Paper - Further observations on the anatomy of the brain in the monotremata

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Smith GE. Further observations on the anatomy of the brain in the monotremata. (1899) J Anat Physiol. 33(2): 309–342.

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This historic 1899 paper by Smith is an early description of the platypus brain.



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Further Observations on the Anatomy of the Brain in the Monotremata

By G. Elliot Smith, B.A. (Cantab.), M.D. (Syd.), St John’s College, Cambridge. (PLATES X.—XII.)

IT had been my intention not to add to the number of communications which I have already contributed on this and allied subjects during the last four years, until I might be in a position to Write a. fairly‘ comprehensive monograph upon the whole central nervous system in the Monotremata and Marsupialia. But many of the statements which have been made in Professor Ziehen’s recent memoir upon this subject 1 cannot be allowed to remain unchallenged. In a Work which covers such a broad field, it is only natural that some errors of observation and interpretation should be made; and as the misinterpretation of the features of the brain in such an important order as the Monotremata is bound to have an influence upon the interpretation of the morphology in other groups of animals, no time should be lost in correcting such errors.

In his memoir Professor Ziehen honours my Work by referring to it on numerous occasions. In many places he points out What he considers errors on my part; but as We shall see in the course of this contribution, most of these discrepancies are to be explained by the fact that Ziehen’s material did not permit him to appreciate the true state of affairs, so that in imputing errors to me, he thereby proclaims on many occasions the inaccuracy of his own observations. The most peculiar instances of this are to be found in his description of the pans Varolii in Ornithorhg/nchus, in which he clearly overlooked the greater part of the true pans, and accused me of committing this mistake; and, again, in the confusion of the cranial nerves in Ornithorhg/nchus, which has led him to label the roots of the hypoglossal nerve “ seitl. gemischten Systems,” by which he means the spinal accessory series of nerve roots.

1 Theodor Ziehen, “ Das Centralnerrensystem der Monotremen und Marsup2'alz'er.” I. Theil. Makr. Anat. Semon’s Zool. Forschungsreisen, iii. Jenaische .Denkschrz'ft., vi. 1897.


Then, again, in other places he attributes to me statements of which I am completely innocent. The most noteworthy instance of this is to be found on page 7, in which he imputes to me the ridiculous statement that the locus pmgfomtus is identical in structure With the pyriform lobe, and then proceeds to disprove this fiction of his own imagination. '


In the present contribution I shall take advantage of the occasion which has thus been imposed upon me, and give a short account of the features of the brain in Ormworhynchus. Ziehen gives an excellent description of the gross anatomy of the brain in Echidna, but his account of the organ in Ornithorhynchus is not nearly so satisfactory, and hence I shall devotemost of my attention to the latter, and merely add comparative notes"to supplement Ziehen’s observations on the former. a .

I shall postpone the discussion of the brain in the Marsupialia until a later occasion, because the limits imposed upon this contribution forbid the adequate consideration of this subject here. As Ziehen does not attempt to give a full account of the spinal cord, I shall also postpone the consideration of this part of the Prototherian nervous system for the present.

General Account or the Brain in Ornithorhynchus

A brief account of the three figures which accompany this communication will convey a good general idea of the features of the brain in Ornithorhynchus. Then we shall be in a position to devote special attention to the parts of the subject which are of special interest and significance. ‘

I would not have been able to make these figures so complete as they are now if the generosity of my friends Professor J. T. Wilson and Mr James P. Hill of the University of Sydney had not kept me well supplied with numerous Well preserved brains of Platypus, in addition to large numbers of brains and spinal cords of Echidna and various Marsupials, the study of which has indirectly influenced my Work on 0rm'thorh3/nchus. To Professor Parsons of St Thomas’s Hospital I am also indebted for the head of an Ormlthorhynchus, which enabled me to make a dissection of the cranial nerves, and verify my diagnoses of them, which were so strangely at variance with Ziehen’sWork. To these three gentlemen I must express my sincerest thanks for their kindness.


The cerebrum of Ornithorhynchus, when viewed from above or from below, is of the conventional heart-shape, and presents a delusive resemblance, at -first sight, to that of a bird. When viewed from above, the cerebellum presents a broad lozengeshaped outline behind the cerebral hemispheres, and is slightly overlapped by the latter. Contrary to the opinion of Ziehen, the cerebellum completely hides the fourth ventricle, so that the opening of the central canal into the latter cannot be seen unless the cerebellum is displaced.


The dorsal aspect of the brain of Ornithorhynchus has been represented at various times by Meckel and Turner} but the best idea of the exact shape of the cerebral hemispheres and olfactory bulbs and their relation the one to the other will be gained from an examination of Grervais’ excellent figure of a cranial cast? In the recent text-book of Zoology by Parker and Haswell there is also a good figure?


These figures seem to indicate to what extent Ziehen’s figure (22) of the dorsal surface fails to represent the actual appearance, although I must admit that.the latter is the only figure which represents the central part of the cerebellum With any degree of accuracy. The projecting floccular lobes have been omitted from all these figures.


Fig. 1 represents a View of the ventral surface of the brain of Ornithorhynchus in which the kg/pophysis, the lobusflocculi on the left side, many of the cranial nerves on the right side, and the bulging of the left choroid plexus of the fourth ventricle have been removed in order to represent other features more clearly.


In front we find the ellipsoidal bulbus olfactorvkz/,3, into the cephalic pole of which, a compact bundle of nerve fibres, the olfactory nerve, is inserted. The bulb is attached to the under surface of the apex of the cerebral hemisphere by the flattened pedunculus olfactorms. This appears to split into two bands, Which diverge and embrace an elliptical protuberance of grey substance,

1 This Journal, vols. xxvi. and xxx.

2 Paul Gervais’ Mémoire sur les Formes Oérébrales propres aux Jllarszopiaux, Nouvelles Arch. du Mus., T. v., pl. 14, fig. 17.

3 Parker and Haswell, Text-book of Zoology, vol. ii. p. 547, fig. 1136. VOL. XXXIII. (N.s. VOL. XIII.) x 31


the ta/Jberculmn 0lfa.ct0r~imn. The mesial of the two bands immediately disappears from View in this figure by extending on to the inner surface of the hemisphere. The lateral band is continued backward on the Ventral surface of the hemisphere as the lobz/as pg/7°z;fbr~72z2's, which describes a sinuous course, and consists of an anterior are lying along the outer margin of the tvzobcirciizlizmz, olfccczforizmz, and a posterior arc which curves nearer the mesial plane, and becomes swollen to form a tubercle (lying in the figure below the pointing line to the corp?/3 7ncm2»mz'Zla7°c), just before the lobe disappears under cover of the hind-brain. The outer margin of the pyriform lobe is formed in itswhole length by the fissum 9'/z;£naZriz's, which anteriorly becomes continuous with the cleft which separates the olfactory bulb and its stalk from the apex of the cerebral hemisphere. The anterior are of the lobus pg/rfiomnvfs is also bounded on its mesial side by a deep cleft (fiss. cc), which is often called the endorhinal fissure.

Behind the zfubcirciizdmn olfactovrmm we find a quadrilateral depressed area, which constitutes the locus per/”oi7’atii2¢s aint'icv2z«3. It is separated from the floor of the ’tween-brain by an oblique furrow, which forms the anterior limit of a little mesial triangular area, which is bounded posteriorly by the optic chiasma. Retzius has described this little triangular pouch in the brain of many mammals and in the human foetus under the name “ area to7°7n«i72.ccl'is,” which I have adopted here} Its mesial portion is formed by the ventral extremity of the Z(m2'2'na ter7m)nalis.


Behind the optic chiasma the floor of the ’tween-brain is clepressed into an elongated sac-like If“lLZ70’I‘ ci7w7°cum in the mesial plane, and in its posterior part we see the cut-lips of the z'72fzm(ZiZmlmn (b), from which the /cg/pophy/32's has been removed.

On each side of the tuber cincrezmv, the sloping surface of the hy/potltalanzrus may be seen disappearing under the Zobus pf/7% fornzis. Emerging from under cover of the latter, one may see on each side the small oculo-motor nerve (III) proceeding obliquely across the izig/]90t/zailmrms toward the tube?‘ c2}ne7'emn. Behind the infundibulum we may see the ‘covyms 7ncm'mzz'll(w’e projecting in the depths of the deep cleft, which is caused by the head-bend in the embryo.

The large area of the base of the cerebral hemisphere, which lies upon the outer side of the fissura rhiualis, is the pallium. It is completely devoid of true fissures or sulci. Opposite the middle of the anterior arc of the lobus pyriformis a shallow furrow (f) proceeds obliquely outward and backward. In itthe middle cerebral artery is placed, and it may be analogous to the fossa S3/livid of other mammals. Opposite the hypothalamic region the ventral surface of the lobus pyriformis, and the adjacent area of pallium, which is shaded (Is), is grooved to lodge the ' ophthalmic and maxillary branches of the huge trigeminal nerve, as they proceed forward to their respective foramina in the floor of the cranium.

