Talk:Paper - An experimental investigation of the motor cortex and pyramidal tract of echidna aculeata (1939)

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AN EXPERIMENTAL INVESTIGATION OF THE MOTOR CORTEX AND PYRAMIDAL TRACT OF ECHIDNA ACULEATA

By F. GOLDBY University of Adelaide

‘Tue possession of an area of cortex, which on electrical stimulation gives rise to movement in contralateral parts of the body, is well known to be a common and probably universal characteristic of the mammalian forebrain. It is equally well known that this so-called ‘“‘motor area” contains cells whose axons form a long projection tract, ending, partly in the brain stem and partly in the spinal cord, in immediate or very close relationship with the neurones of the “final common path”’.

In some of the more primitive mammals, e.g. the American opossum, Didelphys virginiana, the excitable area of the cortex lacks clear definition, and will evoke movements only in approximately the forequarters of the body (Gray & Turner, 1924). The cortico-bulbar and cortico-spinal tracts in this and some other mammals have been shown to be small and probably to extend no farther than the upper cervical segments of the spinal cord (Turner, 1924). . Further, in modern reptiles, unequivocal evidence for the presence of an excitable cortical area is lacking, and there is no evidence at all for the presence of cortico-bulbar or cortico-spinal tracts.

These observations have made the condition of the surviving monotremes of particular interest. Although highly specialized animals, they retain many reptilian characteristics. One may well enquire, therefore, whether in the brain they have yet developed an excitable motor area, with the corresponding connexions with the brain stem and spinal cord, and if so, whether these structures are even more rudimentary than in primitive metatherian and eutherian mammals.

Although several descriptions, more or less complete, of the appearance and structure of the nervous system of both Ornithorhynchus and Echidna have appeared, they are nearly all based on the examination of normal material prepared by the usual neuro-histological techniques. Experimental studies have been few, and are in fact limited to two published papers. Martin (1898) published an account of the motor cortex of Ornithorhynchus, and Abbie (1938) a more detailed account in both Ornithorhynchus and Echidna. It is the object of the present communication to place on record the results of some further experiments on the Echidna. The work falls into two subdivisions: (i) an examination of the cerebral cortex by the method of electrical stimulation, the results of which will be seen to be, in the main, confirmatory of the work of

Anatomy LXxmI 33 510 F. Goldby

Abbie, and (ii) an examination of the connexions of the excitable area by the Marchi technique. It appears that this technique has not before been used successfully on either monotreme.

MATERIAL AND METHODS (i) Electrical stimulation of the cerebral cortex

The apparatus used was that described by Myers (1936) and used by Tower (1936) in her work on the extrapyramidal motor areas of the cat. Alternating current at 50 cycles was drawn from the mains and connected to a transformer with an output at 4, 8 or 16 V. The current from the transformer was allowed to flow continuously through a resistance coil of about 80 ohms, one terminal of which was connected to the indifferent electrode applied to the skin of the animal’s abdomen. The stimulating electrode, a platinum wire, was connected to a variable contact on the coils of the resistance, so that the stimulating voltage could be varied at will from 0 to 4, 8 or 16 V.

The operations were all conducted under open ether anaesthesia, the depth of which was just enough to produce moderate relaxation and to abolish the reaction of “rolling up” which is so sensitive in the unanaesthetized animal. The ordinary precautions were taken to avoid injury to the cortex or its blood supply, and the field under investigation was kept moist with warm Ringer's solution, except during actual stimulation.

The cortex was stimulated on one side in fifteen animals. In all but three the primary object of the experiment was the infliction of localized lesions with subsequent study of the resulting degenerations. Except in these three, therefore, only limited areas of cortex were exposed and explored. Results obtained from such limited exposures have a very restricted value, chiefly as confirmatory or otherwise of results obtained when a more complete exposure is possible. The number of hemispheres (three) in which exploration was reasonably complete is, of course, small. The results acquire greater significance, however, since they are consistent with the larger number of incomplete explorations, and with those of Abbie (1938) on eight specimens in which the whole of one or both hemispheres was exposed in each experiment.

