Paper - Development and homology of the mammalian cerebellar fissures 2
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Bradley OC. Development and Homology of the Mammalian Cerebellar Fissures: Part II. (1903) J Anat Physiol 37: 221-240.13. PMID 17232554
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On the Development and Homology of the Mammalian Cerebellar Fissures II
Professor of Anatomy, Royal Veterinary College, Edinburgh. (Plates XII-XVI.).
- The work, of which the present paper is the outcome, was done by the writer as a Research Student of the University of Edinburgh.
The cerebella which remain to be described are all built on much more complicated lines than are those which have been passed under view in the foregoing sections. This being so the examination of the development of the fissures in an animal possessing a richly fissured and foliated cerebellum in adult life will greatly aid in the task of recognising homologies. Therefore pig embryos will be examined, with a view to noting the time and order of appearance of the various fissures,
40 days embryo
52 mm. long (figs. 54, 55 and 56).
At this stage the cerebellum of the pig embryo bears a certain likeness to that of the rabbit on the 20th day of gestation. No fissures are visible to the naked eye, but when sections are made and microscopically examined there is noticed a somewhat thin liplike plate projecting from the lower posterior corner of the section of the cerebellar lamina (fig. 56). This is comparable in every respect to the same feature in the rabbit’s brain on the 20th day, and there develops a homologous lobe in connection with it.
44 days embryo
64 mm. long (figs. 57, 58 and 59)
Development has proceeded rapidly during the interval betweën the last stage and the present. A naked-eye examination shows a sufficiently clear distinction between the future vermis and hemispheres. There is also visible on the anterior slope a fissure (IL.) of considerable length (fig. 58). Microscopie sagittal sections show fissure IV. as before, and fissure II. of some depth. There are also possibly faint indications of two other fissures in that part of the vermis lying between II. and IV. There is as yet no trace of a separation of a paraflocculus from the hemisphere.
48 days embryo
80 mm. long (figs. 60, 61 and 62)
Development has again progressed rapidly ; indeed, it is something of a misfortune that a stage intermediate between 44 and 48 days could not be obtained. But though this is a misfortune, it is not one which offers any insuperable difficulty in the solution of the problem before us.
An examination of a 48 days cerebellum reveals a fissure (IL) which is prolonged for some distance into the hemisphere. Below it the two othér fissures are faintly marked. These develop into fissures I. and c. On the posterior slope there are two faint fissures in the vermis. Subsequent development shows that these become fissures III. and d. In the hemisphere there is an indication of a fissure, which, growing inwards from the lateral part of this portion of the cerebellum, ultimately demarcates the paraflocculus. Another faint foreshadowing of a fissure is also seen indenting the margin of the hemisphere anterior to the one just mentioned. This latter, growing inwards, ultimately forms part of fissure a (fig. 60, a).
Microscopie sections afford additional evidence as to the actuality of the faint depressions seen with the naked eye (fig. 62). They also show that a number of fissures are about to complicate that portion of lobe À which lies below fissure c (lobule A,). Lobe E has increased in volume, and is now, in consequence, sharply defined from the posterior medullary velum. A flocculus is becoming evident, and its development from the boundary of the lateral recess is clearly indicated. Its boundaries are not as yet rigidly set down, but it reveals itself as a thickening and bulging in the region in which, in the future, it is to become conspicuous (fig. 61).
Embryo 86 mm long
In the cerebellum of an embryo of 86 mm. in length (of which the age is not certainly known, but is estimated at about 50 days) the anterior surface is quite richly fissured. Fissure IT. now reaches the extreme margin of the hemisphere, and fissure I. almost does 80. On the posterior slope, fissure &« runs completely across the cerebellum, but is shallow at the junction of vermis and hemisphere. Fissure III. crosses the vermis and invades the groove between it and the hemisphere. The fissure which is about to cut off the paraflocculus is deep, and is growing inwards towards fissure III. of the vermis, with which it finally becomes continuous.
Fissure d is, if anything, rather longer than fissure III. The parafloceulus forms a distinct projection, and is now clearly separated from the flocculus. Sections show that lobe B is becoming divided by a shallow transverse fissure.
51 days embryo
88 mm. long (figs. 63, 64 and 65)
The difference between this and the above stage is only one of depth of fissures.
