III.
We have now seen how protoplasm exists in a large body, sharing the work of living amongst specialized cells, and how it responds as a whole to the influences exerted upon it by its surroundings. The next thing to consider is how it is situated with regard to matter which does not form part of its own body; how protected from, and how put into communication with, the rest of the universe.
With regard to the former, we have seen that in the single cells, constituting unicellular organisms, there is always a bounding membrane of denser texture than the rest of the protoplasm. As the cell develops its capabilities, we have a shell or case of non-living matter secreted around it, with apertures for communication with the outside world, and increasingly effective protection is provided as protoplasm, whether in the single cell or the body, leaves the water, and has to face the inclemencies of terrestrial life.
Diagram 48.—Showing the Formation of the Skin.
Diagram 49.—Structure of Skin.
In the schematic embryo ([Diagram 6]) and other diagrams contained in this volume, the skin has so far been represented as consisting of a single layer of living cells; but we must now admit that the skin of man is quite different. Such a covering would be no protection from heat, cold, or irritating chemicals, while, in order to prevent its drying up, it would have to be kept moist with slime, and we should look very like frogs. In order that an adequate defence may be provided for the body, this layer of cells divides tangentially, forming two layers. The inner of these two then divides tangentially again, and a second layer is interposed between the innermost and that first formed. The skin now consists of three layers, and so the process is repeated until it is several layers thick. ([See Diagram 48.]) It is the innermost and best-nourished layer which keeps dividing; the other layers, as they get pushed outwards, are only reached by a little lymph which filters between the cells, and are eventually starved even of that. As they get pushed away from the dividing layer, however, they set to work to surround themselves with a horny wall, which thickens and thickens, until eventually there is hardly any cell left. ([See Diagram 49.]) Finally the cells die and the horny envelopes form a dead cuticle, protecting the living layers beneath, and are ultimately sloughed off when their successors are ready to replace them.
Diagram 50.—Showing the Development of Hair.
Not even a horny layer of dead cells is, however, always sufficient protection, and the growing layer has sometimes to supplement it by hair or feathers. How hair is developed is shown in the accompanying diagram ([50]). The growing layer sends a strand straight downwards into the connective tissue, which forms the basement of the skin. The cells in the middle of this strand, which behaves like ordinary skin, are the least well nourished, and accordingly die and leave a tube. This tube, if no further development took place, might become a sweat gland; but if it is to give rise to a hair it becomes cup-shaped at the base, enclosing a small loop of bloodvessel. The cells just above the capillary, being better nourished than the rest, grow more rapidly than their neighbours, and the result is that a column of cells which we know as a hair pushes its way up through the tube. ([See Diagram 50.])
This outer layer comes everywhere between the main bulk of the body and the outer world. Hair and sweat glands do not by any means represent its only modifications. Teeth are formed from it in somewhat the same manner as hair, while we have already seen that it gives rise to the whole nervous system.
The next thing which we have to consider is how knowledge of the external world reaches the central nervous system. Sensations of touch, temperature, and pain are fairly easy to understand, since the nerves which convey such impressions have numerous endings in the skin. End organs of nerves in the joints and muscles doubtless enable the animal to perceive and estimate strain and resistance in moving or lifting things. But the power of perceiving the chemical peculiarities of things; light, involving the formation of visual images, which we call seeing; sound; and position and equilibrium, it is not possible for the whole surface of the body to possess. The principle of division of labour is extended to the task of perception as well as to that of motion; and cells, with their property of responding to light, vibration, chemical stimulation, etc., are grouped together to form special organs, connected with the central nervous system by special nerves.
Perhaps the most important factor which can influence protoplasm is the chemical nature of its surroundings; and in [the first essay], on the general nature of protoplasm, we touched upon the way in which it is drawn towards some substances, and repelled by others.
In the body there are two sets of cells deputed to act for the rest in this particular. One set is situated in the membrane lining the nose, over which the air we breathe passes; and these cells examine our gaseous surroundings, and warn us, by what we term ‘smell,’ whether the atmosphere is fit for us or we had better seek a purer. The other set is for the examination of liquids. Against these we are protected by our skin, and, as we do not absorb anything through it, it is devoid of the power of examining the things it touches. But with our food it is different; we must have the power of testing that. Accordingly, there are Customs officers in our mouth in the form of little groups of cells, which report upon the liquids and solids moistened by saliva, and enable the animal to reject pernicious imports. Thus, the stimulation of a small portion of the protoplasm composing a body is transmitted over the whole, and is able to awake in it the necessary response.
