Principles of General Physiology

Descartes left some extremely rough sketches, one of which, as copied by Clerselier, "with his best ability," as he says, is given in Fig. 155. The figures with the letter G at the bottom, reproduced in Fig. 154, are by a M. Gutschoven of Louvain. Clerselier made the acquaintance of this gentleman and, finding him to be thoroughly familiar with the views of Descartes from his conversations with the philosopher, commissioned him to draw more figures in order that the text should be more easily understood. A further series of drawings were obtained from a M. de la Forge, whose notes were also added to the book. I have not thought it necessary, to reproduce M. de la Forge's figure.

The description of the figures is sufficiently interesting to give it in Descartes' own words and I think that it must be admitted that Gutschoven's diagram makes them more intelligible : " Voyez apres cela comment le tuyau, ou petit nerf, bf, se va rendre dans le muscle D, que je suppose estre 1'un de ceux qui meuvent 1'oeil ; et comment y estant il se divise en plusieurs branches, composees d'une peau lache, qui se peut etendre, on e'largir et retrecir, selon la quantite des Esprits Animaux qui y entrent, ou qui en sortent, et dont les rameaux ou les fibres sont tellement disposees, que lors que les Esprits Animaux entrent dedans, ils font que tout le corps du muscle s'enfle et s'accourcit, et ainsi qu'il tire 1'oeil auquel il eat attache ; comma au contraire lors qu'ils en ressortent ce muscle se desenfle et se rallonge.

" De plus, voyez qu'outre le tuyau b f, il y en a encore un autre, a S9avoir e f, par ou les Esprits Animaux peuvent entrer dans le muscle D, et un autre, & scavoir d g, par oii ils en peuvent sortir. Et que tout de mesme le muscle E, que je suppose servir <\ mouvoir 1'oeil tout au contraire du precedent, recoit les Esprits Animaux du cerveau par le tuyau c g, et du muscle D par d g, et les renvoye vers D par e f. Et pensez qu'encore qu'il n'y ait aucun passage evident, par ou les esprits contenus dans les deux muscles D et E, en puissent sortir, si ce n'est pour entrer de 1'un dans 1'autre ; toutesfois, parce que leurs parties sont fort petites, et mesme qu'elles se subtilisent sans cesse de plus en plus par la force de leur agitation, il s'en dichappe tousiours quelques-unes au travers les peaux et des chairs de ces muscles, mais qu'en revanche il y en revient tousiours aussi quelques autres par les deux tuyaux b f , c g.

" Enfin voyez qu'entre les deux tuyaux b f, e f, il y a une certaine petite peau H f i, qui separe ces deux tuyaux, et qui leur sert comme de porte, laquelle a deux replis H et i, tellement disposez, que lors que les Esprits Animaux qui tendent a descendre de b vers H, ont plus de force que ceux qui tendent a monter d'e vers i, ils abbaissent et ouvrent cette peau, donnant ainsi moyen a ceux qui sont dans le muscle E, de couler tres promptement avec eux vers D. Mais lors que ceux qui tendent a monter d'e vers i sont plus forts, ou seulement lors qu'ils sont aussi forts que les autres, ils haussent et ferment cette peau H f i, et ainsi s'empechent eux-mesmes de sortir hors du muscle E ; au lieu que s'ils n'ont pas de part et d'autre assez de force pour la pousser, elle demeure naturellement entr' ouverte. Et enfin que si quelques fois les esprits contenus dans le muscle D, tendent a en sortir par b f e, ou d f b, le reply H se peut etendre, et leur en boucher le passage. Et que tout de mesme entre les deux tuyaux c g, d g, il y a une petite peau ou valvule g, semblable & la precedente, qui demeure naturellement entr' ouverte, et qui peut estre fermee par les esprits qui viennent du tuyau d g, et ouverte par ceux qui viennent de c g.