55 days embryo
100 mm. long (fig. 66)
To the naked eye the fissures have obviously deepened since the 51st day, but no new ones can be made out. Sections, however, show that a fissure, 6, has begun to invade that part of lobe C which is in the vermis. It seems likely that this fissure first made its appearance, on the anterior slope of the hemisphere, about the 48th day (fig. 61), and that the two parts gradually grew together in the vermis. It is interesting to notice at what an early period fissure a came into existence, and how comparatively late fissure b is in making its appearance in the vermis. This should be compared with the constancy of the former fissure in the cerebella of the type of the rabbit, and the inconstancy or difficulty of determination of fissure b in the cerebella of the same order of complexity.
The fissures in lobule A, are now of considerable depth. Lobule A, retains its comparatively small size. Lobe B is larger, and contains a moderately deep fissure, which is the forerunner of a like feature in the adult brain.
59 days embryo
118 mm. long (figs. 67, 68 and 69)
As in the rabbit, the anterior part of the pig’s cerebellum has advanced more rapidly than the posterior part during the earlier stages of development. By the 59th day the anterior surface is bearing a strong resemblance to the adult condition, but the posterior part is still comparatively simple. Fissure à is now of some depth and can readily be recognised by the unaïded eye. Fissure a has gained considerably in depth. Fissure III. has become continuous with the lateral fissures, which, making an early appearance, first indicated the limits of the paraflocculus.
Fissure d is of great lateral extent, being indeed the longest fissure of the cerebellum at this stage (with the possible doubtful exception of fissure IL, which has a curved course). Fissure d, it should be noted, is growing forwards into the paraflocculus, which is, by it, being divided into an upper and a lower part, connected together in front (fig. 67). It is desired to emphasise the fact that there is a strong, well marked connection between lobe D and the paraflocculus This connection at this stage is not confined to the part of lobe D above fissure d (lobule D,), but belongs to the entire lobe. Nothing could show more clearly that the paraflocculus and lobe D are parts of one and the same morphologie unit. This point is illustrated much better in the pig than it was in the rabbit.
The paraflocculus has enlarged, and its anterior surface shows signs of foliation (fig. 68).
65 days embryo
132 mm. long (figs. 70, 71, 72, 73 and 74)
The anterior surface has now very closely approached the adult condition, both in its external appearance and also in those features which can only be adéquately appreciated by means of sagittal sections. |
Fissure IL. is of great depth, its lowest part being not far removed from the summit of the roof of the 4th ventricle (fig. 74). Lobe B shows definite evidence of its future bipartite condition. Lobule A, has now lost its former arrangement of indefinitely arranged folia, and has collected them into three sub-lobules such as are found in the adult brain. Fissure b is now of some depth, and fissure a makes an important landmark on the posterior slope. Fissure d is deeper than fissure III, and both parts of lobe D are becoming foliated (fig. 74). Lobe E remains relatively small and simple, but is now separated from the posterior medullary velum by a conspicuous fissure.
The paraflocculus is now divided into two parts, both of which are now foliated. The whole lobule now closely resembles the same lobule in the adult squirrel. The division into two parts has obviously been brought about by an extension in a forward direction of fissure d. This extension was beginning in the previous stage. The upper part of the paraflocculus is connected with lobule D, by a rounded nonfoliated ridge. The connection between lobule D, and the lower half of the paraflocculus has almost become obliterated, but it should be kept in mind that such a connection did at one time exist. The flocculus is small and, to the naked eye, not yet provided with folia. On examining microscopic sections, however, slight fissures are found to exist.
70 days embryo
150 mm. long (fig. 75)
Except in richness of foliation, no marked change has occurred in that part of the cerebellum which is anterior to fissure II. The posterior portion of the organ, however, has now entered into a more active phase of development, and is rapidly assuming the adult appearance. That part of lobe C which is anterior to fissure a (lobules C, and C,) has grown considerably in a lateral direction. Further, the vermis portion has also grown so much in an antero-posterior direction that it can no longer be accommodated in the strict mesial plane, but has become distorted by being thrust over to one side. Fissure b is now a very important feature. It extends all the way across the cerebellum. Lobule C, has also altered considerably in appearance. It no longer forms a band of practically uniform width, running from one margin of the cerebellum to the other. It now fails to extend as far laterally as the more anterior part of lobe C. Its vermis portion has increased in volume in a sagittal direction, and, like that part of the vermis immediately in front of it, is now distorted by being pushed to one side. The hemisphere portions, too, have enlarged in a sagittal direction, and are now in the form of rounded masses, connected with the vermis by a comparatively narrow isthmus. This lobule has therefore come to resemble that of the squirrel.