The threshold stimulus was measured in volts. It was found that with 1 V., 0-1 mA. flowed through the stimulating circuit, with 2 V., 0-4 mA. Most of the threshold values for the excitable cortex in the different experiments lay between these limits. The apparatus was tested on cats and used in a series of experiments on the phalanger, Trichosurus vulpecula. The lowest threshold for the motor cortex of the cat was found to be 0-9 V., 0-1mA., and for the opossum 1-1 V. and 0-1mA. In all these experiments the anaesthetic was “open” ether. It is obvious that factors that can be controlled only’ very imperfectly or not at all, such as the depth of anaesthesia, the general condition of the animal and so on, may lead to considerable variations in the threshold value obtained in any individual experiment. The figures therefore can have Motor Cortex and Pyramidal Tract of Echidna 511

little value except as indicating that the threshold for the motor cortex of three very different mammals is in each case of the same order of magnitude. The actual figures obtained are closer than one would have expected.

(ii) Degeneration experiments

In all, ten experiments were performed with the object of tracing efferent fibres from the cortex by the Marchi method, but only one of these has proved successful. Of the ten, two died under the anaesthetic, and four survived the operation by less than 10 days. Of the remaining four, one died on the 27th day after operation, and when found was too decomposed to be of any value. Sections were prepared from three specimens: Echidna XIV, which died on the 13th day after operation, and was fixed within an hour of death, Echidna II, killed on the 28th day, and Echidna XIII, on the 44th day after operation.

Two techniques were used in the preparation of the material. One was the ordinary Marchi technique with preliminary fixation in 10 % formalin, followed by 10 days’ treatment in 3 % potassium bichromate, and finally by 21 days in the osmic acid-bichromate mixture. The other was the modification described by Swank & Davenport (1935). Here potassium bichromate is not used (it is replaced by potassium chlorate), so that the tissue does not become brittle and can be cut more easily. In addition artefact formation is less, and the sections will take a good counterstain with cresyl violet. This modification was tried in a preliminary experiment on a cat, and gave very satisfactory results.

In each specimen the whole brain was cut into slices 5 mm. or less in thickness, and similar slices were taken from three levels of the spinal cord: lower cervical, mid-thoracic and from the lumbo-sacral enlargement. The blocks were embedded in celloidin or paraffin, and as a general rule every fifth section mounted, in some regions more, in others less. The paraffin blocks were cut at 20, the celloidin at 40 py.

Echidna XIV, treated by the Swank & Davenport modification and cut in celloidin, gave entirely negative results. It had survived an extensive cortical lesion only 18 days. A serious disadvantage of the technique showed itself in this specimen, namely, patchy but intense staining of normal myelin sheaths. This can be avoided, according to the authors, by modifying the technique in certain directions to suit the material.

Echidna II, killed 28 days after operation, was treated in the same way, but cut in paraffin. Paraffin is not reeommended by Swank & Davenport, but I had found it satisfactory in the trial experiment on the cat. It is of course much more convenient. Again the result was entirely negative.

In Echidna XIII, killed on the 44th day from operation, alternate blocks were treated by the Swank & Davenport modification and by the ordinary Marchi technique outlined above; paraffin embedding was used for the former and celloidin for the latter on account of the brittleness of the tissue. Every block on which the Swank & Davenport modification was used showed no

33-2 512 F. Goldby

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osmic acid staining; all the others showed unequivocal positive results which are reported in the later part of this communication. The extent of the successful blocks is shown by the stippled bands in Fig. 2. It was fortunate that they included the crucial regions of the lesion itself, with the formation of .the cerebral peduncle, the level of the decussation, and the transition from brain stem to spinal cord.

It is probable that the failure of the Swank & Davenport method was due to the solubility of the products of degeneration in cedar wood oil and hot paraffin, and that the success in the preliminary experiment on a cat was an unfortunate accident. Similar results have been obtained in a parallel series of experiments on the phalanger, T'richosurus vulpecula. Potassium bichromate inhibits this solubility, but makes celloidin embedding advisable on account of its hardening action. The main disadvantage of the Swank & Davenport method, its tendency to stain normal fibres, could probably be overcome by experience. When the experimental animal is of an unusual type and can be obtained only in small numbers, it would seem best to keep to the standard Marchi technique.

