Principles of General Physiology

The different methods of focussing this dioptric mechanism, accommodation for near or distant objects, are given in the text. The only known photo-chemical substance present in the retina is the visual purple. It is sensitive to nearly the whole of the visible spectrum, but whether it consists of one substance only, or of several, or whether other photo-chemical substances are present is not yet known. In order to account for colour vision, the photo-chemical changes produced by light of one wave length must differ from those produced by another wave length, so that different receptors may be stimulated.

That visual purple is, at least, one of the photo-chemically active substances of the retina, is shown by the fact that the light absorbed by it in different parts of the spectrum, the threshold stimulus necessary to produce sensation in the peripheral parts of the retina, and the bleaching effect of the light on the pigment, all follow the same curve. Other properties of the visual purple are described in the text. There are certain characteristic electrical changes produced by light acting on the retina. The actual curve obtained experimentally is more or less complex, but can be analysed into a compound of three or more simple curves, each of which has an opposite direction at incidence and at disappearance of light. But these components have no connection with the three hypothetical sensations of the Young-Helmholtz theory. - Since the electrical change in the Cephalopod is less complex in nature, and the nerve elements in this case are separated from the eye itself, it appears likely that some of the complexity of the electrical change in the vertebrate eye is due to these nerve elements.

There is reason to believe that there are either six or seven primary colour sensations in man, as described by Newton. Further, the whole spectrum does not consist of an indefinite number of gradations of visible tint, but of a seines of patches, each of which, when isolated, appears to be of a uniform colour (monochromatic). The number of these areas is about sixteen to twenty in normal sighted people, but the precise number differs according to circumstances.

The mechanism of the receptors for sound consists of a membrane, stretched so as to be in a state of tension transversely only. Its transverse measurement increases regularly from one end to the other, so that it is capable of resonance to different rates of sound vibrations at different regions. The vibrations of the different regions are intensified by the structures known as Corti's organ, in order to be able to excite the nerve fibres of each region. Degenerations can be produced in localised areas by exposure to a particular note for a long time.

The receptors for sound appear to have arisen somewhat late in the course of evolution. Periodic impulses, if intense, may affect touch receptors without causing a sensation of sound, which requires an appropriate cerebral aiiulv^T. There are certain organs, " statocysts," present in practically all animals from the jelly-fish upwards, and indeed there are similar structures in the higher plants, whose function it is to enable their possessors to appreciate their position with respect to the direction of gravity. This is done by the presence of a loose particle or particles in a sac, which press upon different receptor endings according to the position of the sac. By introduction of iron filings, an animal can be made sensitive to the direction of magnetic force.

In addition to statocyst organs, the vertebrate possesses a system of three canals, the semicircular canals, or labyrinth, on each side. These are arranged to correspond with the three dimensions of space and are capable of detecting rapid movements and appreciating their direction. This is done by the aid of the inertia and internal friction of the liquid filling them. Sensitive hairs, attached to receptor cells on the walls, are drawn through the liquid and bent, owing to the fact that the liquid does not immediately follow the movement of the walls of the tube containing it.

Sensations derived from the labyrinth play a large part in the maintenance of the tonic contraction of the muscles necessary for posture. Attention is called to the manner in which combinations of sensations from different receptors are used for the purpose of forming complex notions, such as those of space and time, etc. Plants, also, have developed means of intensifying and determining the direction of external forces, especially those of gravity and of light. The region of a growing root, for example, sensitive to gravity is not identical with that in which the response takes place. The mechanism appears to be the same as that of the animal statocysts, grains of starch being generally the movable particles.

The direction of light is appreciated by the leaves of plants owing to refraction by the outer ends of the epidermic cells, these ends being shaped as lenses. By this means the spot most brightly illuminated on the base of the cell differs in position according to the direction from which light rays enter the epidermis. Certain plants possess structures very sensitive to touch, and in many cases a rapid movement of an organ results from a slight stimulus.