1 G. Retzius, “ Das Zlfcnschenlz-z'm,” 1896.



A deep depression (g) is found in the pallium on the ventral portion of its caudal border, and it will be noticed that upon the right’ side the lobus flocculi of the cerebellum lies in the corresponding ‘ floccular fossa,’ as we may call the indentation.

As the brain lies in the cranium, the lobus flocculi is placed‘ in a peculiar little bony cage, formed by the petrous temporal. It is really this bony structure which is lodged in the floccular fossa. Upon the ventral surface of the hind-brain we find an enormous swelling on each side of the mesial plane and a deep intervening vallecula. As this swelling is intimately associated with thefifth nerve, I have called it the tuberculum quinti. The anteriorextremity of this tubercle is full and rounded, and is separated from the cerebral hemisphere merely by a fold of dura mater, which forms part of the teutorium cercbelli.

In the caudal direction the tuberculum guinti gradually tapers, and hence appears as a large conical mass, lying upon the lateral aspect of the medulla oblongata. The pans Varolvji is a narrow strand which crosses the t-ubercula at their widest and most prominent part, and dips into the deep vallecula '£ntértubercularis. Here its antero-posterior extent becomes considerably increased, and its cephalic extremity projects forward toward the corpus mammillare as a beak-like process, which we may call the rostrum poutis. Emerging from the interval between the pre—pontal part of the tubcrculum quinti and the rostrum, we find the motor root of the fifth nerve, which proceeds transversely outward across the tubercle to join the ramus muudibularis of the fifth nerve, as is shown in the enlarged scheme (fig. 1).


The sensory root of the fifth nerve is subdivided into two parts, a lateral (the ram/as mandwualamls) and a mesial (the ramas ma;eilla7°z's)—the conjoined maxillary and ophthalmic branches——which are inserted separately into the tubereulum qainti, in from‘ of the pans. [Compare ‘the left side of the fig. 1 with the scheme fig. 1.] The large projecting lobus floceulziw of the cerebellum is seen lying upon the pons and the posterior part of the cerebral hemisphere on the right side, while on the left side its cut-stalk (h) is seen just on the outer side of the seventh and eighth nerves, which are attached to the medulla oblongata in their usual situation. Immediately below these nerves we find a line of roots belonging to the ninth, tenth, and eleventh nerves, which present the usual arrangement and situation. On the right side the choroidal fold (c) of the fourth ventricle may be seen projecting beyond the lateral margin of the medulla on the cerebellum. The delicate sixth nerve emerges from the vallecala 'inte7'tabe7*culam's a short distance behind the pons.

The anterior pyramidal tracts are flat and insignificant, and the decussating fibres are not collected into compact bundles, but are scattered over a length of 5 mm. The hypoglossal nerve (XII) arises in the usual manner by a series of roots. which emerge at the lateral margin of the pyramids. These roots extend as low as the ventral root (c1) of the first cervical nerve, which emerges at the lateral border of a very distinct anterior column.


Fig. 2 represents that surface of the brain of Omitlzlorhynchus which is exposed by a mesial sagittal section. In front we again see the bulb?/.3 olfactomlas projecting beyond the apex of the hemisphere, from which it is separated by a deep cleft. Behind the lower part of the bulb we see the white mesial surface of the olfactory peduncle continued obliquely upward above the tabereulam olfactomtm into continuity with the white district to which I gave the name area moecommissaralis. The latter proceeds backward as far as the thickened lamina terminalis, which contains the two round bundles of fibres, which constitute the dorsal and the ventral commissures. The lamina terminalis bends backward near its ventral extremity to reach the optic chiasma, and we can now understand how this part of the lamina termelnalis forms the area term1Inalz's in front of the optic chiasma upon the base of the brain (fig. 1). In the interval between the tubcrculum olfactorium and the lamina tcrminalis the precommissural area becomes continuous with the locus pcrforatus anticus (fig. 1).


Immediately in front of the upper extremity of the lamina tcrminalis the precommissural area becomes continuous with the cephalic extremity of the fascia dentata, which makes its appearance as a fusiform band above and in front of the commissura dorsalis. The fascia clcntata is limited on its dorsal side by the hippocampal fissure, which separates it from the broad area of pallium. The pallium forms the greater part of the mesial surface of the hemisphere which extends backward over the optic thalamus, mcsenccphalon, and part of the cerebellum. From the cephalic extremity of the fissura hippocampi a shallow furrow proceeds obliquely forward to the deep cleft which separates the olfactory bulb from the pallium. This furrow separates the pallium from the precommissural area, and may be distinguished as the sulcus limitans pallii.


The anterior cerebral artery, after crossing the locus pcrforatus anticus, proceeds upwards across the area proecommissuralis, upon which it produces a furrow (m), and then reaches the hippocampal fissure, in which it proceeds backward. It gives off three very constant branches, which produce deep depressions in the pallium (m) as they traverse its surface to reach the dorsal aspect of the hemisphere.


The thickened lamina temninalis (containing the two commissures of the cerebral hemispheres) forms the anterior wall of the third ventricle. The roof of this cavity is formed by a thin epithelial fold, which has been represented semi-diagrammatically in the figure (tcla) in order to avoid confusion. Its complex folds, which form the choroid plexus of the third ventricle, have not been represented, for the sake of clearness. The epithelial roof is attached to the cephalic corner of the upper thickened part of the lamina tcrminalis. In other words, it proceeds forwards, and forms a small rcccssus superior, which extends above the commissura superior incthe same manner as is common in the Reptilia and Amphibia. w


Posteriorly, the roof is attached to the little cord of transverse fibres which constitute the commissum habenularum or commvlssura superior of Osborn, and which is attached to the anterior extremity of the little solid cpiphg/sis, into which only a‘very slight recessus pinealis extends.

The floor of the ventricle is formed by the horizontal lower part of the lamina termlnalis and the sac-like tuber cinercum, to which the conical bilobed hypophysis is attached posteridrly. [The downward extension of the tuber cinereum is slightly exaggerated in fig. 2.] Extending upward from the posterior part of the kg/pophg/sis, we find a thin lamina ascending to the large oval section of the corpus mammillarc. This thin inframammillary lamina corresponds to the cmincntia saccularls of G. Retzius, which, according to him, represents the saccus rasculosus of fishes}

The corpus mammillare becomes continuous dorsally with the mesially fused tegmental mass, which arches upward and forms the floor of the aguocductus Sylmll.

This anterior surface of the fused tcgmenta, together with the corpus mamnrlllare and what we may call the lamina saccularis, form the great part of the posterior wall of the third ventricle. The dorsal lip of the wide open anterior mouth of the Sylvian aqueduct is formed by the cornmissura posterior.

The cavity of the third ventricle has the customary vertical slit-like form. Immediately in front of the wide mouth of the aguccductus S3/lrivl we find the large mass of the commvlsssura mollis bridging across the cavity. In the interval between it and the epzpltg/sis we find upon the lateral wall of the ventricle the large pear-shaped bulging of the ganglion habenulae (habenul). From the apex of the latter the large cord—like stria medullartls extends forward and arches downward, being here separated from the commissura dorsalis by the foramen Jllonrol. Around the lower margin of the foramen of Monro it appears to be continuous with the columna fornlcis, but below this point neither of these fibre-bundles is visible in the mesial wall of the ventricle. [For the actual relations of these two tracts of fibres in this region, see fig. 6 in my memoir in last year’s volume of this Journal, vol. xxxii. p. 36.] .


1 Gustav Retzius, “Das Mcnschenh1.'rn,” 1896, and Biologisch. Unters-uch., 1895.

Above the foramen of Monro we can see the fimbria extending backward from the region of the dorsal commissure to reach the margin of the hippocampal formation. Above the area termiualis there is‘ a depression in the lateral wall of the third ventricle; this is the rcccssus opticus. The floor of the tuber ciucrcum is also bulged out laterally in a sac-like manner. There is a vertical furrow on the lateral wall of the ventricle below the commissura mollis.

The roof of the aqueduct of Sylvius is formed by the quadri geminal plate, and the broad flattened anterior and the small posterior quadrigeminal bodies may be seen projecting upon the lateral side of the middle line. The cerebellum has been drawn backward in the figure in order to expose the posterior quadrigeminal body.

The tegmeu mcscncepkali is joined to the ventral surface of the cerebellum by means of the vclum medullarc in the ordinary manner. The minute fourth nerves (which are not shown in this figure) present the usual relations.


A very deep cleft separates the floor of the third ventricle from the region of the pons Varolii. In the roof of this cleft we find a large corpus (or ganglion) iutcrpcduuculare, which is opposite to the corpus mammillare. The small third nerve, which is 11ot shown in this figure, issues in its usual position immediately in front of the interpeduncular body. The rostrum of the pons comes into contact with the corpus z°utcrped~uucularc. Emerging in front of the pons we see the large ramus maarillarwis and the smaller motor root of the fifth nerve.