Diagram 51.
Diagram 52.
Diagram 53.
Diagram 54.
Diagram 55.
Diagram 56.
So much for the chemical sense organs; they are comparatively simple. But between a single cell, which always makes towards or always hurries out of a ray of light passing through the water in which it swims, and an animal with eyes capable of recognising the colour, shape, size, and distance of objects in space, there really does seem to be a wide gulf. It is not, however, too wide to be bridged.
After the single-cell stage has been passed, and we have beasts consisting of an inner layer of cells which is digestive in function, and an outer layer which is protective, motor, and sensory, the power of perceiving light is doubtless vested in the outer layer. When we get beasts consisting of three layers progressing along the straight path of development which leads to man, we find the outer layer becoming too opaque for this purpose, and the torch is handed on to the sensory tube derived from it. ([See Diagram 5.]) As more and more protection is required, the skin thickens, and the neural tube comes to lie deeper, as in [Diagram 51]. In order not to lose the light altogether, it has to throw out buds, which concentrate in themselves the peculiar faculty of perceiving it, and at the same time little pits are formed in the skin just over them to help the light to reach them. ([See Diagram 52.]) In [Diagram 53] both the nervous elements and the integumentary are developing their possibilities; and in [Diagram 54] a large surface has been prepared for the reception of light, and a lens formed to focus the rays upon it. [Diagrams 55 and 56] give the concluding stages in the development of the eye: the formation of the cornea and its protecting eyelids. The two cavities are filled with clear liquids, and the whole eyeball supported by connective tissue.
So fascinating is everything connected with the eye that the temptation to describe it in detail is great; but in a book of rough outlines, and in consideration of the many important matters yet awaiting their turn, we must confine ourselves to briefly mentioning a few of the more important points concerning it. The light is focussed by the lens upon the nervous curtain at the back, and produces there a picture, as in the photographic camera. Thus we perceive the shape of objects. The different rays of the spectrum affect different elements in this curtain or retina, whereby we get sensations of colour. Finally, the clearness of the picture, its size, the degree of convergence of the two eyes, and the effort of focussing—for the curvature of the surface of the lens can be altered—enable us to estimate the size and distance of an object. And now, though it would take volumes to do justice to the physiology of vision, we must pass on to deal equally briefly with the functions of that no less important organ, the ear.
The essential part of the ear is a membranous bag, formed by the pouching in of the outer layer of cells—as shown in Figs. 1, 2, and 3 of [Diagram 57]—which comes to lie in a bony chamber beneath the skull, and assumes the somewhat complicated shape depicted in Fig. 4. We have not time, nor is it for our purpose necessary, to trace all the steps in the development of the ear, either external or internal, nor need we spend much time upon its structure, beyond indicating its position. But its position, which is shown in [Diagram 58], must be grasped in order to understand how it is influenced by sound.
Diagram 57.—Showing Development of the Membranous Labyrinth of the Ear.
U, Utricle; C, cochlea; S, saccule; S.C., semicircular canals.
It will be seen that the membranous bag, which is fitly termed the labyrinth, is situated in a bony cavity which fits so closely as to be termed the bony labyrinth (C). The membranous labyrinth is filled with a liquid, called endolymph, and the bony labyrinth (C) is also filled with a liquid, called perilymph, in which the membranous bag swims. All this is called the inner ear. The inner ear communicates with a second cavity—the middle ear (B)—by two apertures in the bony wall, which are closed by membranes. The middle ear is full, not of liquid, but of air, and is separated from the external ear, the cavity marked A, which is open to the external world, by another membrane called the tympanum, or drum, of the ear. The middle ear is connected by a tube with the throat, so that the pressure of the air on both sides of the drum may be the same.
Diagram 58.—Showing the Position of the Ear.
A., Outer ear; B., middle ear; C., inner ear.