Since Echidna II was embedded in paraffin, its negative evidence is of no value. With respect to the time necessary to produce degenerative changes that can be stained, it can be said only that it is longer than 18 and shorter than 44 days. It is probable that the longer period is advisable, although it adds considerably to the difficulty of the work on account of the heavy postoperative mortality.

The material is adequate for the study of the course of a long projection tract, extending throughout most or a considerable part of the nervous system. It is not adequate for the detailed study of short fibres intrinsic in the forebrain. Such evidence as could be found concerning the latter will be reported, but it must of necessity be incomplete. The usual criteria have been used in assessing the sections. Staining has been accepted as evidence of degeneration resulting from the lesion only when it was unilateral and consistently present throughout a series of sections. On these grounds, and allowing for the gaps in the series, it may be taken that positive evidence is reliable, but that negative

-evidence has comparatively little value. Artefact formation was not very conspicuous, especially in the forebrain and brain stem. It was more marked in the spinal cord, and particularly in the spinal nerve roots, which would have been subjected to some stretching and. other minor trauma in removing the cord. .

ELECTRICAL STIMULATION OF THE CEREBRAL CORTEX

In all the earlier experiments only the anterior half of one hemisphere was exposed, i.e. cortex which lay anterior to sulcus f. This region was inexcitable except with strong stimuli varying from 8 to 5 V. The results. of such strong stimulation were fairly uniform. Over the whole area, but more marked posteriorly and medially than elsewhere, they were retraction of the head, Motor Cortex and Pyramidal Tract of Echidna 513

accompanied by erection or depression of the spines over a transverse band about the level of the forelimbs. The erection of the spines was sometimes more marked on the heterolateral side. The only other movement seen in these experiments was obtained more easily over the anterior part of the frontal lobe and consisted of a depression of the snout with rotation of the head in one direction or the other, occasionally with slight movements of the jaws. In experiments in which nearly the whole hemisphere was exposed, these movements could be obtained on strong stimulation from practically the whole dorsal and lateral surface. They were also obtained from a necrotic area of cortex in the frontal lobe of an animal, anaesthetized with “dial”, that had been operated on four weeks previously. There is therefore no doubt that they are due to the spread of current beyond the cortex, probably to subcortical structures. They were not obtained by stimulating the dura, skull or other adjacent non-nervous structures. They could be obtained at a slightly lower threshold from the subcortical white matter.

Between the sulci « and B quite different results were obtained. The threshold stimulus in the three animals where exposure was nearly complete, was 1-5 V. in one and 1-1 V. in the two others. The excitable area between these sulci fell short of the mid-dorsal line by about 1 cm. and extended downwards to about the level of the pseudo-sylvian fissure. Cortex below this level was not exposed. The order and extent of the representation of movements of different parts of the body were found to be the same as those described and figured by Abbie (1938). The area from which movements of the hindlimbs were obtained, extended posterior to the upper part of sulcus « in one case. In another, the whole anterior boundary of the excitable area fell short of sulcus B by about 1 mm., a finding easily explicable in terms of the variability of this sulcus.

The movements themselves, not very accurately localized within the area in question, were often difficult and sometimes impossible to separate from one another. Movements of the tail (seen in only two animals), were combined with hindlimb movements; movements of the trunk with those of the limbs, usually the hindlimbs; and of the head and neck with those of the forelimbs. By keeping strictly to a threshold stimulus one could nearly always isolate

‘movements of either fore- or hindlimb, but only a very slight increase in the strength of stimulus led to extension of the movement either to the trunk or to the other limb. The hindlimb movements were constantly heterolateral, as also were the few movements of the tail and trunk. The forelimb movements showed a marked tendency to be bilateral, but were always more pronounced on, and sometimes confined to, the heterolateral side.

