Principles of General Physiology

Those functions which we know as " mental " have their seat in the cerebrum, especially in the cortex or pallium, if the philosophers will pardon the use of the word " seat " in this connection. It is, then, at first sight, a somewhat surprising fact that, by stimulation of certain limited regions of the cortex, definite, localised movements of limbs, face, trunk and so on can be evoked. The area which has these properties is known, for convenience, as the " motor area." Arising from this we have the system of long neurones known as the "pyramidal tract," con-

sisting of the axones of giant pyramidal cells, the " Betz cells," of a particular layer in the grey matter of the cortex. These axones form synapses with certain inter- Shows the elaborate system of association neurones, arranged as parallel or alternative paths between the primary sensory neurone" (S) and the final common paths (M). mediate neurones as far away as the distant end of the spinal cord and ultimately with the motor neurones of the final common path. These Betz cells are found to disappear when the axones forming the pyramidal tract are cut through. Fig. 148 (Holmes and May, 1909) shows this fact. The view that the pyramidal fibres do

not form synapses directly with the motor neurones of the cord, as was supposed at one time, but with intermediate neurones more on the afferent side, appears to be favoured by most neurologists at the present time. It should also be mentioned that the motor area has a different histological structure from that of other, non-excitable regions. The first definite proof that movements are evoked by electrical stimulation of particular regions of the cortex was given by Fritsch and Hitzig (1870) and was a very important advance in knowledge. It was supposed by many that " mental " functions were independent of the material constitution of the nervous system and insufficient credit is given to Gall for having propounded a more scientific view. It is true that his system was based on very superficial considerations, but it was Auguste Comte (1877, 3, 565-570) who first drew attention to the philosophical importance of his work.

When we call the area of the cortex from which movements can be excited, the motor area, it is not to be supposed that we use the words in the same sense as when applied to spinal motor neurones in the ventral horn of the grey matter. What we stimulate in the former case appears to be some part of a certain complex system of neurones, the activity of which implies a particular movement. So that, if we regard all that part of a complex arc up to the final motor neurone as belonging to the afferent side, we may speak of these cortical areas as " sensorimotor " or " kincesthetic " in accordance with the view of Bastian (see his book of 1880, pp. 584-588).

The work of Graham Brown and Sherrington (1913) shows that destruction of the motor cortex does not produce permanent paralysis of even delicate voluntary movements of the part whose " area " has been removed, even in an animal as high as the chimpanzee. The arm area on the left side was removed, with the usual result of paralysis of the right arm. In the course of four and a half months, recovery was so complete that no difference could be detected in the behaviour of the two arms. Now there are three explanations that might be suggested for the recovery.

1. Regeneration of the area destroyed. This is excluded by the fact that, six and a half months after the first operation, another operation was performed and the area in question was. found to be completely inexcitable. 2. Taking over of the movements of both arms by the corresponding area on the normal side of the cortex. To test this, four and a half months after the first operation, the arm area was destroyed on the right side. Although the immediate result of this was paralysis of the left arm, there was no change in the movements of the right arm, which had recovered from the previous paralysis. In two months more, complete recovery of both arms had taken place.

3. The post-central convolution, itself not motor, that is, not excitable by electrical stimulation, might have taken over the function of the arm area immediately in front of it. Two months after the second operation, this convolution was removed. At the operation it was found to be, as usual, inexcitable and its removal did not cause paralysis of voluntary movement, although for two or three weeks after the removal there was weakness in some movements, but this completely disappeared later.

The reactions to be obtained by stimulation of the motor cortex are, compared with those of spinal reflexes, much more modified by slight variations in the condition of the animal, blood supply, narcosis, etc. A systematic investigation of the reaction to be obtained by electrical stimulation of cortical points was made by Graham Brown and Sherrington (1912). They took two points, one giving primary flexion at the elbow, the other primary extension. The two antagonistic muscles, supinator longus and the humeral head of the triceps, were connected to levers for tracing. The effects obtained were very complex. Variable latency, various after-actions, such as rebound, tonic and clonic, mutual relations of great diversity as regards the pair of antagonists, show the high complexity of cortical reactions. Inhibition appears more prominent than excitation and seems to be independent of simultaneous excitation of the antagonist muscle, thus differing from typical reciprocal innervation of spinal reflexes to