Now, the object of this arrangement is that the ear may be able to fulfil one of its principal duties, namely, the perception of sound. Sound, as the reader is doubtless aware, is transmitted through the air as waves of condensation and rarefaction, due to the swinging backwards and forwards of its particles; it resembles the passing on of a bump along a line of trucks on the railway when the engine runs up against the end one preparatory to coupling. The magnitude of this oscillation we perceive as the loudness, the frequency as the pitch of a note. Now, when the waves of sound strike against the drum of the ear, they cause it to vibrate backwards and forwards also. Supposing there was no middle ear, and the sound waves beat directly upon the membranous windows of the inner ear, these could not be made to vibrate, as there is liquid behind them, and liquids are incompressible; so, in order that the movements of the drum may be transmitted to the liquids of the inner ear, they are carried across the middle ear by a chain of small bones, by which their extent is curtailed, but their force increased, and brought to bear upon one only of the two openings. The consequence of this is that the membrane closing it is able to vibrate and pass on the vibrations to the liquid within, since when it is pushed in, the membrane covering the other hole is pushed out.
Diagram 59.—The Semicircular Canals.
Exactly how the different parts of the membranous labyrinth contribute to our perception of sound we do not quite know. It appears as though the difference of pressure in saccule and utricle originally conveyed to the brain a sensation of noise without any idea of quality, while the cochlea was developed later to analyze sounds and give information as to pitch and tone. Whether the rest of the labyrinth has any longer a part to play in the perception of sound, we cannot say with certainty; but it seems pretty certain that the cochlea is the organ for receiving musical impressions. Here, again, though, we are at a loss, for we do not know with certainty how the cochlea acts. In shape it is a long tube, and in the head is coiled spirally—like a snail’s shell to look at. Along its whole length is a ridge of cells with short hairs projecting from their inner surface into the liquid it contains; and to the cells along this ridge a branch of the auditory nerve is distributed. But as to whether one of the cells along this keyboard responds to each of the notes we can distinguish, or whether they are affected as a whole, physiologists are not yet agreed.
At least one other important duty the ear performs; it tells us in what position we are, and how our whole head moves or is moved. On the top of the saccule, in [Diagram 57, Fig. 4], there are shown three little loops which are called the semicircular canals. They are shown again more clearly by themselves in [Diagram 59].
Fig. 1 shows their position with regard to each other. It will be seen that two of them are vertical, with their loops forming a right angle with one another, and that the other is horizontal—in fact, that they lie in the three planes of space. Fig. 2 shows the structure of one of them; it has a swelling at one end (a), and a knob projecting into it where the nerve joins it (b). In Fig. 3 is shown a section through this knob, which gives the key to the use of these structures. A little head of cells projects from the wall of the canal into its lumen, and from these cells hairs bristle out into a dome-like covering of jelly, weighted, to prevent its moving too easily, with small particles of lime. Now, if you take up a round vessel full of liquid—say a bowl of gold-fish—and give it a twist round, you will notice that, though the bowl turns, the water inside does not; the fish remain in their old position. If there were a rod projecting from the side of the bowl, it would, of course, move with it, and if a fish came in its way would strike against it. This is the principle of the semicircular canal. For if we turn our head, the tube of the canal turns, passing over the liquid in it, which of course does not move, though it appears to flow in the opposite direction. The consequence is that the hairs on the side of the knob in the direction in which the head is being moved are pressed upon by the dome of jelly, which, as it floats in the liquid, tends to remain where it is. The nerves, stimulated in this way, inform the animal generally of the movement.
These little organs are very important to us, though we have our eyes to correct our ideas of position, and they are still more so to the fish, which dart and turn in the wide expanse of the ocean, and the birds and bats, which wheel about in the air. There are, however, some occasions when we do not feel inclined to bless them; for, inasmuch as they faithfully report every roll and plunge of a ship to a person on board, it is they which are mainly responsible for sea-sickness.
And now that we have seen how the body lies with regard to the external world; how it is efficiently protected from its surroundings; how it is placed in communication with them; and have briefly examined the organs by which it makes its chemical and physical investigations, looks out into space, and is kept aware of what is going on therein, we may return to the means whereby it responds as a whole to the stimuli thus reported—the central nervous system—and try to learn how the right response is brought about.