In the depths of the interval between the corpus mammillarc and the pons we can see the tubercle of the pyriform lobe [which is not labelled]. The anterior part of the pyriform lobe can be seen in front of the tuber ciucrcum.

I shall not enter into a detailed account of the cerebellum in this place, but shall deal with it in a separate paper subsequently. We may merely note in passing how markedly it differs from the conformation of the organ in all the Meta- and Eutheria. T y . .

Fig. 3 represents the lateral aspect of the brain of 0ru'£thorhyuchus, in which the postero-inferior part of the ovoid cerebral hemisphere has been dissected away together with part of the optic thalamus [leaving the cut surface 3/ exposed] in order show the lateral surface of the brain stem which is overlapped by the hemisphere.


Once again we see the lmlbus olfactomas projecting forward beyond the apex of the hemisphere proper. But this wall of the bulb is deeply invaginated along. an oblique line (ac) in a manner which I have described and figured elsewhere} Analogous involutions of the whole thickness of the wall of the bulb producing ‘ complete fissures ’ arenot unknown among placental mammals; for I have recently described such an involution upon the ventral surface of the bulb in the African Aard-vark (Org/cterop-213).?

Behind the bulb we see the anterior part of the pyriform lobe, its surface whitened by the thin layer of fibres of the tractas olfactomus. The pyriform lobe disappears from view immediately below the fossa S3/lm'i (f). Below the pyriform lobe we again see the tabewculam olfactomatm as a narrow fusiform area.

Beginning at. the bottom of the deep cleft between the olfactory bulb and the hemisphere, a deep groove ascends obliquely across the pallium, and divides into a branching system of channels,which become gradually fainter. These are caused by the peculiar large vein which drains away most of the blood from the brain, and which I have called the oena magna cercbvri. The groove which it produces I have called the salcas oenaz magnce. The distribution of these vessels has been admirably represented by Gervais’ figure 3 of a cranial cast.

Behind the upper part of the cut surface of the optic thalamus (y) We see the coppora qaadmgevrtina. Upon the posterior part of the lateral surface of the broad fiat natcs We find a peculiar little knob. Behind and on a lower level than the nates, we find the relatively small and inconspicuous testes, overhung by the projecting beak-like process of the cerebellum. Extending obliquely forward from the testes to the cut surface of the optic thalamus We can see the posterior brachtam (which is not labelled). Below the braclciam and testes We see the lateral surface of the mid-brain, which is placed very obliquely, or even almost vertically. The ventral or cephalic half of this surface is covered by a thin white coat of longitudinal fibres, which represent the pcs of the crus cc7°cbri_ and the lcmmlscus, which to the naked eye are indistinguishable the one from the other. For so diminutive are the proportions of the crus ccrcbri, that its fibres do not form any definite projection on the surface. The mdst dorsal (caudal) of this group of fibres, which probably belong to the lcmmlscus, proceed toward the posterior brachium.


1 Elliot Smith, Anat. Anz._, Bd. xi. N1‘. 6, 1895, p. 165, figs. 5 and 6.

2 Elliot Smith, “ The Brain in Edentata,” Trans. Linn. Society, London, 2nd set-ies,—“ Zoology,” vol. vii., 1898. B 1 3 P. Gervais, lac. cz't., pl. 14, fig. 17.


The delicate trochlear nerve (IV) may be seen issuing in the usual manner behind the nates, and proceeding forward on the surface of the tegmentum. sThe larger oculo-motor nerve (III) may be seen to issue from the angle between the crus ccrcbri and the cut surface (y) of the thalamus. Immediately in front of the figure III we can see the lateral surface of the corpus mammillam below the cut surface of the thalamus, and below this again we can see the cvmucutvla sacculams of Retzius

The oculo-motor nerve, in its ventral course to reach the situation in which we have seen it on the base of the brain (fig. 1, III), proceeds across the lateral surfaces of the corpus mamm1.'llcw°c and the cmiueutia saccularis. The statement of Ziehen, that in the Monotremes the third nerve appears to spring relatively far forward in the brain} is not true. In both Monotremes the third nerve arises and follows a course immediately in front of the corpus 'iute7°pcnluucula7°c, exactly as it does in other mammals. In fact K6lliker’s figure 636 2 might have been drawn from a section of this region in Oruithorhg/uchus or Eclm'dua,'instead of in the rabbit (Lmus). The lateral surface of the mesencephalon or tegmentum is directly continuous with the large swelling of the tubcmulum quoluti. But a deep cleft separates the latter from the ventral part of the tcgmcutum, where this is coated by the fibres of the crus cc7*cb7°7I and lcmmscus.

The part of the cerebral hemisphere that has been removed in this preparation would be lodged in a deep vallecula, 65 mm. deep and 35 mm. wide, which intervenes between the tuberculum guinti and the optic thalamus.

1 Th.‘ Ziehen, loc. c7It., Jena. Denksch., p. 147. _ 2 A. Kolliker, “ Handb. dcr Gcwcbclehrc dcs Mcns.-hen,” II. Hilfte, Leipzig, 1896, p. 505.


Inferiorly, We can see the insertions of the two ‘huge sensory divisions of the fifth nerve into their tubercle immediately in front of the pons. The latter ascends between the mmus mandibularis and the facial (VII) and auditory (VIII) nerves to enter the cerebellum under cover of the lolms flocculi. [The nerves VII and VIII, which lie in the shadow of the flocc'ulus, have unfortunately become obscured in the reproduction of the drawing.] The line of roots of the glossopharyngeal (IX) and vagus (X) nerves may be seen in their usual position along a line joining the facial (VII) nerve to the spinal accessory (XI). The hypoglossal nerve (XII) may be seen coming around the tuberculum quinti in order to associate itself with the IX, X, and XI nerves, and all four nerves leave the cranium by the one large ‘ jugular ’ foramen.

On the Superficial Origins of the Cranial Nerves

The first nerve is in Ormlthorhynchus a single definite bundle of fibres inserted into the apex of the bulbus olfactorius in the usual Saurian manner.

In Echidna we find the nerve split up, as is customary in the mammalia, into bundles of fibres which ascend through a cribriform plate, and are inserted into the ventral surface of a large flat foot-like bulbus olfactorius.

The optic nerves are very small in both Monotremes, and especially in 0rnitho'rhynchus, and the third, fourth, and sixth nerves are of correspondingly diminutive sizes, but present the typical superficial origins. I

The trigeminal nerve is peculiar in both Monotremes in being inserted Wholly in front of the pans. Garner’s statement that they are inserted behind the pons is obviously erroneous} Otherwise the nerve in Echidna presents the usual mammalian features. The extraordinary peculiarities of the fifth nerve in Platypus will be described in detail further on in this account.


1 Robert Garner, British Ass. Reports, 1858, p. 123,-" Zoology.”

The superficial origins of the facial and auditory nerves are found in the usual situation in both Monotremes. Ziehen could not find the facialis in 07°m°th0rhynchus, and suggested_ that it may have been pushed nearer the middle line by the enormous development of the fifth nerve} This is not the case, for the nerve is placed in close contact to the mesial side of the auditory nerve at the lower border of the pons. , In his figure of the ventral surface of the brain of Ornithorhynchus Ziehen labels the roots of "the hypoglossal (XII) nerve “Wurzellinie des seitl. gemischten Systems,”2 and explains in the text that by this name he refers to the accessorius (page 49). In the same figure he refers to a point some distance external to the so-called accessomlus by the figure ‘ XII ’. This is obviously a misprint. The hypoglossal nerve in the brains of both Omvjtkorlzg/nchu.9 and Echidna appear after the usual manner in the groove at the lateral border of the pyramidal tract (fig. 1). Ziehen does not represent the vagus and glossopharyngeal nerves in this figure, and his account of them in the text is vague and unsatisfactory (pp. 49 and 50). But elsewhere he makes this extraordinary statement :—“ Die nerven des seitlichen gemischten Systems entspringen dementsprechend—-wiederum namentlich bei 0rm'thorhynchus—viel weiter ventromedial wiirts als bei den iibrigen Saugern.” 3


He does not give any hint as to‘ whether this vague statement refers to the accessorius only, or to the IX, X, and XI nerves. It harmonises to such an extent with his suggestion concerning the facialis that perhaps he may refer to the IX and X nerves also. To this we may add that the ninth, tenth, and eleventh nerves are arranged in both Monotremes in the same manner as they are disposed in other mammals, and the enormous development of the fifth nerve has not appreciably altered their positions (see figs. 1 and 3). The eleventh nerve can be followed with a lens as far as the neighbourhood of the fifth cervical nerve roots, and in 0-7'm'th07°hg/nchus its extent is 30 mm.

1 Ziehen, loc. cz°t., p. 47. T 9‘ Ziehen, loc. cz't., fig. 23, p. 34. _ 3 Ziehen, Anat. Anz., xiii. Bd., No. 6, 1897, p. 173.