THIS word really implies a state of persistent excitation and is appropriate enough as applied to the condition of a nerve centre when sending out a constant stream of impulses which maintain some effector organ in a state of activity. As we shall see presently, however, it does not so well apply 'to the case of smooth muscle, which may, as it seems, remain in a shortened state without necessarily being in a state of excitation. Since it is to this case that the name has most commonly been applied, and the last word has not yet been said as to the nature of the process, we may retain the name for both kinds of phenomena.

As indicated, there are three sets of phenomena to be taken account of, although perhaps only for convenience of description, (i.) The prolonged state of contraction of smooth muscle, which is automatic, or independent of the receipt of excitatory impulses from nerve centres, (ii.) That shown under certain conditions by the cross-striated, skeletal muscle, of which mention has already been made as "decerebrate rigidity" (page 417), and is dependent on stimuli from the centres, disappearing when these are cut off. (iii.) The state of some nerve centres themselves, in which they appear to give out constantly nerve impulses apart from the receipt of messages from receptor organs. The discharges of such centres are frequently rhythmic, as in the case of the respiratory centre.

We will take first the case of smooth muscle, with its natural " tonus." It is a general property of this kind of tissue, wherever met with, to maintain itself in a certain degree of shortening apart from impulses from nerve centres. It is also, almost invariably, provided with two kinds of nerves — a set which increase the tone, excitatory, and a set which diminish it, inhibitory. This peripheral tonus may also show itself as rhythmic changes, as in the case of the heart muscle. As has been remarked above, this structure behaves as smooth muscle.

In the first place, what is the evidence of a natural, inherent tonus in smooth muscle, apart from the obvious activity of the heart 1 The Blood Vessels. — Goltz was the first to point out that the dilatation of the blood vessels, which results from section of their constrictor nerves, on account of the cutting off of continuous impulses from the vaso-constrictor centre, is not so great as that produced by stimulation of dilator nerves (Goltz, Freusberg, and Gergens, 1875, p. 62). Thus, after section of the vaso-constrictors, a state of moderate contraction still remains, which can be further reduced by stimulation of dilator nerves. This moderate tonus, left by section of constrictors, increases in a few days, and becomes nearly equal to the original one in some weeks, although the nerves may not have regenerated (Goltz and Freusberg, 1876, p. 175).

In the frog, after pithing, rhythmic contraction of arterioles is present, while the vessels can still be dilated by the action of carbon dioxide (Bayliss, 1901, 1), showing that they were previously in a state of contraction. In the mammal, after destruction of the spinal cord, although all nervous influence is thereby cut off, the arterial pressure remains at 30 to 50 mm. of mercury. Maewilliam (1902) found that excised mammalian arteries pa-- readily into a state of contraction, which seems to depend upon a supply of oxygen. They can be relaxed by carbon dioxide. Similar observations on the effect of oxviri-n and carbon dioxide were made by Severini (1878, p. 93, and 1881) on the mesenteric vessels of the frog.

Reaction to Stretching. — The denervated smooth muscle of the earthworm was shown by Straub (1900) to respond to stretching by a contraction (see Fig. 132, page 436). The stomach of the frog behaves similarly (Winkler, 1898). The question of the behaviour of the arterioles will be discussed in Chapter XXIII. The Muscles of the Chromatophores of the Cephalopod. — Hofmann (1907, 3) showed that there are no peripheral ganglia in this case, but that the ton us returns after section of the nerves. He is inclined to attribute it to an cfl'cct <»f carbon dioxide in moderate concentration.

The Adductor Muscle of Anodonta. — Pavlov (1885, pp. 21, 22) showed that the tonus of this muscle is not due to nervous impulses from ganglion cells, simv it does not disappear when the visceral ganglion is removed, and there artno ganglion cells in the muscle itself. But stimulation of the nerves from tinvisceral ganglion to the muscle, after the ganglion itself has been cut out, cau-r- This reference to tlu> adductor muscle of the Mollusc leads us to consider some noteworthy peculiarities, which are most easily inves- Fic;. 169. DIAGRAM TO ILLUSTRATE A CATCH OK RATCHET tigated in these organisms, MECHANISM. — The upper piece can be pushed in the although they set-in to be direction of the arrow and the total length of the niore or less present in all model shortened in this way. But the upper piece .-, ,