The movements obtained from the hindlimbs were varied. Extension of the whole limb was seen most often, sometimes combined with abduction and ‘medial rotation. Flexion of the limb was seen less often. In two cases extension was limited to the ankle joint and digits, and in one to the enlarged second digit alone (cf. Abbie, 1988, p. 148). 514 F. Goldby

In the forelimbs two types of movement were seen with about equal frequency: a flexion forwards of the shoulder, lifting the whole limb; or a complex movement consisting of weak extension of the limb combined with medial rotation and abduction. Isolated movements of the digits were not seen in the forelimb.

The movements of the tail were simple elevation or flexion to the opposite side; of the trunk, flexion, with the concavity towards the opposite side; and of the head and neck, elevation or flexion to one side or the other.

This description is based mainly on the three animals in which the cortical exposure was most complete. In eight other experiments, in which limited exposures were made of parts of the cortex outside the area between the sulci a and 8, the results of stimulation were uniformly negative, except with the high voltages already mentioned. Four others, with limited exposures including part of the area between the sulci « and 8, all gave positive results. Hindlimb movements were obtained most often because the upper part of the excitable area is more easily accessible. In two of these high thresholds were observed, 2-5 and 7 V. respectively. It is probable that 2-5 V. is within the limits of experimental error, and it may have been due to greater depth of anaesthesia, but it is difficult to account similarly for the very high reading of 7V.

In this series of experiments all regions of the cortex have been explored except for the posterior and ventral part of the temporal lobe, the temporal lobe below the level of the pseudo-sylvian fissure, and the whole medial surface of the hemisphere. It must be remembered too, that many of the sulci are very deep, so that a considerable area of cortex lies below the surface and is inaccessible to direct stimulation. No movements of the eyelids, jaws or tongue were obtained from the cortex in any experiment where the probability of spread of stimulus beyond the cortex could be excluded. This finding must be accepted only in relation to the limitations just mentioned.

It is clear that these results differ only in detail from those of Abbie (1938). They appear to show, for example, some predominance of extensor movements, and also of complex movements involving several segments of a limb. A predominance of extensor movements may be of significance in view of the fact that the normal resting posture of this animal is one of general flexion. Taken generally, however, differences are of a minor character and probably the result of unimportant variations in technique. One may conclude, therefore, that the Echidna possesses an electrically excitable ‘motor area” of cortex, the threshold stimulus for which is of the same order as that for the motor area of other mammals. The area as a whole can be defined with considerable precision ; there is an ill-defined localization within it for movements of the tail, hindlimb, trunk, forelimb and head, in that order, from above downwards. The regions from which these different movements can be elicited overlap considerably. The completeness with which movements of nearly the whole body are represented is remarkable, but the variety of movement is not great. These findings Motor Cortex and Pyramidal Tract of Echidna 515

differ from those in many primitive mammals; the most important points of difference are: (i) the completeness with which the whole body is represented, and (ii) the fact that the hindlimbs have at least as extensive and detailed a representation as the forelimbs.

FIBRE DEGENERATIONS FOLLOWING INJURY TO THE MOTOR CORTEX

The superficial extent of the cortical injury in Echidna XIII can be seen in Figs. 1 and 2. It occupies that part of the cortex between sulci « and B from which movements of the hindlimb and tail can be elicited, and encroaches on the region for movements of the trunk and forelimb. At the operation the region in question was identified by electrical stimulation, and shown to yield movements of both hind- and forelimb. In Figs. 4 and 5 the depth of the lesion is shown. It has opened the lateral ventricle and caused some damage to the alveus fibres of the hippocampus in its ventro-medial wall. There is no injury to the corpus striatum or other basal structures.

The following degenerations could be observed in the stained sections:

(i) Association fibres

A very fine diffuse degeneration in the deepest layers of the cortex and in the white matter immediately subjacent could be seen surrounding the lesion on all sides; medially it extended from the lesion into the medial wall of the hemisphere, and laterally and ventrally for between 1 and 2 mm. from the lesion (Fig. 4). Anteriorly, it could be followed a little beyond the level of Fig. 3 (see Fig. 2). This anterior extension is narrow transversely. It does not reach to the dorso-medial border of the hemisphere, and extends very little further ventrally than the level of the lower border of the lesion (Fig. 3). It can be distinguished from degeneration in the external capsule, being more superficial and generally of a finer and more diffuse nature. Behind the lesion it spreads out on the dorsal and medial aspects of the occipital lobe, and in this situation is more pronounced, containing a fair number of coarse granules.

