This section is from the book "Health", by W. H. Coefield.
Now, as to the sense of smell,-
You will remember that I told you in the last lecture, that of the twelve pairs of cranial nerves that leave the brain the first pair are devoted to the sense of smell, and have no other office to perform.
These nerves lie along the upper surface of one of the bones of which the skull is made up, which bone is pierced with a large number of small holes ; branches from the nerves pass through these small holes down into the upper part of the cavities of the nose, and they are distributed to the mucous membrane which lines those upper cavities.
I told you, when speaking about breathing, that the breathing or respiratory passage passed through the nostrils along the lower part of the cavities of the nose into the cavity at the back of the mouth, which we call the pharynx. The nerves which belong to the special sense of smell are, then, distributed to the mucous membrane of cavities which are in close connection with the respiratory passages in the nose, being, in fact, just above them and communicating directly with them, and as the bodies that are capable of exciting the sense of smell come to us in the air that we breathe, you will see that that is precisely the position in which you would expect the extremities of those nerves to be placed.
As the air passes along the lower cavities of the nose some small quantity of it gets into the upper cavities ; if the air that we breathe contains a considerable quantity of particles that are capable of exciting these nerves, they convey to the brain the sensation of smell If, for instance, the air is loaded with attar of roses, then as it passes along the respiratory passage sufficient particles of that scent will get into the upper cavities of the nose, and will stimulate the extremities of those nerves through the epithelium which covers them. If the air is only lightly charged with particles, we draw air forcibly into the nose, so that more of it than usual gets into the upper cavities of the nose, and brings the particles that are capable of irritating the nerves of smell into contact with the mucous membrane ; but mind the extremities of these nerves are not directly exposed to the particles which produce the sensations of smell; the extremities of these nerves end in mucous membrane, which is covered by the epithelium, and when substances are brought into the nose that are capable of destroying the epithelium, such as strong acid vapours, they do not produce a sensation of smell, but one of pain.
Now you see in these senses that I have described we have had direct contact of the sensory apparatus with external objects.
But the organs of the other two senses are contrivances for taking note of certain kinds of movements which occur outside of us, and enabling such movements to produce stimuli which travel to the brain along the nerves which belong to the senses of sight and hearing.
We will take the sense of sight first, because it is one easily understood, and most interesting. The organs of the sense of sight are two bodies which are nearly but not quite globular, composed, in fact, each of portions of two spheroids, and they are situated in cavities which are called orbits, which are walled, as it were, by bones belonging to the skull and face. These cavities, of course, are open in front, and have each an aperture at the back of them, through which the nerve which goes to the eye from the brain passes-the nerve which I mentioned in the last lecture as the nerve belonging to the second pair of nerves-the optic nerves.
The eyeball is protected by means of the bony walls of the cavity in which it is placed, and by the arched projection of part of the frontal bone; it is protected by the covering of the latter with fat, skin, muscle, and even hair in the eyebrows; it is protected by fat at the back, and by muscles which surround it, and by means of which it is moved ; it is protected in front by two lids, partly composed of tough cartilaginous substance, partly composed of muscle and skin; and, again, it is still further protected by the fringes of hair which we call the eyelashes. This is, roughly speaking, the way in which the organ of sight is protected from injury by external violence. The eyeball itself consists of certain layers or coats on the outside, enclosing a cavity which is divided into two, as will be described presently. The greater part of it is invested with a tough, whitish, opaque, hard, coat, which is called, from its hardness, the sclerotic coat. This coat has an aperture at the back, through which the optic nerve passes into the interior of the eyeball,, and is in continuation with the fibrous sheath of that nerve. This coat does not extend over the whole eyeball, but in the front part it is replaced by a transparent substance or coat, which is called the cornea. Inside the sclerotic there is a finer coat which is very largely supplied with blood-vessels, and which contains several layers of dark-coloured pigment ; in this coat travel blood-vessels, by means of which the greater part of the structures in the eyeball are nourished, and also important nerves which go to the front part of the eyeball, and that coat goes by the name of the choroid coat. It, as it were, lines the hard outside coat, or the sclerotic coat, and at the front part, very nearly where the opaque sclerotic coat joins the transparent cornea, the choroid coat divides into two parts, one of which ends in a kind of fringe, while the other is continuous with a curtain placed a short distance behind the cornea : this curtain goes by the name of the iris, and has in the middle a circular aperture called the pupil. The iris is largely supplied with blood-vessels and with nerves, and it contains muscular fibres within it. How that acts, and what are its uses, I shall describe in a few minutes.
Inside the choroid coat we find a thin white film, which is continuous with the optic nerve. The optic nerve passes through the hard sclerotic coat on the outside, passes through the coat containing blood-vessels and pigment, called the choroid coat, and then it is expanded, and its fibres are distributed in this fine white structure, which goes by the name* of the retina, and which lines the inner surface of the choroid membrane pretty nearly up to the place at which the sclerotic coat joins the cornea. These are the coats, then, roughly described, which surround the remaining structures that are inside the eyeball.