cannot be moved back again, unless the two pieces smooth muscle, and perhaps are intentionally separated from one another by the even in skeletal muscle, as we depth of a tooth. shall see later. I use the word "catch" as a translation of von Uexkull's name "Sperrung,"' but it is a matter of difficulty to find one which suggests the complete meaning of the German word. Before trying to explain the idea, we will examine a few experimental facts. The strength with which a bivalve mollusc holds its shells together is known to every one who has tried to open an oyster by merely pulling the shells apart. On the face of it, there is nothing to suggest that this fact may not be dm- to the reflex contraction of a powerful muscle. It is found, however, that weights may be arranged to pull continuously, and yet the shells remain firmly closed against a considerable force for many days. To take an example, it requires a tension to be exerted by each square centimetre of the adductor of Dioxinia exoleta equivalent to the weight of 2,400 g. in order to close the shells against the elastic cushion which forces them open. Yet the animal can do this for twenty to thirty days continuously without evidence of fatigue (Pamas, 1910). Consideration of such facts led Griitzner (1904) to suggest that the muscle fibres cannot be exerting tensile stress by a continuous excitatory process, but that the fibres must be " hooked up " in some way, by a kind of arrangement similar to a ratchet, and kept in the position to which the shortening process brought them. If we raise a weight to a certain height and hold it suspended, we have seen that cotisadentble work has to be done all the time and that fatigue soon results. But if a bolt is shot out under the weight, so as to support it, it remains in the raised position without any further expenditure of energy on our part.

The next experiment is one on Pecten which I will give in the words of von Uexkiill (1912, p. 311). "If one takes a normal Pecten out of the water, it gives two or three flaps with its shells before permanently closing them. While it is open, a piece of wood is pushed between the shells, which then close and hit upon the wood with so powerful a crash that their edges are splintered. The wood is then held as in a vice. One can, however, pull it out by twisting it about backwards and forwards, and then one is surprised to see that the shells remain motionless, just as would the jaws of a vice if an object clamped between them had been forced out. The shell movement shows not the least degree of elasticity. The muscular fibres seem to have been suddenly frozen solid." If one next tries to open the shell, no effect can be produced, but even the pressure of a finger is sufficient to press them nearer together, and in this position they remain fixed again, so that they cannot be brought back. The nearest mechanical illustration that can be given is that of two racks with saw teeth, as in Fig. 1 69 ; these will glide over one another if pulled in the direction of the arrow, but resist any pull in the opposite direction. The fact that the animal itself can allow the shells to open, shows that the " catch " can be removed by some means. This, as we shall see presently, is done by " inhibition " from the central nervous system. In the model, it might be supposed to be effected by separation of the two racks to the extent of the depth of a tooth. The device of Fig. 170 may perhaps assist in understanding the process.

If a flat piece of soft iron be arranged so as to be able to move around an axis at one end, and an electromagnet fixed at a short distance above it, on sending a current through the coils of the magnet the weight of the piece of iron is raised ; but, in order to hold it up, energy must be continually FIG. 170. APPARATUS TO ILLUSTRATE THE MECHANISM OF A TONUS MUSCLE. — If the upper electro-magnet is actuated by closing the lower key on the right, the iron lever, with the weight attached, is raised. In this, it pushes back the thin vertical steel spring, until the end of the lever has passed the tooth. When this happens, the spring flies back and the lever is now supported on the top of the tooth, so that the magnet may now be put out of action without the weight falling. To release the weight, so that it may fall again, the electro-magnet on the left must be actuated by means of the upper key. This attracts the spring and draws away the support from the lever, which then falls until it reaches the pointed support. Finally, the circuit of the second electro-magnet is broken, the spring flies back and the apparatus is in its original state. The two keys may be supposed to be nerve centres.