This superficial degeneration seems to indicate the presence of association fibres, running predominantly in an antero-posterior direction. The coarser degeneration in the occipital lobe may be the result of damage to some of the fibres of the optic radiations. On account of the gaps in the series no more definite statement can be made about their distribution.

(ii) The external capsule

Degenerated fibres can be seen in the posterior 1/2 or 2/3 of the external capsule (they are shown in Figs. 3, 4 and 5). Many of these fibres run ventrally and anteriorly to enter the posterior part of the anterior commissure. They cross to the opposite hemisphere and are distributed to an area of cortex corresponding approximately to the lesion, but rather more extensive. Whether some have any other destination could not be determined from this material. 516 F. Goldby

Fig.

Fig.

Fig.

Fig. 3.

1. A dorsal view of the brain of Echidna XIII, drawn with the camera lucida, to show the position of the lesion. About natural size. The lettering and terminology of the sulci and fissures in this and subsequent figures are taken from Elliot Smith (1902); only those that are relevant to the description in the text are marked.

2. A right lateral view of the brain of Echidna XIII drawn with the camera lucida and about natural size. The lesion is obliquely shaded. The stippled bands show the position of the blocks in which Marchi staining was satisfactory (see text, p. 512). The oblique lines above the figure show the approximate levels from which Figs. 3-9 are drawn.

3. A transverse section about the middle of the anterior commissure ( x 2-5). This and other sections were drawn with the Edinger projection apparatus. In some places, owing to the buckling of the thin slices of brain, or to breaking of the very brittle tissue, the sections are not quite complete. Where this is so, the outline has been filled in with an interrupted line. Motor Cortex and Pyramidal Tract of Echidna 517

St.Med. Assn.F. (D)

Opt.Tr. B Rh.F.

Fig. 4. A transverse section through the anterior part of the lesion. x 2-5.

Assn.F. (D)

Rh.F.

Fig. 5. A transverse section through the posterior part of the lesion and showing the beginning of the cerebral peduncle. x 2-5. 518 F. Goldby

(iii) Cortico-thalamic connexions

Fine diffuse degeneration is seen in the lateral part of the thalamus and in some bundles of fibres that are entering it (Figs. 4 and 5). From Abbie’s figures (1934) it would appear to be confined mainly to his pars lateralis of the ventral nucleus of the thalamus.

(iv) The alveus system

There is marked degeneration in thealveus and fimbria anterior to the lesion. Posterior to the lesion degeneration is insignificant. From the fimbria degenerated fibres can be traced into the precommissural fornix system and into the column of the fornix on the same side. A few appear to cross and can be seen among the alveus fibres of the opposite side.

(v) Long projection fibres

As in many mammals, the internal capsule is not a compact mass of fibres, but is broken up into a large number of fascicles of varying sizes that run independently through the corpus striatum. They are seen in Fig. 8, anterior to the lesion, where they show no degenerative changes. In Fig. 4, a section through the anterior part of the lesion, they are larger, and many, coming from the part of the cortex that has been destroyed, show intense degeneration. In Fig. 5, near the posterior border of the lesion, they can be seen again, joining the beginning of the cerebral peduncle. Others, still in the corpus striatum, are more compactly arranged here, so that they form an illdefined internal capsule between the corpus striatum and the thalamus. It is probable that more degeneration should be seen here than is shown, but there was a small patch, just in this region, where the osmic acid had failed to penetrate. The continuity of degenerated bundles of fibres through the corpus striatum could easily be traced in sections from the anterior part of this block, of which Fig. 4 is an example.