The two parts of lobe D have also enlarged, and their folia are more numerous. The connection between lobule D, and the corresponding part of the paraflocculus is still smooth. Lobe E remains small, and to the naked eye appears to have no connection with the flocculus beyond that established by means of the posterior medullary velum. But microscopic sections show that there is still a low smooth ridge running between the two structures.
The parañlocculus has not increased much in size, and, because of the lateral expansion of lobe C, is now not 80 prominent a feature on the lateral surface of the hemisphere. The flocculus is still small, and to the naked eye smooth.
Embryo 165 mm. long
age unknown (figs. 76, 77, 78, 79 and 80)
This is the last embryonic stage which it is necessary to examine, as it brings us within a short distance of the condition of the adult cerebellum. Lobule A, is now certainly composed of three sub-lobules, the uppermost of which has beyond doubt an extension into the hemisphere. One single small folium still adheres to the anterior medullary velum, and therefore may possibly be looked upon as an attenuated example of a lingula. Lobule A, is relatively small, and has a rather shallow fissure dividing it into two parts. Fissure IT. begins on the dorsal surface of the vermis; curving forwards at the lateral boundary of the vermis, it runs obliquely down the anterior surface. Lobe B is divided into upper and lower portions by a fairly deep fissure, whose advent has been noted in earlier stages. Lobe C has again made great advances. So much is this the case that lobule C, is very considerably distorted. Lobule C, is now clearly divided into three parts— one in the vermis and one in each hemisphere—connected by narrow bands. The connection between lobule D, and the upper part of the paraflocculus is becoming slightly marked by fissures, and has become in part hidden by the posterior extremity of lobe C.
The paraflocculus is now quite complicated, from the presence of numerous folia; but there is no difficulty in recognising its constitution as two tiers. The flocculus is now foliated.
(figs. 81, 82, 83 and 84)
Having traced the development of the fissures and lobes up to an advanced stage, it does not seem necessary to give an additional detailed description of the adult organ. It will suffice to briefly indicate the changes which have occurred since the 165 mm. stage.
The cerebellum anterior to fissure II. has not undergone any radical change. It has taken additional folia upon itself, but that is all In the posterior part of the cerebellum more decided changes have occurred. Fissure b is now very distinct crossing vermis and hemisphere, and reaching the border of the latter. A further displacement of the vermis portion of lobe C has taken place, so that in the adult brain fissure à is decidedly oblique. The connections between the vermis and hemisphere portions of lobule C, have become very much reduced. The upper part of lobe D has shared in the general distortion of this region of the vermis. Its connection with the paraflocculus now consists of a transversely foliated ridge (fig. 83). Lobule D, has merely increased in size and become more thickly foliated. Lobe E remains very small and inconspicuous (fig. 84).
In many cerebella the paraflocculus has become a somewhat jumbled collection of folia, but in most brains it has retained a closer resemblance to its earlier condition. There is usually little difficulty in tracing its two-tiered character, but it appears as though the lower tier had been turned forwards at its posterior end. The flocculus in the adult is in the form of a row of vertically placed folia, and runs in an antero-posterior direction, immediately below the paraflocculus. Its extremities only are visible when the cerebellum is looked at from before or from behind.
Having now learnt the characters of the fissures and lobes in the pig, we are in a position to examine those cerebella which are constructed after a similar plan.
Mustela furo (figs. 85, 86, 87 and 88). — In this animal is a good example of the backward retreat that fissure II. makes in some of the more complex cerebella. The vermis is about equally voluminous in front of and behind this fissure, this being the result of an increase in the number of lobules in the more anterior section of the vermis.
Lobe A is divided into two slightly unequal parts by a fissure, e, which is almost entirely visible when the cerebellum is looked at from the front, and which reaches the margin of the hemisphere. Lobule A, is divided into two parts, each carrÿing two or three folia. Lobule A, is also divided into two portions, but the fissure is not so deep as that in lobule A. Lobe B is cut by a curved fissure which almost reaches its lateral boundaries. It will be seen that lobes À and B are very similar to the corresponding lobes in the pig, except that the lower component of A is divided into two instead of three sub-lobules.