"En suite dequoy il est aise & entendre que si les Esprits Animaux qui sont dans le cerveau ne tendent point, ou presque point, a couler par les tuyaux b f, c g, les deux petites peaux ou valvules f et g demeurent entr' ouvertes, et ainsi que les deux muscles D et E, sont laches et sans action ; dautant que les Esprits Animaux qu'ils contiennent, passent librement de 1'un dans 1'autre, prenant leur cours d'e par f, vers d, et reciproquement de d par g vers e. Mais si les esprits qui sont dans le cerveau tendent a entrer avec quelque force dans les deux tuyaux b f, c g, et que cette force soit egale des deux costez, ils ferment aussi - tost les deux passages g et f, et enflent les deux muscles D et E autant qu'ils peuvent, leur faisant par ce moyen tenir et arrester 1'oeil ferme en la situation qu'ils le trouvent.

" Puis si ces Esprits qui viennent du cerveau tendent i couler avee plus de force par b f que par c g, ils ferment le petite peau g, et ouvrent f, et ce plus ou moins, selon qu'ils agissent plus ou moins fort ; avi moyen de quoy les Esprits contenus dans le muscle E se vont rendre dans le muscle D, par le canal e f ; et ce plus ou moins viste, selon que la peau f est plus ou moins ouverte : Si bien que le muscle D, d'ou ces esprits ne peuvent sortir, s'accourcit, et E se rallonge ; et ainsi 1'oeil est tourne vers D. Comme au contraire, si les esprits qui sont dans le cerveau tendent a couler avec plus de force par c g que par b f, ils ferment la petite peau f, et ouvrent g ; en sorte que les esprits du muscle D retournent aussi tost par le canal d g dans le muscle E, qui par ce moyen s'accourcit, et retire 1'oeil de son coste.

"Car vous S9avez bien que ces Esprits, estant comme un vent ou une flamme tres subtile, ne peuvent manquer de couler tres promptement d'un muscle dans 1'autre, si tost qu'ils y trouvent quelque passage ; encore qu'il n'y ait aucune autre puissance qui les y porte, que la seule inclination qu'ils ont a continuer leur mouvement, suivant les loix de la Nature. Et vous scavez outre cela, qu'encore qu'ils soient fort mobiles et subtils, ils ne laissent pas d'avoir la force d'enfler et de roidir les muscles ou ils sont enfermez ; ainsi que 1'air qui est dans un balon le durcit, et fait tendre les peaux qui le contiennent." (Clerselier's edition, pp. 15-20.)

It will be remembered how Descartes looked upon the material bodies of man and animals as pure machines, using, in fact, the word itself. In man, this machine is made use of by the soul, which enters into relation with it at the pineal gland. Other animals, which have no souls, are therefore nothing but machines. The cries made by a dog when injured are no more than the noise made by a machine when a part of it breaks off and gets into the wheels. The object of the " Traite de 1'Homme " is to show how the working of the human body can be explained on purely mechanical principles. According to Stensen (Steno), who lived from 1631-1686, and whose name is familiar in the denomination of the duct of the parotid gland, Descartes did not pretend to expound the actual structure of man's body, but to describe a machine capable of performing all its functions (quoted by Foster, 1901, p. 62).

It will scarcely escape the notice of the reader how.closely the method of description of the innervation of the eye muscles, as given by Descartes, approaches the ' ' drainage " views of Macdougall and von Uexkiill, if we read "neurin," "nerve^energy," "lomis" or "excitation" in place of " Esprits Animaux." The essential difference between the present view of reciprocal innervation and that of Descartes is that the latter placed the mechanism in peripheral structures, whereas we know now, by experiment, that it is in the nerve centres.