Degenerated fibres in the cerebral peduncle in the mid-brain are shown in Fig. 6, at the level of the inferior corpora quadrigemina. They are scattered throughout the peduncle, but are more numerous towards its medial side. A little posterior to this level, but still anterior to the root of the trigeminal nerve, the decussation begins (Fig. 7). It lies mainly in that part of the pons, named by Elliot Smith, the rostrum, which projects forwards between the roots of the two trigeminal nerves. The interlacing bundles of normal and degenerated fibres are very clearly seen here. This decussation is surrounded by the cells of the pontine nuclei, a few of which are scattered among its fibres. The decussation and the pontine nuclei are covered superficially by a sheath of transversely directed fibres in which no degeneration occurs. These fibres can be traced into the middle cerebellar peduncle which runs into the cerebellum entirely behind the trigeminal nerve root. The distribution of the cells of the pontine nuclei and the destination of the superficial transverse pontine fibres were observed in Motor Cortex and Pyramidal Tract of Echidna 519

Fig. 10. Fig. 11.

Fig. 6. A section through the inferior corpora quadrigemina and the rostrum of the pons. x5.

Fig. 7. A section through the pontine decussation of the cortico-spinal tract. x5.

Fig. 8. A section through the medulla, showing some rootlets of the hypoglossal nerve, and the degeneration of cortico-spinal fibres in the ‘‘Zonalbiindel”. x5.

Fig. 9. A section at the level of the foramen magnum. x 7-5.

Fig. 10. A section through the 7th cervical segment of the spinal cord. x 7-5.

Fig. 11. A section through the 24th segment of the spinal cord. x 7-5. 520 . F. Goldby

two series of sections, transverse and sagittal, of the normal brain stem. The sections were stained alternately by toluidine blue and by Weil’s modification of the Weigert technique.

The decussation appears to be complete, and posterior to the pons degenerated fibres can be traced on the opposite side in the “‘ Zonalbiindel” of Kolliker (1901) or the “tractus temporo-trigeminalis”’ of Abbie (1984). They are shown here in Fig. 8 at the level of the exit of the hypoglossal nerve. They lie external to the fibres of the spinal root of the trigeminal nerve and are covered by the external arcuate fibres. There is some intermingling of degenerated fibres with those of the spinal root of the trigeminal nerve.

At the junction between the brain stem and the spinal cord the degenerated tract occupies a similar position, in close relationship with what is now the posterior horn of grey matter (Fig. 9). Its fibres are beginning to enter the lateral columns of the spinal cord and are shown in their final position in Fig. 10. They lie quite superficially in the most posterior part of the lateral column, and at this level (the 7th cervical segment) the tract is limited anteriorly by a longitudinal groove on the surface of the cord. The degenerated tract was seen in the same position in the 6th thoracic segment and again in the lumbo-sacral enlargement (Fig. 11). In this part of the cord it is smaller and less clearly defined. This last section was taken from the level of the 24th spinal nerve root. The Echidna has seven cervical, sixteen thoracic and two or three lumbar vertebrae, so that in terms of vertebrae the section corresponds to the 16th thoracic. No sections were cut behind this level.

DISCUSSION

The results of the degeneration experiment fall into two categories, (i) those which concern intrinsic fibres of the forebrain, and (ii) those concerning projection fibres which run to the brain stem and spinal cord. Results in the second category have a closer relation to the stimulation experiments and form the main subject of this communication. They will be considered first.

The cerebral injury involved an area of cortex from which movements of both fore- and hindlimbs were elicited by electrical stimulation. The most obvious interpretation of the degeneration that resulted, is to say that the injury has destroyed certain neurones in this cortical area, the axons of which form a long cortico-spinal tract; and that, although this tract differs in its position in the brain stem and in the level at which it decussates, it represents the pyramidal tract of other mammals. It is particularly well developed in the Echidna, reaching at least as far down as the 24th segment of the spinal cord. Its extent is indeed its most surprising feature, but is consistent with the observation of Abbie that movements of the tail and hindlimbs can be observed on stimulating the appropriate cortical area, and that they are abolished after section of the lower thoracic spinal cord.