Lobe C forms à very considerable constituent of the hemisphere. It has fissures a and 6, but the lobules in the vermis between a and b and « and IIL are comparatively simple; 1e. they are not developed to such an extent that their accommodation necessitates distortion of the vermis The connection between vermis and hemisphere segments of lobule C, is very narrow, as in the pig, and partly or wholly concealed. Lobes D and E are confined to the vermis; and D is divided into two lobules by a fissure, d.
The paraflocculus is arranged in the form of two tiers of folia joined together anteriorly. From the lower tier a lobulus petrosus projects for some distance. The connection between paraflocculus and lobe D cannot be made out in the adult. It is somewhat difficult to satisfactorily distinguish a flocculus, but it is apparently present, and visible when the cerebellum is viewed from the side or from behind.
Mustela erminea and M. vulgaris have both been examined, but they so closely resemble M. furo that no further description is necessary.
Meles taxus (figs. 89, 90, 91 and 92). — As compared with lobe B, lobe A is smaller in the badger than it is in the pig. Only a comparatively small portion of it is visible on the anterior surface of the cerebellum. Lobule A, is also small. Below fissure c there are two groups of folia, that group lying more inferiorly being further partially divided.
Lobe B is large, and divided by a deep fissure into upper and lower lobules, each of which is again somewhat deeply indented by a fissure (fig. 92).
In lobe C, fissure b extends to the border of the hemisphere, as it does in the pig (fig. 90). Lobule C, consists of a vermis portion, whose folia—unlike those of the pig—run transversely ; and à hemisphere part, considerably removed from the vermis, because of the large development of those parts of lobule C, which belong to the hemisphere. The three segments of lobule C, are very unequal in size, the hemisphere portions being very extensive. There is practically no distortion of lobule C, in the vermis (fig. 91). Lobes D and E call for no special remark. The double character of the paraflocculus is very evident, the two portions being arranged in an oblique plane, and very clearly continuous in front (figs. 89 and 90). The connection between paraflocculus and vermis is very difficult to establish. In the brain examined, a very prominent lobulus petrosus was present on the right side, and was received into a fossa formed by the temporal bone. On the left side the corresponding lobule was curved forwards underneath the lower part of the paraflocculus (fig. 89). The question arises as to the possibility of the lobulus petrosus always representing the posterior extremity of the lower portion of the paraflocculus. This may be the case. If we accept this as being a true interpretation of the facts, then we should consider that, as the paraflocculus increases in size in different animals, it tends to press forwards, since the lobulus petrosus is often found in cerebella having small paraflocculi.
The flocculus consists of a single folium iying between the lateral recess of the ventricle and the most posterior part of the paraflocculus.
Canis familiaris (figs. 93, 94, 95 and 96)
The anterior part of the cerebellum of the dog does not differ very materially in the arrangement of its fissures and the disposition of its lobes from the corresponding part of the badger’s cerebellum. In lobes C and D, however, there are differences of sufficient magnitude to warrant mention. Fissure b is present in a position very similar to that of the badger. It can readily be followed across the vermis and hemisphere to the border of the latter, running almost parallel to fissure IT. Lobule C, has a very considerably distorted vermis portion, and its hemisphere dependencies show several fissures of some depth, which. give the impression that it consists of several distinct sub-lobules. The central segment of lobule C, is also much twisted, and on superficial examination appears to have no connection with those vertically elongated masses which form its hemisphere segments. On opening up the groove between vermis and hemisphere, however, the connection can be distinguished. The displacement and sinuousness of the vermis in lobules C, and C, only appears after birth. In a new-born dog the vermis is perfectly straight and its folia entirely transverse.
Lobule D is connected to the upper part of the paraflocculus by a low white ridge, which can only be discovered by removing the lowest and most posterior part of lobe C. The rest of lobe D and lobe E call for no remark.
The paraflocculus is relatively larger than that of the badger, to which it bears a close resemblance in the manner in which its two tiers are arranged. It has not, however, a lobulus petrosus; or, at any rate, there is not more than the merest attempt at the formation of one, this occurring at the posterior end of the lower tier, and being only occasionally present. The flocculus is small and consists of a few folia, placed, under cover of the paraflocculus, at the most anterior limit of the lateral recess of the ventricle (fig. 95).