The Region of the Medulla Oblongata and Pons Varolii

The peculiarities of this region of the brain in Ornithorhg/nchus may be mainly attributed to the following factors :

(1) The enormous development of the trigeminal nerve.

(2) The diminutive proportions of the bundle of fibres which forms the pans Varolvké.

(3) The peculiar relation of the roots of the fifth nerve and the associated masses of grey matter to the pons.

(4) The diminutive proportions of the pyramidal tracts and the scattering of its decussating fibres. '

In Eckidmt the trigeminal nerve is also large, but in comparison with that of Platypus it seems small. In Echidna the pons is not much bigger than it is in Ovwithorhynchus, and the pyramidal tracts present similar features in the two genera.


Recent research has clearly demonstrated that a large proportion of the sensory fibres of the fifth nerve terminate in the neighbourhood of their insertion into the hind—brain by forming synapses with the nerve cells of a mass of substantia gelatinosa which collects around the incoming fibres.-1 In the brain of Ormlthorhynchus this mass of substantia gelatsinosa (which, with a total disregard for accuracy of expression, is commonly called the ‘sensory nucleus’ or even the ‘origin’ of the fifth nerve) attains enormous dimensions in correspondence with the large size of the nerve. So great is this mass of substantvja gelatinosa that it cannot be hidden under the diminutive pons (as it is in other mammals), but bulges out as a prominent swelling both on the caudal and cephalic sides of the pons; and as this swelling presents relations in Platypus for which no exact analogy is found in any other animal, not even in Echidna, I have called it the t~ube7°oulum qmIn25i.

Many of the fibres of the fifth nerve do not end in this principal synaptic ganglion (if I may coin a new expression) of the nerve, but descend through the medulla and upper part of the spinal cord, and end freely in a downward continuation of the principal synaptic ganglion, which produces upon the surface a swelling usually known as the tuberculum cinereum Rolcmdvl. The spinal root of the fifth nerve is arranged on a similar plan in Ornithorkynchus, and the tubercle of Rolando forms a huge pyramidal swelling, the apex of which is placed upon the lateral aspect of the upper part of the spinal cord. VVe readily recognise (figs. 1, 2, and 3) that the tubercle of Rolando in Platypus is identical with the post-pontal part of the tuberculum Quinta’.


1 S. Ramon y Cajal, “Beitrag zmn Stadium dcr Mcdulla Oblongata,” etc., German translation by Bresler, Leipsig, 1896. 2 This Journal, vol. xxx. p. 487.


The latter attains such large dimensions that it forms the greater part of the ventral surface of the medulla. A transverse section through the medulla oblongata opposite the greatest prominence‘ of the tuberculum gwinti presents a very peculiar appearance. For it consists of two large circular masses 8 mm. in diameter, joined the one to the other by a bridge 5'5 mm. thick and 2 mm. wide, in such a manner that the dorsal surface, which is the floor of the fourth ventricle, is flat.


The trigeminal nerve has in 07°mIth07'hynchus three distinct roots, which I have distinguished as the mmus mandibularils, mmus maaallaris, and radix nzotorius respectively. The mandibular root is inserted quite separately from the rest of the sensory root (figs. 1 and 3, V. mm. mcmdib.) into the tuberculum gmlntvl immediately in front of the lateral half of the pons, and measures at the situation of its insertion 6'5 mm.+ 15 mm. (in a specimen which had been preserved in M1"1ller’s fluid and subsequently in alcohol). The maxillary trunk represents the conjoined roots of the nervus maxillamls and the nemms ophthalmricus of the trigeminal; but as both these nerves are destined mainly for the supply of the upper jaw, I have called the trunk the ramus mazzcillarvls (figs. 1, 2, and 3). It is inserted into the rounded apex of the pre-pontal part of the tuberculum quinti, slightly in front and to the mesial side of the mmws mandibularis (fig. 1, V. mm. ma:mIll.). But just in front of their insertion these fibres are collected into a much more compact bundle, which presents a cross-section of 6 mm. X 4 mm. (figs. 1’, 2, and 3). This maxillary root proceeds directly forward, and produces a-deep groove (fig. 1, Is) upon the ventral surface of the hemisphere, involving both the pallium and the lobus pyr73fo7°mz's. While the mmus maa3z'lla.7°7Is is placed in this groove, which it makes for itself, it divides into two branches —a smaller mesial ophthalmic nerve and a larger lateral maxillary nerve-—which Meckel has accurately represented} These two nerves almost immediately leave the cranium through their appropriate foramina, and their further course can be readily understood from the examination of even a macerated skull. It will then be seen that the ophthalmic nerve, as it passes forward in the temporo-orbital fossa, is separated by merely a thin plate of bone from the olfactory bulb, which is contained in its own recess of the cranial cavity. And the maxillary nerve is placed in the temporo-orbitalfossa immediately on the ventro-lateral aspect of the ophthalmic nerve. The relation of these -two branches of the trigeminal nerve bulb to the olfactory bulb is well shown in a figure which Dr Hill has given} In his description of the brain Hill says“To the outer side of the [olfactory] bulb lies the optic nerve, and to the outer side of this again the immense fifth nerve” (page 375 . “Optic” is obviously a misprint for “ophthalmic” in this quotation. I


1 Meckel, “Ornithorhynchi paradoxi Descriptio Anatomica,” Leipzig, 1826, tab. vii. fig. 4.


The situation of the motor root of the trigeminal nerve in 0rm'tho'rh3/nclms is very peculiar. Its superficial origin is situated on the deep surface of the tuberculum guintvl, that is, in the deep groove which separates this swelling from the mesencephalon. But, in order to reach the surface, it proceeds around the mesial border of the projecting pre-pontal boss of the tuberculum qmhztri, where the mmws maa:~illcw"»is is inserted into the latter (fig. 2, V. motor). It then proceeds transversely in a lateral direction across the tubcrculum (fig. 1), immediately in front of and parallel to the pons, and ultimately reaches and joins the mmus manclibulawis just as this is leaving the cranium through the fommen ovalc. In the scheme (1’) which accompanies figure 1 the relations of the three roots of the trigeminal nerve, each to the others, and to the tube7'cul2.cm qmlntri, is shown on an enlarged scale.

The two sensory roots of the nerve appear to be quite separate the one from the other, and hence there is not a single Gasserian ganglion, in the ordinary sense, in 07°m'th07'hg/ncheos. But the nerve cells from which the fibres composing these roots arise are scattered among the fibres of the two roots in their intracranial course. In the brain of a foetal 07"/m'th07°hg/nchus I found a definite Gasserian ganglion?

1 Alex. Hill, “On the Cerebrum of 0rm'.'horhynchus paradoxus,” Philosophical T/'ansactz'0ns of the Royal Sr)cz'ct3/, 1893, B. Plate '20, fig. 1.

2 “The Brain of a Foetal Ornithorhynchus,” Quart. Jour. of Micros. Sc., vol. xxxix, N.S., pl. ii. figs. 1, 11, 1'2, and 13.


The pons Varolvivl in Ormfhorhync/1/u,s is a small compact strand, of transverse fibres, which arch across the large hill-like prominencesof the tubercula guinti, and dip down into the deep valley which intervenes between these eminences. In this sulcus rlntelrtuberczolaris, as we may call the valley, the pons is produced in the cephalic direction into a beak, and here it attains its maximum sagittal diameter of 5 mm. When the pons attains the mesial border of the tubcrculum. guinti, its antero-posterior diameter has become reduced to 4 mm., and when it has reached the lateral border of the tulmrculum its sagittal measurement has become further reduced to merely 2 mm. At the situation where the sagittal diameter is 3 mm, the thickness of the pons is scarcely 05 mm. Upon the lateral aspect of the tuberculum qmnti it ascends between the mmus man.dz'bzo/arts of the fifth nerve and the eighth nerve to the cerebellum as its small middle peduncle. The mesial part of the lobus flocculi rests upon the middle peduncle in this situation, and produces a slight depression upon it (figs. 1 and 3);

The scanty literature of this subject contains a most extraordinary series of errors of observation and interpretation.

In 1826 Meckel gave anexcellent figure of the base of the brain, in which he represented and correctly named the pans, and the “eminentia cui trigeminus laxe insidet”: he also represented, but did not name, the motor root of the fifth.‘

Twenty-one years later Owen adopted, with very slight modi-fications, the figure in question, but he committed the strange error of calling the pons the “corpus tmpez0z'des,” and the motor root of the fifth the pans! Thus he says :—-“ Their [the tuberoula Quinta’ of this communication] anterior extremities are crossed by large trapezoid bodies (figured by Meckel as the pons Varolii), and anterior to those is the true ‘ nodus encephali,’ which is narrow, in correspondence with the small lateral lobes of the cerebellum”?

Eleven years later Garner erroneously described the trigeminal nerve as attached behind the pons to the medulla in both

Monotremes.3

1'Meckel, lac. cz't., tab. vii. fig. 4. ‘3 R. Owen, “Monet-rematrt,” Todd’s C'ycl0p0cdia of Anatomy, vol. iii., 1847, p. 383, fig. 181. 3 R. Garner, British Association Reports, 1858, p. 123.