Several views have been expressed concerning the position of the pyramidal tract and its decussation in the Echidna, all based on normal histological Motor Cortex and Pyramidal Tract of Echidna 521

material. These experiments confirm the description given by Fuse (1926a), and more recently by Addens & Kurotsu (1936) and Yamada (1938), all of whom state that Kolliker’s “ Zonalbiindel”’ is a laterally situated pyramidal tract which has decussated in the pons. Fuse (1926c) classified it as his second or mesencephalic type of decussation. Kolliker’s original suggestion, that the ‘‘Zonalbiindel”’ is an ascending sensory tract, is therefore wrong, and Abbie’s term for it, the ‘‘tractus temporo-trigeminalis’’, is inappropriate. They do not support the tentative suggestion put forward by Ziehen (1897 and 1908) and later amplified by Abbie (1934), that the pyramidal tract is small, and situated on the ventral surface of the medulla “in the position characteristic of all mammals” (Abbie). Neither do they support the suggestion put forward by Kolliker & Ziehen, and reported by Abbie as a definite finding, that the decussating fibres seen in the antero-median sulcus in the region of junction between the medulla and spinal cord, are pyramidal in nature.

The interpretation given above is no doubt in the main correct, but certain reservations must be made. In the first place the cortical injury was deep and destroyed the subjacent white matter down to the ventricle. Although in view of the results of electrical stimulation it is highly probable that the degenerated tract arose in the cortical area destroyed, the degeneration experiment alone does not prove it. It might have arisen in some other part of the cortex and have been interrupted as fibres of passage in the sub-cortical white matter. This is not very likely, as cortical projection fibres in general seem to take a short and direct course before they enter the corpus striatum, which was not injured.

Secondly, it was only the area of cortex concerned in movements of the forelimbs and parts of the body posterior to them that was damaged. The area for movements of the head and neck lies more laterally and ventrally and its projection fibres appear to run straight into the corpus striatum and to have escaped injury. It is probable that not the whole pyramidal tract but only its cortico-spinal part was damaged, and the resulting degeneration affects exclusively that part. It is therefore possible that cortico-bulbar fibres take a different course in the brain stem, in the position, for example, that Abbie (1984) described for the pyramidal tract. Provided they were few in number, or non-myelinated, they might also be accompanied by cortico-spinal fibres, the whole pyramidal system being duplicated as Addens & Kurotsu suggest. It is relevant to say here that Yamada (1988) has been unable to confirm in normal material the presence of the ventrally situated pyramidal tract of Abbie and of Addens & Kurotsu. The last two authors themselves were unable to trace this ill-defined tract cranially into the forebrain or caudally into the spinal cord.

It may be concluded, finally, that a well developed myelinated cerebrospinal tract exists in the Echidna, and that it almost certainly arises from the cerebral cortex between the sulci « and 8. After a short course in the cerebral peduncle it decussates completely in the upper part of the pons. It then runs caudally in KOlliker’s ‘‘ Zonalbiindel”’, and is continued in the lateral columns of the spinal cord at least as far as the 24th spinal segment. In view of the doubt 522 F. Goldby

that exists about the interpretation, from normal material, of a small and illdefined tract close to the mid-ventral line of the medulla, it would be inadvisable to postulate a duplication of the pyramidal tract or a separate course for the cortico-bulbar fibres. The final interpretation of this tract will depend on experimental investigation. This will probably be difficult, as the tract in question is small, consists at best of finely myelinated fibres, and is perhaps predominantly non-myelinated.

It is not intended here to discuss at any length the theoretical implications of these findings. It is of interest to note, however, that a high decussation of the pyramidal tract is particularly characteristic of a small number of highly specialized mammals, all probably of great phylogenetic age. Fuse (1926b), among others, has described a decussation just caudal to the pons in some bats and edentates. There is a tendency in some of them for fibres from this high decussation to take up a lateral position in the medulla, e.g. in an armadillo, Lysiurus unicinctus, and the pangolin, Manis tricuspis (Fuse, 1926b). In none is the decussation so high as in the Echidna, nor does the tract, after decussation, lie in such an extreme lateral position. It is unfortunate that clear information about the condition in Ornithorhynchus is lacking.