Canis vulpes (figs. 97, 98 and 99)
The general shape of the cerebellum of the fox is very different from that found in the dog. The fox’s cerebellum has a greater vertical height in comparison with its antero-posterior diameter. Its anterior surface is depressed for the reception of the corpora quadrigemina, and its posterior surface is also concave from above downwards. The posterior concavity is rendered all the more obvious because of the backward projection of lobe D over the medulla. This projection is confined to lobule D,, and is so great that this lobule can be seen very distinctly when the cerebellum is viewed from above. These differences being recognised, the cerebellum of the fox otherwise resembles that of the dog. The only points to which it seems necessary to draw attention are two, as follows: The vermis in the region of lobules C, and C, is possibly a little shorter in an antero-posterior direction, and somewhat less distorted in form. The lower part of the paraflocculus carries a definite lobulus petrosus (figs. 97 and 98).
The flocculus is small in the fox, and only just visible from behind (fig. 98).
Felis domestica (figs. 100, 101 and 102)
In the domestic cat the anterior part of the cerebellum is so similar to the same portion in the dog, both as regards its superficial characters and also its appearance in sections, that no detailed description is needed. The most important features are those presented by the organ when viewed from behind. Several cerebella of the cat have been examined, and in all a very striking character is the extreme to which the distortion of the central portions of lobules C, and C, is carried (figs. 100 and 101). In the brain from which the figures were made this distortion is very marked, possibly more so than is the case in the average cerebellum ; but they serve to show to what lengths this twisting of the vermis may go. It will be observed that lobules C, and C,; are arranged in the form of an S-shaped curve, the bends of which are very abrupt. This curvature of the vermis is continued into lobe D, but here its bends are not so sudden (fig. 101). There can be little doubt that this exaggerated dis-. placement of the vermis is to be interpreted as meaning that, in the cat, lobes C and D are relatively more developed (s0 far as those parts of them which belong to the vermis are concerned) than is the case in the other mammals examined. The lateral parts of lobule C, are relatively smaller in the cat than in the dog, badger, or fox (fig. 101). They do not extend 80 far downwards as to blot out the connection between paraflocculus and the vermis. This connection is in the form of one or two folia, resting upon the medulla below, and in contact with the lowest part of lobule C, above.
The paraflocculus resembles that structure in the dog. There is considerable difficulty in distinguishing a flocculus with any degree of certainty in the adult animal. That it is present is undoubted from the observations made by Stroud on its development. But its clear definition in the embryo appears to become obscured at a later date.
Goat and Sheep
(figs. 103, 104 and 105)
In many respects the cerebellum of ungulates departs, in the way of details, from the plan found in those carnivora just described.
When viewed from the front, the cerebella of the goat and sheep show fissures c, I., II. and b very distinctly (fig. 103), all of these reaching the margins of the hemisphere. Fissure c crosses the vermis almost perfectly transversely. Lobule A, has only a very imperfectly developed hemisphere portion; indeed it is doubtful if the hemispheres can be considered to extend into this region. Fissure I, possibly a little shallower than c, has a curved direction. Fissure IL is very acutely curved, as in the dog. Lobule A, and lobe B are almost entirelÿ confined to the vermis, their lateral prolongations being very small Indeed, in this region it is difficult to set definite bounds between the vermis and the hemispheres. There is some amount of lateral displacement, with consequent curvature, in the vermis in lobules C, and C,, but this is not greater in amount than that found in the dog.
In the sheep and goat, and in ungulates generally, the lateral divisions of lobule C, are not nearly so large as they are in the carnivora. In the carnivora their uppermost ends are commonly visible, either on one or both sides, when the cerebellum is regarded from the front. This has never been found to obtain in those ungulates which have been examined for the purposes of this research. Again, these lobules do not reach so far down as to touch the medulla, other than in exceptional cases. The result of this vertical abbreviation is to allow of the connection of the paraflocculus to be traced directly to the vermis, as is the case in the simpler forms of cerebellum (fig. 104). As we have seen, this connection is easily made out in the adult pig. In the sheep and goat, however, it is not quite so evident on à superficial examination ; it is necessary to open up the groove between vermis and hemisphere.
The form of lobule D, is somewhat peculiar in both the sheep and goat (fig. 104). It has a central, well developed portion in the vermis, and smaller offshoots reaching into the hemispheres, a constriction of greater or less tenuity intervening.