In 1896 the present writer correctly represented this region of the brain in 0rnith07°liynchio3.1

This figure, however, does not meet with the approval of Ziehen, who says concerning it :———“ Die Abbildung der Hirnbasis von Ornithorlzynchus, bei Elliot Smith (JOQLT. of Anat. and Pkg/3., vol. xxx. p. 487, fig. 12), ist wohl schwerlich ganz nat11rtreu. Aiich reickt der Pans waiter nach worn als Smith angiebt. Vergl. meine Darstellung, p. 45 fi°.” 2 (The italics are mine.)

When Ziehen so positively aflirms that my figure is scarcely correct, one naturally expects to" find some definite statements to indicate the error. But on referring to page 45 of Ziehen’s memoir, as he directs, we find the paragraph in question beginning thus :—“ Die Feststellung der Grenze der Briicke bietet sehr grosse Schwierigkeit. Nicht einmal der einzelnen Hirnnerven lassen sich sofort ohne Zweifel identifiziren.” Upon the next page he correctly indicates the anterior limits of the mesial part of the pons, and adds, “Die mikroskopische Untersuchung bestatigt diese Deutung.” He then proceeds to describe a thin transverse layer of fibres which crosses the tube9'c~ul¢.mi quinti, but avoids expressing an opinion with the remark :-—“ Die definitive 'Entscheidung iiber die Frage, ob diese Faserschicht thatsachlich der Ponsformation homolog ist, bleibt natiirlich der mikroskopischen Untersuchung vorbehalten,” which hardly accords with the previous quotation from the same page. He then describes the anterior part of the tuberculnm guinti, and is unable to find transverse fibres upon its surface. But he adds that the “ abfallende F lache ” of this tubercle is covered (bedeckt) with transverse fibres. He says that the anterior edge of this layer of transverse fibres crosses the crus cerebri (Hirnschenkel) and is continued into the projecting mesial rostrum of the pans. To this he adds :—“ Es kann keinen Zweifel unterliegen, dass damit der vorderer Rand des Pons vollstandig gegeben ist. Diese vordere Rand liegt also auf der Basalflache nicht frei, sondern ist wenigstens seitlich ganz in der tiefen Spalte zwischen Occipitalhirn und Tuberculum cinereum versteckt.”

He then formulates the view that the large swelling which has appeared upon the lateral side. of the pontine region has separated the pontine fibres into two groups, one of which has been displaced backward. and the other forward into the deep cleft between the cerebral hemisphere and the »tube'rculum gmlnti.


1 G. Elliot Smith, this Journal, vol. xxx. fig. 12, p. 487. 2 Th. Ziehen, loc. ci°t., Jena. Dcnkschr., p. 147.


It is difficult to understand to which transverse fibres Ziehen refers as being thus hidden from view in the cleft between the tuberculum guinti and the cerebral hemisphere. The only definite bundle of fibres I can find in this situation is the motor root of the trigeminal nerve (mlde supra). Although these fibres come into close topographical relationship with the beak-like anterior projection of the pans (Ziehen’s “ Zapfen ”), the fifth nerve of course does not give fibres to the mstmm! In Ziehen’s remarkable figure 23 (page 34), the motor root of the trigeminal is indicated as “ ein kleiner Rest vom Trigeminus,” but no attempt has been made to represent the mesial portion of the pons, and it is not clear from the text where Ziehen would place the posterior boundary of the pons in the middle line. In this figure he accurately represents the lateral portion of the pons, but does not name it; but he calls the pre-pontal part of the tuberculum qwintvl the “ pons ”! I presume that this extraordinary figure explains what Ziehen means when he says later on that the pans reaches further forward than I represented it (p. 14). But if the pons occupies the situation represented in this figure, what does Ziehen mean when he says, concerning the region which I have called the pre-pontal region of the tuberculum quinti,——«“ Auf dem Kamm lassen sich mit unbewaffnetem Auge und auch mit der Lupe querlaufende N ervenfasern nicht sicher nachweisen ” (p. 46)? It is obvious ‘that Ziehen has completely failed to recognise the pons, and has himself fallen into an error of a similar nature to that which he wrongly attributed to me. If he had recognised the true pons, he would hardly have represented the sixth nerve perforating the pons, as he does in his above figure 23. The true position of this nerve I have represented in figs. 1 and 2.


In further proof of Ziehen’s complete failure to appreciate the real state ofaffairs, I might quote his remark on page 43 :“ Dabei ergiebt sich zugleich als bald die Thatsache, dass eine freiliegende Briicke im gewohnlichen Sinne fehlt. Der vordere Ponsrand . . . . ganz in die Tiefe gedrangt.”

voL. XXXIII. (N.S. voL XIII.) Y


I have already referred to Owen’s peculiar error in mistaking the motor root of the trigeminal nerve for the pons. In this connection’ it is interesting to quote a note of Ziehen’s with regard to this point :—“ Die distale Querfaserung deutet er [Owen] als Trapezkorper, _die proximale wie vlch [sic I1] als Briicke.” Verb. sap. ‘ ‘ . '

There is in Ornithorhynchus a compact andwell-defined pons,

which is placed in an exposed position upon the ventral surface of the hind-brain (figs. 1, 2, and 3). i y

I have entered somewhat fu.lly into the consideration of this point, because Ziehen, in the full knowledge that my observations had been made upon an abundant supply of fresh material,

has unjustly attributed to me a gross error in confusing one of the most obvious features of the brain.

The pons Varolii in Echidna closely resembles that of Om?/itho-»

rhy/nckus in shape and general arrangement, but is slightly larger in the former. Thus in the mesial plane the pons varies in its sagittal diameter from 6 mm. to 7 '5 mm. in different brains, but it rapidly shrinks, so that when it turns upward to

become the middle peduncle of the cerebellum its diameter‘

is barely 25 mm. in some brains, although in others it is as much as 3 mm. in diameter. Ziehen has accurately described and represented the pons in Ecimidna, and hence it is difficult to understand his observations upon Ormithorhynchus when we consider that the pons is so strikingly similar in the two brains. After referring to the pons in Echidna, Ziehen says :——“ Die Verhaltnisse bei 07°m'th07°h3/nchus sind so complicirt und so einartig, dass sie zu einem messenden Vergleich sich nicht eignen ” (p. 146).

Ziehen’s difficulty in understanding the arrangement of the pons in 07"77/ithorhg/nchzos is probably due to the fact that the tuberculum gwinti projects on the cephalic side of the pons. In order to distinguish the special condition which we find in Platypus, I have introduced this new term. But the tuberculum Quinta’ is merely a tuberculum cinereum Rolcmdi, which is not confined to the post-pontal region of the hind-brain.


In Echidna the tuberculum cinereum is much smaller and the pons is slightly larger, and hence the former does not project forward beyond the latter, as is the case in 0rm'thorhynchus. In other words, Echddna. 1 presents the ordinary mammalian type of tuberculum cvlneareum Rolando’. " 1 But Ziehen, who distinguishes the tuberculum quinti by the name ‘ tuberculum cinereum ’ in Ornéthorhg/nchus, calls the body which contains the spinal root of the fifth nerve in Echiolna the ‘ emvlnentia olivaris’ (p. 32). And yet in the same memoir this writer adversely criticises Owen for committing the same error in Ormltkorhynchus. Thus on page 47 we find the remark :— “Die hintersten Theil des Tuberculum cinereum hat er [Owen] falschlich als Eminentia olivaris gedeutet.” I may add that both Meckel and Owen clearly recognised this swelling in Platypus as an appendage of the fifth nerve, although they employed the term e7m'nent73a olivaris, which is associated with the topographically analogous but not homologous region of the human brain.


1 The italics are mine.


Thus, after giving an excellent description of the tube7°culum 1

qztinti, Meckel adds concerning it :—-“ Magnae nervi trigemini evolutioni respondere videtur.” 1

The swelling which Ziehen describes as ‘ eminentéa olivaris ’ in Echidna. is the true tuberculum cinereum Rolandi, and its arrangement is exactly analogous to the larger post-pontal part of the tuiberc-ulum qmlntvl of Platypus. Ziehen’s error is all the more remarkable, seeing that he says that his statements are founded upon “zahlreiche, vorzugsweise nach Pal gefarbte Schnittserien alterer und jiingerer Tiere und Embryonen.” 2 He describes the eminentia olivaris as a large swelling 45 mm. wide, which is placed immediately upon the lateral side of the roots of the hypoglossal nerve. On the same page, in a description of the dorsal aspect of the medulla, we learn that “ Tuberculum cinereum Rolandi, Tuberculum cuneatum und Clava sind sehr schwach entwickelt ” ; 3 but in his contemporary communication to the Anatomischer Anzeiger he states (speaking of Eclmldna) that “ die spinale Quintuswurzel ist stark ventralwarts verschoben und stellt einen Ziemlich schmalen, aber sehr langen streifen dar” (p. 173).