supplied to the magnet, and this energy is dissipated as heat. This part of the process corresponds to the behaviour of the sartorius muscle of the frog in A. V. Hill's experiments (page 450). Suppose, however, that a thin strip of steel spring, with a little projection on it, is arranged at right angles vertically at the free end of the piece of iron, and in such a position that the projection together with the spring can be pushed back by the iron weight as it rises. When the weight has passed over the projecting bit, the spring flies back underneath the weight, and the latter Avill remain suspended when the current is switched off the magnet. In order to allow it to fall again, a second electro-magnet is fixed at the back of the spring and, when this magnet is actuated, the support is drawn back with the spring and the weight falls. This last mechanism corresponds to the impulses from the central nervous system, which release the adductor muscle of the mollusc.

Now the muscle which has this remarkable property must be able also to shorten actively and, indeed, if tested by pressing a hard ohject upon it, it is found to be quite soft when relaxed and as hard as cartilage when in the contracted state. If Fig. 171 be examined, it will be seen that the adductor muscle consists of t\\o distinct parts, a larger round part, which has a clear appearance and consist v ot' cross striated fibres, and a smaller more opaque part, of smooth fibres. In order to distinguish them, we may call, with von Uexkiill, the former the "motor"

FIG. 171. ANATOMY OF PECTEN. — RIGHT VALVE AND RIGHT LOBE OF MANTLE REMOVED. (Reproduced by permission of the Liverpool Marine Biology Committee. For the meaning of the remaining letters, see the original monograph by W. J. Dakin, 1909, pi. 2, Fig. 1.) muscle and the latter the "catch" muscle. The reason is that, if the "catch muscle be removed, the motor muscle can be excited to contraction and keeps the shells closed as long as the stimulation lasts; but, as soon as this ceases, the elastic hinge causes them to open again. If the " catch " muscle be cut through while the shell is closed, the other muscle is unable to keep it closed ; whereas, if the motor muscle be divided, the shell remains closed. Thus the large motor muscle seems to bring the shells together quickly and for a moment ; the smaller catch muscle then holds them fast in the position to which they have been brought by the

contraction of the other. In its contraction it follows up, as it were, that of the other muscle and fixes the whole system at the point where it stops. Another aspect of the phenomenon is pointed out by Parnas (1910). If we try to pull apart the shells of Pecten by means of weights, we find that, as already mentioned, a very large weight is required to do so. If, however, we hang a considerably less weight on the shells when open, they are unable to close against it. Thus the muscle is able to hold up a weight which it cannot raise.

If the nerves from the visceral ganglia to the muscles are cut through while the shell is open, stimulation of the muscle ends of the nerves will produce contraction but no maintenance beyond the duration of the stimulation. On the other hand, a remai'kable fact shows itself if these nerves be cut while the catch mechanism is at work, the shells being closed ; there is no relaxation, neither can stimulation of the nerves remove the catch. The catch muscle remains permanently at the length it had at the moment when the nerves were cut.

But there are other nerve cords, one on each side, which connect the visceral ganglia with the cerebral mass, and electrical stimulation of these nerves is able to control the catch muscle in both directions. That of the right side causes its inhibition, so that the shell opens. That of the left side causes shortening and catch action, so that the shell is permanently closed. These various observations are due to von Uexkiill, who regards them as a confirmation of his view that " excitation " is not a wave of change passing along a nerve, but something which flows liither and thither like a fluid. He speaks of the tonic state in which the catch muscle remains, when its nerves are cut while in that contracted condition, as the "tonus- or excitation-trap." This view has been criticised on a previous page (page 424).

A corresponding phenomenon is described by Veress (1908, pp. 195-196) in the caterpillar of Cossus, after it has spun its cocoon. If extracted from the cocoon and pinned out, it exhibits rhythmical contraction of the muscles of the body wall. These can be inhibited by touching the cuticle. But the interesting point is that at whatever stage of contraction the muscle may be in at the time, it is fixed at that stage for a certain period. The " inhibition " affects only the rhythmical movements ; it does not cause relaxation of the muscle, but merely fixation at that degree of contraction which existed at the moment of the stimulus.