be described in the next chapter. The same cortical point, after rest, yields very nearly the same result as it did on previous occasions ; but, if it be stimulated immediately after a previous response, the result is usually found to be reversed ; that is, a point giving excitation after rest gives inhibition if stimulated again after an excitatory response. Suppose, again, that a point gives extension of the elbow, and that then another point which gives flexion of the elbow is stimulated and finally stimulation of the extension point is repeated. It is usually found that the effect is reversed, giving flexion. In the decerebrate preparation, stimulation of an afferent nerve of the limb observed causes contraction of the flexors and inhibition of the extensors. With the cerebrum intact, the action of the cortical flexion point is augmented by stimulation of such an afferent nerve, and the effect of a cortical extension point is reversed to flexion ; so that, if the latter point were being stimulated and giving its normal extension effect, stimulation of the afferent nerve may reverse the effect to flexion, but the result depends, much on the relative strengths of the two stimulations. If two antagonistic cortical points are stimulated, there is some indication of algebraical summation of the opposed actions. The general conclusion is drawn that one of the special functions of the cortex is to reverse the factors of purely spinal or decerebrate reflexes, when necessary.

In connection with these results, the work of Osborne and Kilvington (1910) is of interest. One of the nerve cords of the left brachial plexus was cut, and its central stump joined to the peripheral stump of the corresponding cord of the right side. After time for regeneration, ten months, stimulation of the right motor area gave movements of both paws, although normally it gives movements of the left only. The left motor area was dead. A point of importance is that the natural co-ordinated movements of the limbs appeared to be quite normal, so that the conclusion is justified that the motor centres of the cortex can change their function. The part of the left motor area in the above experiment must have been assumed by that of the right side. Kennedy (1914) performed similar experiments. He joined together the nerves of the fore limb of the dog in such a way that both extensors and flexors were supplied by the same nerve, while the antagonist nerve was eliminated. After regeneration, the respective cortical centres were stimulated. That of the eliminated nerve was inexcitable, according to the usual rule. The other centre, which would normally have produced either flexion or extension only, according to the point excited, caused contraction of both antagonists, and at no part of the centre could contraction of either group alone be obtained.

Some experiments by Burnett (1912) serve to illustrate further points in the function of the cortex. The behaviour of frogs from which the cerebral hemispheres had been removed was much more machine-like and predictable than that of normal ones, although, on casual observation, there was not much difference to be detected, so long as they were not exposed to any new conditions. The normal and the decerebrate frogs were kept together in the same vivarium, and if flies were put in, the normal frogs were more skilful and accurate in capturing them. On the other hand, if a frog of each kind was removed, placed under a glass jar on the table, and flies added, the decerebrate frog captured them all in a few minutes, while the normal frog spent all his time in trying to escape from the holder or in a crouching position, apparently inhibited by fear.

The cortex of the cerebellum seems to be a supreme centre on the sensory side. With this view of Edinger, the work of Horsley and Clarke (1908) is in accord. No direct result is to be obtained from electrical stimulation. The motor centres in correspondence with the cerebellar cortex are in the dentate and other "basal" nuclei. Since no fresh neurones are formed during the life of an animal, and when the cell body of a neurone is destroyed no regeneration occurs, it will be obvious that

any new acquirement in reflex or association must be due to the formation of new connections between neurones already present. Memory thus implies the more or less permanent establishment of these connections. The possibility of disconnection at a later period must clearly be admitted. It appears that, in the lower organisms, such as insects, a habit may be formed by long training ; so that, for example, they may become able to find their way to food by a complex path. But suppose that the arrangement is altered back to the simple one for a time and then the complex one, to which the new adjustment has been formed, is returned to. It is clear that the length of time the new acquirement lasts can be tested ; and experiments have been made on the cockroach which show that about half an hour is the length of the time which this animal is able to remember what it has learned.

In the higher animals, new associations are formed, so far as we know, only in the cerebral cortex. The experiments of Burnett (1912), already referred to, showed that decerebrate frogs were unable to form even the simplest associations. In the lower animals there appears to be less centralisation. Yerkes (1912) found that an earthworm, which had been caused to form a habit of taking a particular course, did not lose the "memory" when the cerebral ganglia were removed.

With regard to the gratuitous introduction of such expressions as judgment, or decision by some sort of a controlling " mind," which it has been thought by some to be necessary to introduce even into the interpretation of the phenomena shown by some of the simplest nervous systems, the experiments of A. A. Moore (1910) on the starfish are to the point. The central nervous system of this organism is in the form of a ring, from which a nerve passes radially to each arm. If a simple cut be made across this ring, no break is made in the actual possibility of control of each arm by the centre. The fact that the arm next the cut does not co ordinate with the others in the righting movement proves that direct nervous connection across the place cut is necessary for "intelligent" co-operation. Any one arm can initiate impulses which affect strongly only adjacent arms and rapidly decrease as they travel from their point of origin. Yet this simple mechanism is sufficient to account for the complicated righting movements of the animal.