If we admit that the spinal root of the fifth nerve must be placed in the tuberculum crnereum Rolando’, these statements of Ziehen’s are reconcilable neither the one with the other, nor with the condition which we actually find in the brain of Echidna.


1 Meckel, loc. cz't., p. 33. 2 Th. Ziehen, Anat. Anz., Bd. xiii. Nr. 6, 1897, p. 171. 3 Th. Ziehen, Jena. Denlcsch., loc. c2't., p. 32. 330 DR G. ELLIOT SMITH.


But Ziehen’s eminentia olivaris corresponds topographically and structurally to the tuberoulum mlnereum Rolandi of other mammals, and is the homologue of the post-pontal part of the tuberciolum quinti of 07°m'th0rky/nchus, to which it presents the closest resemblance in all points except its size.

In the brains of both Monotremes, the descending root of the fifth nerve, the fibres of which can be seen upon the surface of the tubercle of Rolando, is crossed by a thin layer of transverse fibres, some of which are probably the homologues of the corpus trapezoideum, but there is no compact trapezoid body such as we find in the Meta- and Eutheria.

The tuberoulum qmlnti projects forward in Orwithorhynchus, and becomes flattened against the ventro-caudal surface of the cerebral hemisphere, the tentorrlum cerebelli alone intervening. Hence, if we dissect away the posterior parts of the hemisphere, we find a broad sloping surface of grey substance 10 mm. long and 6 mm. broad, looking forward and upward toward the hemisphere (fig. 3). The ventral extremity embraces the ram/us maxillaris of the fifth nerve, and comes into contact with the rostrum of the pons. Its mesial border is encircled by the motor root of the fifth nerve. The upper part of the tubercle lies in front of the ra-mus mand?:b2L[aTi8, and slopes upward into continuity with the lateral surface of the tegmentum of the mesencephalon. The tu-berculum gmlntvl extends in the dorsal direction in front of the pons as far as the angle between the superior and middle cerebellar peduncles, under cover of the lobus flocculi (fig. 3).

The Fourth Ventricle

The floor of the fourth ventricle in Ornithorhg/nchus presents the usual lozenge-shape a11d measures 15 mm. long. The distance between the columrwe cerebelli (so we may distinguish the combined peduncles which support the cerebellum) is 10 mm.:, but as a lateral recess extends over the restiform body in the usual manner behind the columnar, cerebelli, the fourth ventricle attains a width of almost 15, mm. in this situation.


The floor is flatter than is usually the case in mammals.

The eminentia teres is very clearly defined by a slight sulcus which is parallel to the mesial longitudinal fissure.

Immediately to the inner side of the superior cerebellar peduncle there is a deep depression corresponding to an area about 5 mm. by 3 mm. It is caused by the lower part of the anterior lobe of the cerebellum.

A small ellipsoidal knob, 1 mm. high, 4 mm. long, and 2 mm. broad, is found on the floor of the ventricle at a distance of 3 mm. from the mesial plane, just opposite to the recessus latemlvls. This I consider as the homologue of the body which in the Edentata I have called tuberculmn acusticum medvlale, and corresponds to what is usually called the trigonum acusticum. Ziehen regards it as the t~zI.bm°culum acusticum latemle. In other words, he calls it tubelrculumr acustvlcum (page 47 ), and subsequently explains that it represents What he calls the emvlnentvla lent7f07°mz's in Marsupials (page 148)."

In my recent memoir on the brain in the Edentata I suggested the use of the terms ‘tuberculum acusticum mediale ’ in place of trigonum acustvlcum for the projection which corresponds to the dorsal or vestibular end-station, and the term ‘ tuberculum cwu.st'£cum latemle ’ to apply to the mass of grey substance which we find lying on the restiform body, including not only the lateral nucleus or so-called ‘tuberculum acusticum,’ but also the accessory ganglion; in other words, the cochlear end-stations. These terms were introduced in the hope that the descriptive nature of the names ‘lateral tubercle ’ and ‘mesial tubercle ’ might help to do away with the confusion which the terms ‘trigonum’ and ‘tuberculum’ almost invariably create in the minds of students.

Now the topographical relationship of the tubercle in question to the peduncle of the cerebellum, and its position in the floor of the fourth ventricle, both point to its homology with the mesial and not the lateral tubercle as Ziehen supposes. Then, again, the large bundle of fibres which proceeds to this tubercle follows a course between the spinal root of the fifth nerve and the inferior peduncle of the cerebellum. In other words, it presents the same relations as the vestibular nerve in other animals presents in its course towards the trigonum acustvlcum, mesial tubercle. This view is further supported by the fact that a little crescentic body is found in 0rnz'tko7°h3/nchas upon the dorsal surface of the restiform body, and embracing this peduncle as it bends into the cerebellum. This little body in its relations to the auditory nerve, as well as from its general topographical position, seems to represent a diminutive lateral acoustic tubercle. .

But Ziehen says that the little tubercle in the floor of the ventricle represents the lateral tubercle, and the mesial one is to be found between this tubercle and the middle line. But if the lateral tubercle is so much more prominent than the mesial (trigonam), it is very strange that almost all the fibres of the auditory nerve should‘ proceed to the former by the route which is usually pursued by the root going to the latter.

From such considerations it seems to me probable that the tubercle in the floor of the fourth ventriclerepresents the trigonam acnsticnm ‘or mesial tubercle, and that the tabercalam acasticam of writers (lateral tubercle) is reduced to very diminutive proportions, especially in Echidna.

There are well-defined ligalce and obese’ in both orders. But concerning this structure in Ornvlthorhy/nchas, Ziehen falls into a peculiar error. He says that the obex is “ nur angedeutet” (p. 47), and yet in the figure which illustrates his account (fig. 35) he clearly represents the large obex, but calls it stratum transversum dorsale; and in the text he says that the median fissure in the floor of the fourth ventricle is interrupted in posterior 3 mm. of its extent by a strand of crossing fibres

(p. 47).

The Lamina Terminalis

There are certain features of the lamina terminalels in the Monotremes which are of great morphological interest; and as I have discussed them only very imperfectly previously} and as Ziehen has not added anything new to my earlier fragmentary descriptions, it is necessary to consider these features in some detail here. i


1 This Journal, vol. xxxii.


Fig. 2 clearly shows the manner in which the thin lamina terminalis appears to expand as it ascends in order to form a matrix for the large commissures, and for this reason I spoke of thisthickening of the lamina terminalis as the ‘ commissurebed ’ in one of my earlier contributionsl ,


I propose now to briefly consider the nature of this matrix. The ventricular cavity is everywhere lined by its own epithelium, and naturally the posterior and essential part of the lamina terminalis consists of such an epithelial layer. In the lower part of its extent, as for instance in the region of the area terminalis, this epithelial wall is further thickened and strengthened by a thin layer of tissue, which is mainly of a neuroglial nature. But as it extends dorsally, we find that there is added to the anterior surface of this true lamina terminalis a mass of grey substance, in which numerous nerve cells and fine nerve-fibresare found.

Now, this mass of grey- matter must be derived from one of two sources. It must be formed either by a proliferation of the cells which constitute the primitive lamina terminalis, or it must be derived from the neighbouring mass of grey substance, the surface of which I have called the precommissural area. Now, the fact that the histological structure of this matrix or thickening of the lamina terminalis agrees so closely with that of the neighbouring mass of grey substances, inclines us to believe that this thickening or ‘ commissure-bed ’ is produced by an invasion of the primitive lamina terminalis by nerve cells from the neighbouring (paraterminal) body of grey substance which apply themselves to the anterior surface of the true lamina terminalis. This is what I intended to express by the term ‘ thickened lamina terminalis.’

This View of the nature of the lamina terminalis in the Monotremes is strongly supported by the study of the phases of its development in Marsupials and other mammals, and was first suggested to me by a comparison between the conditions prevailing in the Reptilia with that found here.3 In my early studies of this problem I was led to the belief that this ‘ commissure-bed ’ became converted into the septum lneidnm in Eutheria by its growth and subsequent stretching by the expanding corpus cal '1 This Journal, vol. xxx. p. .190. '3 I bid., vol. xxx., and elsewhere. " I bid., vol. xxx.

losum ;1 but a fuller knowledge of the history of the development of the corpus callosum and lamina terminalis in the Eutheria convinced me of the fallacy of that view. In communicating to this Journal last year what I believe to be the true history of the mode of formation of the septum lucidum from the paraterminal ganglionic mass, the surface of which forms the area prcee0m~missuralis, I unintentionally neglected to announce that my attitude toward this problem had changed, and thereby I rendered myself liable to the charge of inconsistency.


One of the most significant features in this region of the brain is the presence of the little recessus superior, which is of great phylogenetic interest. For here we find reproduced in a mammalian brain a feature which is clearly a persistence of the original Saurian or Pro-mammalian condition. V

The anterior wall of this little supracommissural recess is obviously the homologue of the structure which Burckhardt has called the lamina supraneuroporiea in some of the lowlier vertebrates? Its recognition in a mammalian brain is a fact of some significance to the morphologist.3 The lamina terminalis and the reeessus superior present similar features in Eeliiolna.