Brief reference may be made to some other cases in which similar phenomena are to be seen. The spines of the sea urchin are surrounded at their bases by a circle of about thirty double muscles around each spine. Each of these muscles consists of an inner cord, white and opaque, and an outer one, clear. If we apply a momentary stimulus to the integument near one of the spines, the muscles on that side contract, but only for a moment, and the spine afterwards returns to its original position. If the stimulation be repeated several times, the spine is pulled over and remains so for a considerable time after the stimulation ceases. In this state it opposes much resistance to being displaced. The single stimulus excites only the outer motor muscles ; the repeated stimulation excites the inner catch muscles in addition (von Uexkull, 1909, p. 91).

A mechanism similar to that of Pecten has been described by Jordan (1913) in the case of Holothuria. The phenomena shown by Sipunculus are very instructive. When the body wall of this tubular animal contracts, it forces out the proboscis, exerting a pressure of about six centimetres of mercury. The proboscis is then cut off, the central nervous system removed, and the remains of the body tube tied on to the end of a glass tube. Sea water is then poured into the tube until its level is about half way up the tube. Then the preparation is immersed in sea water. It is found that to whatever depth the body tube is immersed, the meniscus always remains at the same place ; in other words, the capacity of the sac remains constant, although the actual internal pressure must be considerably greater when it is in air than when the weight of the water inside is counterbalanced by that outside. That is, the muscle fibres can maintain the same length in equilibrium with different pressures.

This phenomenon is also to be met with in the urinary bladder of the higher vertebrates, according to the researches of Mosso and Pellacani (1882). It was found that the internal pressure which this organ can withstand without further distension is independent of the actual distension existing at a given time. That is, the length of the muscle fibres of its wall may be very different and yet have the same tension ; or rather, as we should say, in view of the behaviour of the invertebrate muscle, the fibres may be "hooked up" by a catch mechanism at various degrees of shortening. It will be noted that in this case, as in some others to be mentioned later, the motor and catch mechanisms must lie. as far as we know, in the same muscle fibre.

The next question with which we are faced in the consideration of this prolonged " tonic " contraction of smooth muscle is whether the state is associated with any increase of metabolism beyond the normal one. If the muscle is held in the shortened position by a catch or ratchet mechanism, it would appear that increased metabolism is not to be expected, or of a much less degree than in tetanic contraction. Parnas (1910) has, in fact, made experiments which show that, in the bivalve mollusc, none is to be detected. He loaded mussels (Anodonta), whose adductor muscles had an area in section of O3 sq. cm., with a weight of 3,000 g. for three hours and found 110 increase in the respiratory exchange, either during or after the loading. Indeed, if one compares the entire respiratory metabolism of these animals, under the conditions stated, with the increase in that of a skeletal muscle of the mammal, also holding a weight of 3,000 g. per 0'3 sq. cm.. it only amounts to about 0'00003 of the latter, calculated from results on the entire metabolism in man. The anodon muscle uses 0-008 rug. of oxygen per hour, as compared with some 2-8 mg. for the gastrocnemius of the cat (Ver/ar, 1912, 1, p. 248). Take another experiment by Pamas on three specimens of Venus, which consumed 3'222 mg. of oxygen in four hours, or 0'805 mg. per hour. Loaded with 1,000 g. each for three hours, the consumption was 0'786 mg. per hour, and, subsequently, without load for three hours, O'Sll mg. per hour. A Pecten consumed, at rest, 0'672 mg. of oxygen per hour; under a load of 500 g., 0-679 mg. per hour.

Bethe (1911) investigated the question in another way and confirmed the view of Parnas. He found that no evidence was to be obtained of fatigue nor of loss of weight in fasting molluscs holding up a weight for a considerable time. If the consumption of carbohydrate had been comparable with that of cros» striated, skeletal, vertebrate muscle, an amount greater than the weight of the entire animal must have been burnt up. An interesting calculation is made by Bethe on the tonus of the arterioles in a mammal. If the mechanism were like that of the skeletal muscle, one-sixth to one-quarter of the whole resting metabolism of the animal would be in the arterioles.