The paper by Carveth Read (1911) may be referred to in connection with the relations between instinct and intelligence. No reference has been made as yet to the nervous mechanism of speech and those other powers, such as reading and writing, which depend upon it. Early in the evolution of social communities we find means of some sort for the purpose of communication of signals of danger and so on. But very little is possible with inarticulate sounds and it is only when articulate speech, with a great variety of words having definite meanings, commenced that mental evolution made rapid strides.

For the cerebral centres and connections involved, the reader must be referred to the textbooks of Human Physiology and especially to the monograph by Mott (1910). In general terms, these methods have been described incidentally in the preceding pages. They may be broadly divided into those of stimulation and those of destruction of localised areas. The various histological methods of staining different kinds of tissue and of degenerated tracts are also of importance.

For accurate stimulation or destruction of localised spots in the interior of the brain, the "stereotaxic instrument" of R. H. Clarke (Horsley and Clarke, 1908, pp. 19-39) is of great value. This has been recently improved, but details of the latest form of the instrument have not yet been published. PLATE OF THE BASE OF THE HITMAN BRAIN, SHOWING THE CRANIAL NERVES AND THE CIRCLE OF WlLLIS. A, A, A, A, Cerebri quadripartiti anteriores posterioresque 0, O,

O, O, Nervi oculorum pathetici, sive par quartum. H, B, Nervorum par quintum. T, T, T, /, /, Nervorum par sextum. K, K, K, K, Nervi auditorii, et eorum utrinque bini pro- F, cessus, par septimum. L, L, I, I, I, etc., Par vagum, sive octavum, pluribus fibris W,W, (quae deorsum tendentes, in eundem a, a, a, a, truncum coalescunt) qui paulo supra processum occipitis emergit. anteriorem et posteriorem dividitur. Ejus ramus inter duos cerebri lobos incedens. Carotidnm rami anteriores uniti abscedunt, in

cerebri fissuram pergentes. Carotidum rami posteriores uniti, et trunco vertebrali occurrentes. Arteriae vertebrales, et earum tres rami ascen- DUBS glandulse pone infundibulum consitae. Protuberantia annularis qusD a cerebello dimis»a Before passing on to the consideration of certain questions relating to the innervation of the viscera and the blood vessels, a few words may be said as to the blood supply of the brain. As will be seen later, there is no adequate evidence that the cerebral vessels have any vasomotor control ; the importance of the brain is such that its circulation is regulated by the whole of the rest of the body, which is caused to accommodate itself, by constrictor and dilator nerves, to the needs of the brain (Bayliss and Hill, 1895).

A further interesting fact is the way in which, in the higher vertebrates, the main arterial supply is formed by cross connections between all the four arteries taking part, so that the interruption of one source does not deprive the brain of blood. This arrangement, known as the "circle of Willis," is shown in Fig. 149, which is a copy of one of Willis' plates (1680). This plate is of interest for two other reasons. It shows the numeration of the cranial nerves with which the name of Willis is associated, and it was drawn for him by his friend, Christopher Wren, who, as is said in the preface, " eruditissimis snis manibus delineare non fuit gravatus," ''did not think it too much trouble to draw with his skilful hands" many of the plates in the book.

The relation of the nervous supply of the viscera to that of the muscular and skeletal (somatic) components of the organisms was first made clear by the work of Gaskell (1886 and 1889). Gaskell's work on the innervation of the heart led him to see that the sympathetic nervous system, which consists of a chain of ganglia united by nerves with a particular region of the spinal cord, was not a separate nervous system, interchanging fibres with the cerebrospinal system, as had been taught, but was formed by certain fibres given off by the thoracic and upper part of the lumbar regions of the cord. These nerves consist of fine medullated fibres and form the white rami communicantes of the sympathetic ganglia. The grey rami were shown by Gaskell to be in reality peripheral nerves given off by cells in the sympathetic ganglia and distributed to the blood vessels of the spinal cord or its membranes. The white rami, then, form synapses in the ganglia with other neurones which have non-medullated axones, some of which pass to the spinal cord as grey rami, others are distributed to the viscera and blood vessels throughout the organism. The fibres of the white rami, however, do not all lose their medullary sheath in the sympathetic chain, but some of them do so in other, more peripheral, ganglia.