Before leaving the consideration of the lamina terminalis, I must protest against the use of the term ‘eommissura superior,’ which Ziehen applies (p. 37 and elsewhere) to the dorsal or hippocampal commissure. The term ‘ commissura superior ’ has been previously applied by Osborn to the commissura habenularum, and is commonly used in this sense at the present time not only in America and this country, but also among some German writers. VVhen we recall that among many Reptiles and Amphibia the habenular commissure is in close topographical relationship to the hippocampal commissure and that they ha.ve even been mistaken the one for the other on several occasions, the singular inappropriateness of the use of the term ‘eommissura superior ’ in the sense in which Ziehen employs it is obvious. It is for this reason that I have attempted to consistently employ the term ‘ dorsal’ instead of ‘ superior.’


1 R. Burckhardt, Der Bauplan des W irbelthiergehirns, Morph. Arbeiten hrsg. v. G. Schwalbe, Bd. iv., 1894.

2 Compare, in this connection, my account of its condition in the foetus, “ The Brain of a Foetal Ornithorhynchus,” Quart. Journ. Micr. S6,, vol. 39, 1896.

3 “ Eine besondere Fissura bulbi olfactorii lateralis ist scheinbar nicht vorhanden,” loc. cit., p. 36.


The Bulbus Olfactorius

From Ziehen’s account, it is obvious that all his specimens of the brain of Orrmlthorhynchus lacked either the whole or the greater part of the olfactory bulb. In his figures 22, 24, and 26 the whole of the bulb has disappeared, and the structure he labels ‘bulbus olfactorius ’ is the stump of the ‘pedzmculus olfactorius.’

It is difficult to understand also how the bulb could occupy the position in which he represents it in figures 23 and 27. He says that “die Lobi olfactorii springen iiber den Stirntheil der Grrosshirnhemispharen nicht vor," without adding any note to say that Meckel, Owen, Hill, Turner, and the present writer had previously represented it projecting beyond the hemisphere. A glance at the cranial cavity of 07'7b’I:flZ07"]b3/7L0}L’M8 would show him that the bulb must project beyond the hemisphere. I When Ziehen says that the bulb is probably not separated from the rhinencephalon by a furrow,1 one is inclined to believe that the whole of the bulb must have been knocked off his specimen. In one of my specimens in which this had happened the anterior part of the peduncle was flattened so as to resemble the appearance represented in fig. 23.1

The Pyriform Lobe

In both Monotremes the pyriform lobe is separated from the pallium by an unusually deep and well-defined rhinal fissure. In addition the olfactory tract which is placed upon the surface of the anterior part of the lobe consists of a broad scattered series of fibres, which are not collected into a compact strand, as is the case in all other lowly organised mammals. The features of the pyriform. lobe itself present many other peculiarities which distinguish it from that of all other mammals.


1 In the copy of his memoir, which the author kindly sent me, he has added in handwriting, on p. 36, “Vielmehr ist der Bulbus olfactorius gegen das Rhinencephalon nicht scharf abgegrenzt.” To this I may say that there is a distinction as well defined aslthere is in the Rabbit or Cat.


In 07°m'th01°hyuchus the uniformity in the structure of the Whole pyriform lobe long ago convinced me of the absurdity of limiting the term lobus pyr7;for7n.27s only to that part of the lobe which is on the caudal side of the region of the uallecula Sgjluii. In my contributions to this Journal in 1896,1 I therefore suggested that the term should be extended to the whole of the band, which is obviously part of the same histological formation. But there is absolutely no foundation in any of my Writings for the ridiculous statement, which Ziehen2 attributes to me, that the substantrla pevgfomta presents the same structure as the pyriform lobe. .

Ziehen, again, says that my figure of the mesial surface of the hemisphere of 0rm'th07'h3,/uchuis in this Journal 3 is “ nicht ganz correct,” but he does not explain What is Wrong in the figure, but extols his own figure (26, p. 36) as “naturgetreu.”4 I fail to see the justice of this comment, but may observe that if my figure errs, Ziehen’s figure not only repeats the supposed errors, but in addition commits some genuine mistakes, as a comparison with fig. 2 of the present contribution clearly shows.

THE TUBERCULUM OLFACTORIUM.

At the beginning of my investigations in 1894 I sought for some suitable Word in order to define this Well-defined region of the brain, and finally adopted Kolliker’s term ‘tul)e7°— culum olfactorvlum ’ as the most appropriate, and then I proceeded to define its limits more accurately than this had been done previously. In my earlier paper in 1896 the limits of the area were not definitely marked off, but three months later 5

I accurately defined the limits of the region, and separated it from all the surrounding areas. And yet in Ziehen’s memoir, which appeared in December 1897, 'l.e. seventeen months after the latter contribution, We find these remarks :—-“ Ich kann Elliot Smith nicht beistimmen, Wenn letzteres [tuberculum

1 Loo. cz't., vol. xxx. 2 Ziehen, loc. cz't., p. 7, Jena. Denk'sch. 3 Vol. xxx. fig. 1, p. 159. 4 Ziehen, loc. cz't., p. 35. 5 This Journal, vol. xxx., July 1896, p. 468.

olfactommn] mit dem Locus perforatus anticus Vicq d’Azyr’s identificirt (Journ. of Anat. and Physiol, vol. xxx. p. 186). . . . Broca’s espace quadrilatere deckt sich nicht ganz damit.” 1

My second memoir in the July number of the previous year's -Journal clearly shows that I had not confused the areas’ in question, and ‘I submit that my statement to which Ziehen objects is Well founded. I am unable at the present moment to -consult Vicq d’Azyr’s work, but I may quote the following passage from the writings of Broca to justify my original statement:—“ Lorsqu’on examine la face inférieuse de Yhémisphere, -on y apergoit l’espace guadmlatére, limité en avant par les deux racines olfactives blanches, . . . Cet espace quadrilatere, . . est l’analogue de l’espace décrit par Vicq d’Azyr, dans le cerveau humain, sons le nom d’espace pa/rforé. Il est occupé, chez les animaux osmatiques, par une substance grise assez épaisse, -qui s’insere a la fois sur l’angle de separation des deux racines blanches, . . .”

Later We find Broca speaking of the tuberculmn olfact0m'um as “ l’espace quadm'latére,” and the region of the “ area depressa ” or locus pevfomtus he calls the “ bcmdelette diagonale cle «l’espace .quadmIlatére.” Many other instances might be quoted from other Writers to show that the terms ‘quadrilateral space’ and ‘ anterior perforated space’ were loosely employed to include the ‘ tuberculum olfactorium ’ ;l and even to the present day, is it not a fact that we still use Vicq d’Azyr’s term in Human Anatomy in this sense?

In criticising my statement that the tuberculum olfactorowm is larger in Echidna than it is in 07’n'£thorhg/nchus, Ziehen comes to the conclusion that it is relatively larger in the latter.3 In my specimens (preserved in *-M1'iller’s fluid, and subsequently in alcohol) the tuberculum in Eel:/idna varies in breadth from 6'5 mm. to 7 mm. in different specimens, while the sagittal diameter varies from 5'5 to 6 mm.; While in Ornithorhynchus the corresponding measurements are 25 mm., 2°75 mm., and 5 "mm-5'5 mm. But the mere superficial area is not by any means the most important factor in estimating the size of the tubcrculum olfactorium, otherwise We might describe it as: enormous in the anosmatic Cetacea, in which it is practically non-existent. For the corpus striatum comes to the surface in this situation, and may receive only a very thin coating. of cortex, as in the case of ‘ Ornvlthorhgnchus. «But the plump tube-rculum olfuctorium in Echiclna is of a Very difierent nature, and resembles that which is found in the highly macrosmatic Eutheria, instead of being merely a thinly-clad bulging of the nucleus lcnticularis. Here, again, I maintain the accuracy of my original statement.


1 Ziehen, loc. cz't., p. 14, note.

2 P. Broca, Mehnmfres sur le (Jar-veau de l’Homme, Paris, 1888, p. 392. 3 Loc. c73t., p. 142.


Yet once more Ziehen challenges a statement of mine con-cerning this region. We have seen that the fissura cnclorhinallsi (fig. 1, fiss. Ct), which separates the tuberculum olfactorium from the lobus pgrzformis, extends backward and completely separates.

the locus pcrforatus from the pyriform lobe. In Echidna the jissura endorlmlnalis does not extend so far back, and hence the locus perforatus is in free communication With the pyriform lobe, as it is in all other mammals. To this Ziehen remarks, “die Angabe Von Elliot Smith (Jour. of Anal. and Phys, vol. xxx. p. 468) kann ich nicht bestatigen.” 1 Nevertheless, it is a fact.

THE REGION or THE MID-BRAIN AND OPTIC THALAMUS.