Although this reciprocal relation of antagonistic muscles was present to the minds of various previous physiologists in a certain way, as for example to Meltzer (1883, pp. 215-216), it was not until the work of Sherrington (1892, etc.) that our knowledge of the mechanism became clear and definite. The phenomenon is shown in a striking way in Fig. 156 (from the paper by Sherrington, 1909, 2, p. 260). The extensor muscle of the knee (vasto-crureus), in a decerebrate cat, is isolated and connected to a tracing lever, similarly the flexor (semi-tendinosus). These muscles are connected to the nerve centres by their nerves, but the connections of all other muscles which might cause movement of the levers are severed. The upper one (c) of the two signal lines at the bottom of the figure indicates, by its rise, stimulation of the central end of an afferent nerve of the leg of the opposite side (contralateral peroneal) and the lower signal (i) marks stimulation of the corresponding nerve of the leg itself under observation (ipselateral). E marks the myograph tracing of the extensor, F that of the flexor. The lever attached to the extensor writes a few millimetres to the right of the lever attached to the flexor. As the preparation was decerebrate, the extensor muscle was in tonic contraction, but not the flexor, since decerebrate tonus affects the muscles of posture only (Sherrington, 1906, p. 302), which counteract gravity. The first stimulation is that of the contralateral nerve, which produces a flexion reflex. In this reflex we see that, along with the contraction of the flexor muscle, there is a marked inhibition of extensor tone, followed by a rebound (successive spinal induction), as described above. In the third stimulation, that of the ipselateral nerve, producing extension, inhibition of the flexor cannot show itself on account of the fact that the muscle is already in a state of relaxation, but it can be shown, indirectly, that the centres are inhibited. The middle stimulation will be referred to later. It is clear that the afferent nerve fibres proceed to the motor neurones of both the antagonist muscles, but, while exciting the one, they inhibit the other.

Examination of the figures will show also that, in the rebound contraction, sudden inhibition of the flexor contraction coincides with excitation of the .extensor muscle. Similar phenomena are observed in the movements of the eye brought about by stimulation of the cerebral cortex and in movements of the limbs produced in the same way. Sherrington concludes (1906, p. 285) that the seat of the inhibition in these particular reactions from the cortex is not in the cortex itself, but probably at the ultimate synapse with the final common path, that is, the motor neurone. Certain phenomena to be described later, however, indicate that, although the seat is very near to the final synapse, it is very likely in some intermediate synapse. The phenomena referred to relate to the action of strychnine and of chloroform. It is not to be concluded that, in many other cortical reactions, inhibition of one cortical element is not effected by other cortical elements, in fact, there is every reason to believe that this is the case.

Reciprocal co-ordination was observed by Sherrington (1906, p. 285) in the " willed " movements of the eyeballs in the monkey. The external rectus muscle is supplied by the sixth cranial nerve, so that, if on one side the third and fourth nerves, which supply all the other muscles, are cut, any movements of this eye are due only to changes in the state of contraction of the external rectus. If now an object is moved horizontally in such a way that its movement is followed by means of contraction of the external rectus of the normal eye, it is seen that the other eye also follows the movement. Since any movement of this eye must be effected by the external rectus alone, and the movement observed is such as to be brought about by relaxation, it follows that the tonus of its centre must be inhibited in accurate time and step with excitation of the external rectus of the opposite eye. It is therefore to be presumed that a similar process is going on with regard to the internal rectus of the normal eye, which works in conjunction with the external rectus of the other eye.

A kind of reciprocal innervation holds in two cases already dealt with. In the first of these, the " myenteric reflex " of the intestine, although the mechanism is peripheral, it appears to be of reflex nature. In the second, the opening and closing of the claw of the crayfish, the mechanism is not reflex, but of peripheral nature, as we have seen (page 425). A similar case to the latter is that of the action of the sympathetic nerve on the muscles of the iris. Waymouth Reid (1894) showed that, in this dilatation of the pupil, simultaneous contraction of the dilator, radial muscle and inhibition of tone of the sphincter, circular muscle takes place.