Further investigation, with this clue, led to the recognition of two other similar outflows of efferent, ganglionated, visceral nerves, one in the cranial nerves, the other in the sacral region. These latter were originally known as nervi errigentes, but, since they supply the bladder and rectum and other organs of the pelvis also, Gaskell called them " pelvic splanchnics " to show their uniformity with the abdominal splanchnics. These three outflows are separated by two gaps, where the nerve plexuses for the anterior and posterior limbs are found.

Langley (1898, p. 241), in order to obviate the confusion which might be caused by calling the sympathetic nerves which supply the skin, " visceral," proposed the name "autonomic," suggested to him by Professor Jebb. "The word implies a certain degree of independent action, but exercised under the control of a higher power." " The autonomic nervous system means the nervous system of the glands and of the involuntary muscles ; it governs the ' organic ' functions of the body."

It is necessary to be quite clear that the autonomic system includes the sympathetic, since some writers abroad use the name as applying to all the visceral nervous system other than the sympathetic, speaking of sympathetic and autonomic. This practice leads to confusion, as well as being JMijustified by facts. Although, as we shall see in Chapter XXIV., the sympathetic system is distinguished by certain peculiarities not shared by the rest of the autonomic system, it is convenient to have a name for the whole of the visceral system of nerves in contradistinction to the somatic svstem.

We see, then, that the autonomic system is in no sense an independent central nervous system, but an outflow from the cerebrospinal system, distinguished by its connection with neurones outside the central nervous system and by its formation of peripheral plexuses or networks at the places of its distribution. Auerbach's plexus in the intestine has, prima facie, more claim to be called an independent nervous system. Reflexes can be obtained from it, whereas no true reflexes are given by any peripheral ganglion. But, if Gaskell's view of the recent origin of the alimentary canal of the vertebrate be correct, Auerbach's plexus can scarcely be the remains of a primitive peripheral nervous system. It has been pointed out above that it has rather the characters of a synaptic system, and the work of Miss Abel (1909) is of importance also in the decision of the question. She investigated the embryology of this system in the bird and came to the conclusion that its development is secondary to that of the cerebrospinal system. This statement applies also to the whole of the sympathetic system, which shows itself to be derived from the central nervous system, being produced by migration of cells from the spinal cord, some of which pass downwards to the intestinal wall and there form the plexuses of Auerbach and of Meissner. It will be noted that, whereas the somatic nerves are formed by axones growing out from cells in the central nervous system, the autonomic system is formed by chains of cells growing out from the same system and forming axones subsequently.

The system just described is, as will be noticed, entirely efferent. Now, many of the nerves of the sympathetic system, such as the splanchnics, contain a considerable number of sensory fibres. These are not true sympathetic fibres, since they have their trophic centres in the dorsal root ganglia and simply take their course in the sympathetic rami. Certain special neurones have been described by Dogiel (1908, p. 133) as passing from the sympathetic chain to end in networks around some of the cells of the dorsal root ganglion, but their nature is uncertain.

The object of a nervous system is to bring any part of an organism into relation with any other part, without the necessity of direct nervous connections from every part to every other part. It is like a telephone exchange, where each subscriber has a central terminal, which can be put into connection with that of any other subscriber. In the nervous system, however, the channels which bring in messages from parts of the body (afferent fibres, coming from sense organs) are, as a general rule, different from those fibres (efferent) which convey messages outwards to organs in the body, which organs are thus caused to perform some kind of action (effectors). As Pavlov has pointed out, something must be sacrificed in the telephone system, since the same subscriber cannot speak to more than one other subscriber at the same time. The arrangements of the central nervous system are more efficient than this, since the same afferent fibre can at times be connected up with several efferent fibres.

There are thus two aspects under which the nerve centres can be studied. The one is, for the most part, morphological and consists in the following out of the tracts of fibres which connect its various parts together. The other consists in the investigation of the means by which functional connection is established for the performance of different co-ordinated actions. * There is reason to suppose that distinct effectors, as muscle cells, made their appearance in the course of evolution previously to nervous tissue. The receptor, in order to increase the sensitiveness to outer agencies, appears next in close connection with the effector and, as it becomes necessary for effectors at greater and greater distances from the receptor to be acted on, this receptor cell is prolonged in the form of a nerve fibre. Later, an adjuster cell is formed between the receptor and effector, giving the opportunity of connecting up various receptors and effectors together.

The constituent cells of the lowest central nervous systems, as that of the jelly-fish, seem to be in direct protoplasmic continuity with one another, forming a true network. But, very early in evolution, we find that this mode of connection ceases and the cells, now known as " neurones," although in functional continuity, are separated from each other where contact takes place, the " synapse," by a membrane, which plays a very important part in the mechanism of the reactions which take place in nerve centres.