The only observation which I desire to. make upon this region of the brain in the Monotremes is the fact that no definite pro-E jection corresponding to the corpus gcniculatum meclvlale (postal-A cum) is visible in either Monotreme. The corpus gcniculatum latcralc is only faintly giarked in Eclz/iclna, and cannot be cer- tainly recognised macroscopically in Ornithorhynchus.

The large ganglion habcnulcc and the associated ganglion interpeduncularc merely need to be noted.

In Echidnoc the cpiphgsis is a thin-Walled pouch; in Platypus it is a solid mass. l

Ziehen says he has clearly seen a tractus pccluncularrls trans-I vcrsus in 0rn7lthorhynchus.2 I believe that this statement is erroneous. . , _ ‘ . p 1 Loc. cz't., p. 142. 2 Ibid., p. 43.


The Cerebellum

Although Ziehen has been able to give us a minute account of the configuration of the cerebellum in Echidna, the condition of his material unfortunately forbade him giving more than very imperfect notes concerning the organ in Ornitkorkg/nchus.

The full discussion of this organ I must postpone for a further communication; but I may here express my disagreement with Ziehen’s interpretation, and the process of reasoning by which he attained these results.

Conclusion

It is now possible to state in a concise manner the most salient features of the brain in the Monotremata, taking into consideration not only the features discussed in this communication, but in addition those characters which I have previously described.

We may consider these features under two headings :—(A) in comparison with other mammals, and (B) in comparison with reptiles.

A. 1. In comparison with the Marsupialia, Insectivora, Cheiroptera, Edentata, and Rodentia, we must admit that the Monotremata have a large cerebral cortex, the size of which greatly exceeds that of many other mammals.

2. If we admit that the number of fibres issuing from the pallium affords an index of the quality or degree of histological perfection of this cortical area, it is obvious that in the Monotremata the cortex must be of poor quality in comparison with that of all other mammals, because the large cortex gives rise to a very small internal capsule, ems cerebml, and pyramidal tract. ,

3. These observations point to the conclusion that, in order to meet the demands which its zoological position and mode of life imposes upon it, the Monotreme develops a cortex which rapidly increases in quantity, instead of becoming more highly elaborated.

4. This is especially so in the case of Echidna, which, foran animal of its size and lowly status, has an enormous pallium, which exhibits numerous sulci. But it is a very significant fact that the arrangement of these sulci does not conform to the plan which, with relatively slight variations, prevails throughout the large group of Meta- and Eutheria.

5. The features of the pyriform lobe, the tractns olfactorias, and the fisswra rhinahs are quite distinctive .

6. The fact that the supra-’ and precommissural parts of the hippocampal are (especially in Ornithorhynchns) are larger and better developed than the descending part of the arc, is distinctively Prototherian}

7. The absence of a projecting fimbria and the attachment of the choroidal fold at the margin of the fascia dentata are unique features.

8. The round or oval shape of the commissara dorsalds (vel helppocampi) in sagittal section. is characteristic of this order.

9. The attachment of the roof of the fore-brain to the antemlor lip of the thickened upper extremity of the lamina termvlnahs so as to form a supracommissural diverticulum of the third ventricle (recessns snpkwior), is a ‘significant feature which may be shared bysome marsupials (Didelphys? Perameles ? Notoryctes? iPhasc0larctos ?), but is certainly not common to all the marsupials.

10. In comparison with the size of the cerebral cortex, the pans Varolii is small. - 11. The lateral parts of the cerebellum are small, and ‘those regions of the cerebellum which in the Eutheria show a progressive increase in size and complexity are peculiar in the Mono tremes, by reason of their diminutive proportions and the paucity of fissures. - 12. The plan of the cerebellum in the Monotremes is in marked contrast to that which is common to the whole of the Metatheria and Eutheria (wide ‘‘The Brain in Edentata,” Trans. Linn. Society, London, 1898).

13. There is the closest agreement between the structure of the ‘cerebellum in Echidna and 07~n73th07°hynch-us. The fact that in the former the floccular lobe is sessile, and in the latter pedunculated and encapsuled in a special bony case, is of little systemic significance, because such contrasts are not uncommon elsewhere among members of the same family. [I have recently recorded an example of this in the family of .Anteaters, “ The Brain in the Edentata,” loo. cz't.] . .

1 This Journal,’ vol. xxx.


14. The absence of a projecting corpus gemleu-latum mediate, the slight prominence of the testes, the (possible?) diminutive proportions of the lateral acoustic tubercle, and the fact that the corpus trapezoideum is not a definite and compact bundle, all seem to point to a poorly developed central (or cortical) auditory path in the Monotremata 4 in comparison with other mammals. ‘

15. It is impossible to say definitely whether the habits of life of the animals, and the resultant diminution of visual acuity, are sufficient to account for the dwindling of the corpus _qem$-A culatum laterale and the flatness of the nates.

We may safely say that every region of the brain in the Monotremata shows some peculiarity of structure which enables the observer to distinguish it from the corresponding part of any other mammal, and which indicates the wide gap which separates the Prototheria from the Meta-, and Eutheria.

B. 1. The presence of a definite pallium, which produces an internal capsule, crust cerebri, and pyramidal tract, and the existence of a pons, indicate an advance beyond the Sauropsidan to a distinctly mammalian status.

2. The features A. 6, 8, and 9, mentioned above, are distinctly Saurian, but the degree of elaboration of the hippocampal formation is characteristically mammalian.

3. The dwindling of the lateral parts of the cerebellum in'theMonotremata is a slight approximation to the Sauropsidan T condition, although the cerebellum in Monotremes is very much

closer to the mammalian than it is to the Saurian type.

4. The diminutive size of the geniculate bodies and the smallness of the testes in the Monotremata probably indicate that the latter have not yet completely attained to the fully-developed mammalian position, but retain some suggestion of the Saurian status.

5. The olfactory bulb and nerve in 07~ntth07°h3/nchus afford peculiar instance of the persistence of the Saurian type. T Many of the features of the brain are more or less directly the expression of the mode of life of their possessor.

As Omithorhynehus usually ventures out only at dusk, and Echidna is nocturnal in its habits, it is not surprising to find that their visual acuity is diminished, their eyes and optic nerves small, and the parts of the brain associated with them also diminutive.

In neither Monotreme can the auditory nerve be called ‘large,’ nor, at the same time, is it sufficiently small to account for the peculiar features of the tubercula acustica, and of’ the mid-brain. The aquatic mode of life accounts for the relative smallness of the olfactory bulb and lobus pyriformis in Ornitho/rhg/ncims ;‘ and the dominant influence of the sense of smell in a terrestrial, insect-eating mammal sufficiently explains the immense size of the olfactory bulb and the lobus 103/m'fo9~mr£s in Echidna.

But in the aquatic Ormlthorhg/nchus the sense of smell does not exercise the dominant influence, as it does in Eclzxidna. The visual and auditory senses can lay even less claim than the olfactory sense to this role. There can be little doubt that the predominant sense in Ornithorhynchus is that of touch, and the -chief instrument of this sense is the broad expanse of skin which covers the snout, and which is as soft and delicate as a babfs cheek, and infinitely more sensitive, in virtue of its rich supply -of nerve filaments from the enormous trigeminal nerve.

It can easily be conceived that as the animal spends much of its time in dark, subterranean channels, where impressions of -sight and hearing avail little, that the sense of touch should be of most service, and, together with the sense of smell, should be .a guide sufficient to the needs of this strange creature. For although the sense of smell is not predominant, the animal is unquestionably maorosmatic, like all other lowly animals, in -spite of Ziehen's opposition to this view (loo. cit., p. 139, 2nd note).

But when at dusk the animal leaves its burrow and takes to the-' water, the sense of smell is of little avail, and the creature niay be seen swimming about upon the surface of some quiet pool,'vvith its sensitive beak just below the surface of the Water, responsive to every ripple which disturbs the surface. At such times vision and hearing can be but accessories to the more acute sense of touch.

Plate X

FIG 1 — The base of a brain of Ormlthorhynehus. Magnified 3 diameters.

FIG. 1'. — A scheme of the insertion of the fifth nerve. IL . fiss. rhmalzs tuber c,mm._ V motor cbrp. ma/Imm'tl.

, co rp. quad/mg.

sutc. limit. pallii

vel. merlull

. - \ V1 “"tubercu(rum qwium

. hypolghysis 1m’a ‘ ~. 0091). mterped. .


9 _ L ' , ~r - .~' 97 '- 7 FIG. 2.—The surface of the brain of Ornithorhynchus, exposed by a. mesial sagittal section. Magmfied almost 4 dlameters.

Plate XI

lob. , pyriform.. tuberc. olfact. ' corp. mammill. " ems cerebm" lemniscus’ FIG. 3.—The lateral surface of the brain of Ornithorhynchus.

Part of cerebral cortex and optic thalamus (y), Magnified almost 4 diameters.

Plate XII

removed to show the mid-brain.


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