The tonic contraction of muscles concerned in the maintenance of posture has also been shown by Sherrington to be subject to reciprocal innervation. We shall see later that vasomotor and respiratory reflexes follow the same law. The simple case of the antagonistic muscles of the knee joint, acted on reflexly by stimulation of one whole afferent nerve, is not like the normal complex state of affairs, although it shows us the elements out of which the latter is constructed. Any particular motor centre is always more or less under a twofold influence of both excitation and of inhibition. This can be studied by taking a pair of antagonistic muscles and two afferent nerves, one having the opposite reflex effect to the other, as was done by Sherrington (1909, 2) in the work already referred to. If the two nerves are excited simultaneously, the effect on the movement of the joint depends on the relative strength of the two stimuli. By study of the rnyograph tracings, such, as those of Fig. 156, it is seen that the motor centre of each muscle is under a twofold influence ; the discharge of each represents the algebraic sum of the excitatory and inhibitory influences playing upon it. At a particular relative strength, both flexor and extensor centres may discharge, but neither discharge is as great as it would have been if the antagonistic inhibitory influence were absent (see the middle tracing of Fig. 156).

The study of this phenomenon, as Sherrington points out, shows the importance of inhibition, not only as suppressing excitation, but as a delicate adjuster of the intensity of reflex contraction, a method which is probably of frequent occurrence in natural movements. When the intensities of the two opposing influences on the same centre are nearly equally matched, a rhythmic discharge results. An indication of this is seen in the middle tracing of Fig. 156. If the movements of the right and left legs are observed under these conditions, flexors and extensors are seen to be alternating in contraction on the two sides, so that a stepping movement results ; when the one leg is flexed, the other is extended and vice versa. The following explanation is suggested by Sherrington (1913, 2, p. 98): "Reflex inhibition of a centre tends to superinduce in it a state of superactivity, rebound ; and conversely, as has long been known, the reflex excitation of a centre tends to superinduce in it a state of depressed activity, fatigue. It is therefore not surprising that when the two antagonistic influences are concurrently at work on a centre, and are nearly balanced, there should result a rhythmic oscillation of the two ; and presumably the rate of their alternation will depend largely on the nicety of balance, and on the intensity with which the processes are acting." Forbes (1912, 2, p. 287) points out that if we have two opposing forces, an increasing one (A) acting against a constant one (B), and if B is acting in some way to keep potential energy pent up, then, as soon as A becomes greater than B, the accumulated energy is released and becomes kinetic. The important point in the present connection is that, when once the release of energy has begun, it proceeds until more energy is released than is represented by the excess of A over B. A tank into which a stream of water is flowing and provided with an outlet at the bottom, closed by a spring, may serve as a rough illustration.

Forbes further shows that " biogen " molecules acting in accordance with the law of mass action alone, would only give a continuous response to a continuous stimulus. Also that The conversion, by strychnine, of central inhibition into excitation, and that of excitation into inhibition by chloroform, were described above (page 428). As we shall see in the next chapter, Magnus has shown that the extensor tonus of the muscles of " to develop rhythm of discharge there must be an approximation to the all-ornone law."

Graham Brown (1912, pp. 285-286) states that the phenomena observed by him cannot be accounted for either by a "drainage" theory, or by a metabolic one, nor again by the assumption of a self-generated antagonistic stimulus. He suggests that the respective centres of the corresponding muscles of the two sides act reciprocally on each other, the one in excitation inhibiting the other, although, as he admits, the nature of the inhibitory process is not explained thereby.