On the basis of these findings one may suggest that the earliest mammals possessed no more than a common tendency to develop a motor cortex and a pyramidal tract, and that this tendency has been realized in divergent primitive groups, independently, and in slightly different ways. Chiroptera, Edentata and Monotremata are all highly specialized, but probably developed their specializations at a very early period in mammalian evolution. They have escaped extinction by taking to the air, by covering themselves with armour or spines, or by a fortuitous isolation in inaccessible parts of the world. In them, perhaps, are preserved some early experiments in the development of a pyramidal tract, dating from a time when the mammalian nervous system was less stable than it is now.

The degeneration of fibres, intrinsic in the forebrain, needs little discussion. Although adequate for the elucidation of the main facts about the course of a long fibre tract extending throughout practically the whole nervous system, the material is not suitable for investigating the detailed distribution of comparatively short fibres. The degeneration in association fibres, in the anterior _ commissure, and in the alveus and fornix systems is such as would be expected from the nature of the lesion. There is nothing unexpected in it, and it brings to light no new facts of importance. In considering the cortico-thalamic degeneration it must be remembered that the time allowed between the operation and the killing of the animal was long, 44 days. This would be quite long enough in many mammals for retrograde degenerative changes to make their appearance in thalamo-cortical fibres. The degeneration seen here cannot therefore be taken as evidence determining the normal direction of conduction in these fibres. No more can be said than that they run between the cortex and the thalamus. Motor Cortex and Pyramidal Tract of Echidna 523

SUMMARY

A description has been given of the results of the electrical stimulation of the cortex of a number of specimens of Echidna aculeata.

The results in the main confirm those of Abbie (1938). An excitable area, far back between the sulci « and f, is present. In it, movements of the tail, hindlimb, trunk, forelimb and head and neck are represented, in that order, from above downwards. The threshold for effective stimulation is of the same order of magnitude as that for the motor cortex of other mammals.

Destruction of the upper part of this area leads to degeneration in a long cortico-spinal tract which has been demonstrated by the Marchi method. The tract runs through the cerebral peduncle, decussates in the pons, and continues in the “ Zonalbiindel” of Kolliker, superficial to the spinal root of the trigeminal nerve throughout the medulla. It enters the most posterior part of the lateral column of the spinal cord and has been traced as far caudally as the 24th spinal segment. No evidence was found for the presence of a pyramidal tract close to the mid-ventral line of the medulla, nor for a decussation in the usual position at the caudal end of the medulla.

Degeneration in intrinsic fibres of the forebrain, resulting from the same lesion, is also described. It affects association fibres, the anterior commissure, cortico-thalamic connexions and the alveus and fornix systems, but brings to light no new facts of importance.

ACKNOWLEDGEMENTS

It is a pleasure to acknowledge the willing assistance and co-operation of two senior students in the Anatomy Department of the University of Adelaide, who have been acting as part-time technicians. Mr A. D. Packer has assisted at all stages of the technical processes involved, and Mr I. G. Jarrett has been responsible for the preparation of most of the series of sections. I also have to thank the staff of the Physics Department for their help in making and calibrating the apparatus used for electrical stimulation.

ABBREVIATIONS

N.B. The letter (D) placed after any abbreviation indicates that the structure in question shows degenerative changes.

Alv. Alveus Fr. Fornix

Ant.Com. Anterior commissure Inf .C.Q. Inferior corpus quadrigeminum Assn.F. Association fibres Inf.Ol. Inferior olive

C.-Sp.Tr. — Cortico-spinal tract Int.Cap. Internal capsule

C.Str. Corpus striatum L.Lat. Lemniscus lateralis

Cer.Ped. Cerebral peduncle L.Med. Lemniscus medialis

Com.F. Commissural fibres N.C. Nucleus cuneatus

Dec. Decussation N.G. Nucleus gracilis

Ext.Arc.F. External arcuate fibres N.V. Nucleus of the spinal root of the

Ext.Cap. External capsule trigeminal nerve 524 F. Goldby

Opt.Tr. Optic tract S.Med.V. Superior medullary velum Ps.F. Pseudo-sylvian fissure St. Med. Stria medullaris

Rh.F. Rhinal fissure Th. Thalamus

Rost. Rostrum of pons Zb. ‘*Zonalbiindel”

S.Cb.Ped. Superior cerebellar peduncle

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