like a romance. Yet what will the reader think when he comes to consider the human eye? Not that the human eye is peculiar; it is made of the same parts as that of the fish ; it is the human brain which makes the human eye a source of joy and knowledge. We may marvel over the evolutions of the sense of sight and explain it as best we may; as regards the facts of its development in the embryo they are comparatively simple. Three different elements come into conjunction to form it. First there is the dipping down beneath the skin of a bud of epidermis to form the lens. When the bud of epithelium sinks inwards to form the lens the skin closes over the point of subsidence and becomes transparent, thus forming the cornea of the eye. The second element to form the eye grows out from the nerve tube or brain of the embryo. The outgrowth forms a cup round the lens bud; the cup itself becomes the retina and the stalk of the cup is transformed into the optic nerve. The third element is derived from the surrounding tissue of the head—the mesoblast. The cells of the mesoblast swarm within the cup and round the lens and form the transparent jelly within the eyeball. The outer coats of the eye, which enclose and protect the retina, also arise from the mesoblast.
It may seem to the reader that the short account which has been given of the development of the eye only makes its evolution more marvellous and mysterious. It must be remembered, however, that we are witnessing the result of a process which has been at work for millions of years. Indeed, since ever life appeared on the globe, and that is longer ago than the human mind can conceive, the processes have been at work which have ended in the compounding of an eye. It is an instrument which can focus the outer world and is attached to a brain which can partly understand that image. Yet with all its complexity the eye is but a triple compound of skin, brain and intervening tissue. One cannot expect the anatomist to unravel in a few years that which has taken the lifetime of a world to evolve.
We need not stop here to note the fact that the human eyes are set in the face so as to take in a common field of vision. That is not a peculiar human trait; binocular vision is common to the higher primates. Nor need we dilate on the remnant of a third eyelid or nicitating membrane which can be seen as a thin fold between the eyeball and the red caruncula at the inner angle of the eye. Evidently in the human ancestry a functional third eyelid had been present, such as can be seen in the eye of the cat. But allusion must be made to a very remarkable vestige of another eye now completely buried beneath the great mass of our brain. Mention has been made of a bridge of tissue—the corpus callosum—which unites the two cerebral hemispheres. Underneath its posterior end is a small body no larger than a barley or wheat grain, known as the pineal gland. Connected with this vestigial body are remnants of optic nerves and optic ganglia. One has to go down the animal scale to the most primitive form of lizard—Sphenodon— to see this body assume the shape of an eye. It is median, single, and looks out from the crown of the head, and even in the lizard is a remnant of a visual sense we have not discovered so far in its original condition. Yet there sometimes occurs in the roof of the human skull a foramen or opening exactly in the position which the pineal eye occupies in lizards. The place is always marked by two small openings for veins, and the venous blood channel, which sweeps backwards in the roof of the skull, often widens at the spot, and sometimes actually divides, leaving room as it were, for the appearance of a pineal eye —which, however, never comes to the surface.
Quite recently Professor Dendy has described the remnant of a curious organ in the brain of man which may have been connected with some peculiar form of sense organ. For a long time it has been known that certain large fibres occur in the central canal of the spinal cord of fishes. Their function is unknown. They begin in a patch of epithelium situated in the roof of the canal of the stem of the brain. Professor Dendy found that traces of these occurred even in mammals. He found the epithelial organ from which these peculiar fibres arise in the canal of the brain stem of primates. Ultimately he discovered traces of this epithelial organ in the brain of the chimpanzee and of man. We have here a fine example of the persistence of an organ which, in the higher vertebrates, is apparently quite vestigial in nature.