Further details of this interesting question will be found in the papers by Graham Brown (1912), Forbes (1912, 2) and Sherrington (1913, 1). Fig. 157 illustrates the phenomenon well. the limbs is greatly influenced by stimuli from the receptors of the labyrinth and the neck, and that this extensor tonus is associated with inhibition of flexors. Now, in this particular case of reciprocal inhibition, it has been shown by Magnus and Wolf (1913, p. 458), that the inhibitory component cannot be reversed even by so large a dose of strychnine that convulsions made further experiments impossible. These observations were made on triceps brachii and on vasto-crureus isolated. Since it was shown by Sherrington that a small dose of strychnine reverses the inhibitory component of reflexes from afferent nerves of the limb observed, in which these same muscles are employed, it follows that the same muscle in one reflex may respond with reversed inhibition (i.e., excitation), and in another reflex with normal inhibition. Magnus and Wolf rightly draw the conclusion that no " anatomical " scheme of connections can explain this fact. It may be that there are two independent synapses with the final common path, unequally sensitive to strychnine, or, in accordance with the conclusion to which I was led by my observations on vasomotor reflexes, that the drug acts on some intermediate synaptic membranes on the afferent side, synapses which are not part of the path common to the two different reflexes. One of these, that in the reflex arc of the afferent nerve from the limb itself, is more sensitive to the drug than those of the posture reflexes from" labyrinth and neck. But it seems that either hypothesis would suffice.

It will be clear, in any case, what havoc strychnine and tetanus toxin must play with reciprocal innervation in the organism. As Sherrington says (1905, p. 296) : " The sufferer is subjected to a disorder of co-ordination which, though not necessarily of itself accompanied by physical pain, must inflict on the mind, which still remains clear, a torture inexpressibly distressing. Each attempt to execute certain muscular acts of vital importance, such as the taking of food, is defeated because from the attempt results an act exactly the opposite to that intended. The endeavour to open the jaw to take food or drink induces closure of the jaw, because the normal inhibition of the stronger set of muscles— the closing muscles — is by the agent converted into excitation of them. Moreover, the various reflex arcs that cause inhibition of these muscles not only cause excitation of them instead, but are, periodically or more or less constantly, in a state of hyper-excitement ; and yet attempt on the part of the sufferer to restrain, to inhibit, their reflex reaction, instead of relaxing them, only heightens their excitation further, and thus exacerbates a rigidity or a convulsion already in progress." Sherrington thinks it probable that the action of the toxin of rabies lies in a similar effect on the mechanisms regulating swallowing and respiration.

Various reflexes use the same final common path for different purposes or for similar purposes. Afferent arcs which use it for different purposes cannot have possession of it simultaneously and they must take their turns, as it were. One reflex may defer or cut short another. This takes place even if they are both associated with excitation of the motor neurone, if they use the final common path in a different way, as regards time relations, and so on. - Such cases are the flexion reflex and the scratch reflex of the leg muscles. These are " antagonistic " reflexes. " Allied " reflexes act together and frequently reinforce one another.

The function of the receptors of the muscle itself, " proprio-ceptors " as we shall learn to call them, is of importance in this process. According to Sherrington (1906, p. 341, and 1909, 3, p. 155), their function is to cut short a reflex and prepare the arc for another one. Thus, a normal muscle, excited to reflex contraction, can be inhibited by stretching it, whereas, in a muscle deprived of its proprio-ceptive afferent fibres by section of the dorsal roots, this cannot be done.

Certain reflexes may combine together to form a definite co-ordination, which may be either simultaneous or successive. In the latter case, the result of one reaction excites another, and so on. Thus the reflex protrusion of the frog's tongue, excited by the sight of a fly, provides the stimulus (contact with the mucous membrane of the mouth) which causes closure of the mouth, swallowing of the fly, and so on, in series. In such cases as the scratch reflex, where the conditions can be readily controlled, each reflex increases the excitability of the reflex arc for the next succeeding one. The question of compound reflexes is a large one. Sherrington's book (1906, Chapters IV., V. and VI.) should be consulted.

Sinc*e, as we have seen (page 423), the seat of reflex fatigue is not in the final motor neurone itself, it is clear that the possession of this final common path by a new reflex is considerably affected by the fatigue of a previous reflex. The value of this fatigue of an intermediate synapse is to prevent too long possession of an effector by a particular reflex. Fatigue of a certain reflex enables a second one to obtain possession of the effector, although the stimulus exciting the former reflex may be still going on. The final motor neurone is comparatively incapable of fatigue.

Fatigue of a steady reflex, such as the flexion reflex of the knee, is first shown by its becoming rhythmic. Although a reflex arc is soon fatigued, it recovers again fairly rapidly ; its power of responding again may be very considerable even after ten seconds of rest. The diminution of its excitability is gradual, so that a weak stimulus ceases to be effective earlier than a strong one does. A reflex may cease either from fatigue or from inhibition. In a rhythmic reflex, such as the scratch reflex, the difference can be seen. In the former case, the beats become slower, and each beat is more prolonged and sluggish ; in the latter case, there is no change in rate nor in the duration of each beat. They are usually abolished altogether by inhibition, without any previous change, although the amplitude may sometimes be reduced.

Reflexes that protect an animal from injury are usually prepotent, that is, they displace others. The receptors are probably free nerve endings, since any form of nocuous stimulus is capable of exciting nerve fibres, and there is no need of recognition of the kind of stimulus. Great sensibility to small stimuli, so important in the higher senses, would be a disadvantage in this case. The scratch reflex in the spinal animal can be driven from possession of the final common path by a flexion reflex produced by a pin-prick in the foot. These nociceptive reflexes recover first after spinal transection. Thus, the abovementioned flexion reflex can be obtained earlier than that next described, namely,

When the spinal cord has considerably recovered from the shock of section, gentle pressure between the paws will often produce an extension reflex, in which the leg is straightened out. This effect is similar to that which is caused by contact with the ground in walking. Description of the great variety of individual reflexes would be out of place in this book. Those to the viscera, the heart, and the blood vessels are described in other chapters.

We may devote a few words to a peculiar reflex met with in certain Crustacea. If a crab be picked up by one of its ambulatory appendages, it generally, by a powerful muscular contraction, breaks this leg off at a particular place and so obtains freedom. This mechanism was first investigated by Fredericq and more recently by Roskam (1913). The second segment of the leg in the crab consists of two parts, which are distinct members in most Crustacea and united by a movable joint. In this animal, however, in place of a joint, there is a double membrane, whose two components are not very firmly united. In the middle of the membrane there is an aperture, through which the nerve and blood vessels pass. Certain muscles are so arranged that, by a powerful contraction, they separate apart the two layers of the membrane. Thus no soft parts are torn, except the nerve and blood vessels; there is practically no bleeding and the peripheral part of the appendage is rapidly regenerated.

There are two mammalian preparations which are very useful for the study of reflex action. The " decerebrate," described by Sherrington (1898), for which a cat is best, retains all parts of the central nervous system below the posterior colliculi and shows tonic rigidity of extensor muscles. It is therefore valuable for the investigation of inhibition. The other preparation is decapitated and therefore spinal only (Sherrington, 1909, 1). It is important that the operative procedures in both cases, especially the section of the crura or the spinal cord, should be done under deep anaesthesia ; a considerable amount of shock is thus avoided. The vagus nerves should also be divided previously ; unless, in the decerebrate preparation, they are required for the purpose of reflexes.

Pavlov (1910) states that he was struck by the fact that when the physiologist leaves the study of the simpler parts of the central nervous system, which he has investigated by the observation of reflexes, and proceeds to the higher parts, especially to the cerebral cortex, his methods suddenly change. He gives up observation of the relation between external phenomena and the reaction of the organism to them and introduces psychological ideas, derived from his own internal consciousness.

To extend to the higher centres the method of observing what changes in the organism are correlated with external changes might appear too difficult, but Pavlov has succeeded in doing so to a remarkable degree. The method used is that which he calls " conditioned " reflexes. Unfortunately, up to the present, the experimental results are not easy of access, most of the papers being published in Russian, and it is difficult to follow the train of argument apart from the actual facts.