Lobe E is of larger size than in the pig and the carnivora. The paraflocculus and flocculus resemble those parts of the cerebellum of the pig.
Bos taurus (fig. 106). — In the cerebellum of the cow, although the same lines are followed as in the sheep and goat, the arrangement of fissures appears at first sight to be very complicated. This remark applies only to the superior and posterior views, as lobes À and B and lobule OC, are almost identical in form with those parts in the average carnivore or ungulate brain. It may be added that it is impossible to make out any hemisphere in lobe A. Even in lobe B the hemisphere is very attenuated.
On closely examining the posterior part of the cerebellum, it is found that the complexity is more apparent than real, and is due to a distortion which rivals that of the cat’s vermis. Apart from this disturbance of form, there is little to which special attention need be directed. It may be mentioned, however, that the lateral parts of lobule C, commonly extend farther in a downward direction than obtains in the sheep and goat, this extension bringing them almost or quite in contact with the medulla. Not infrequently lobe E is so large and projects s0 far backwards as to be visible as one or two folia on the posterior aspect of the cerebellum.
Equus caballus (figs. 107, 108 and 109)
A very striking feature in the horse’s cerebellum is the comparatively posterior position of fissure IL. Fissures c, I, IL. and b are distinct and deep. Fissure c is of very considerable depth, and fissure I. is almost as deep as fissure IT. (fig. 109). It should be noted—as distinguishing the cerebellum of the horse from that of the sheep and goat, and especially from that of the cow—that lobe À is certainly, though not very strongly, continued into the hemisphere.
The posterior part of the horse’s cerebellum shows one or two points of interest and importance. As in the ungulates already mentioned, the lateral parts of lobule C, are small as compared with the carnivora. In the horse their connections with the vermis are not difficult to follow. There is, further, no difficulty in making out the connecting link between lobule D, and the paraflocculus (fig. 108).
In some specimens lobule D, is continued into the hemisphere for a short distance, but this continuation has only once been found on both sides in the same brain. Its presence, though inconstant, is interesting, as being apparently the remains of that undoubted connection which we have seen to exist between lobule D, and the lower part of the paraflocculus during the embryonic life of the pig. In the majority of animals all trace of this primitive unity is lost as the brain grows into its adult form; but in some, possibly in man, evidences remain.
Lobule E is, if anything, smaller in the horse than it is in the sheep, goat and cow. The paraflocculus shows its two-tier character more clearly than in the other ungulates examined, in this respect resembling the paraflocculus of the carnivora. It should be remembered that in ungulates generally the lower tier shows a tendency to curve forwards at its posterior end. This is so well marked in the horse that there are practically three tiers produced. In an earlier part of this paper the suggestion has been thrown out that possibly the lobulus petrosus of the rabbit, etc. represents only the posterior extremity of the lower part of the paraflocculus of more complex cerebella. It may be asked, further, whether in those animals like the horse, in which the paraflocculus turns forwards at its posterior end, this recurved extremity may not be equivalent to a lobulus petrosus, unenclosed in a special fossa of bone. The supposition that this may be so is strengthened when the condition found in the badger is taken into account. In the cerebellum of Meles taæus, of which a description has already been given, on one side a lobulus petrosus was found; but on the other side the corresponding part of the paraflocculus was turned forwards underneath the lower tier. |
The flocculus is usually easily distinguished in the horse, and is visible from the side and from behind. In some specimens a distinct white ridge, independent of the posterior medullary velum, passes from the flocculus to lobe E of the vermis. This ridge is indicated on the left side of fig. 108. It has not been met with elsewhere than in the horse—possibly because an insufficient number of cerebella have been examined—but its occurrence in this animal is of importance, as showing evidence, in the adult, of the embryonic unity of the structures between which it passes.
The cerebellum of the donkey is 80 like that of the horse in all but the merest details that an extended description is not necessary. It may perhaps be well to say that lobule C, in the hemisphere carries several fairly deep fissures, whose presence give the surface a complex appearance. Lobule D, shows the tendency, remarked in the sheep and goat, to extend into the hemispheres in the form of lateral appendages. The connection of this lobule with the paraflocculus is not so superficially evident as it is in the horse. The flocculus of the donkey has a greater antero-posterior extent than is the case in the horse.
In the foregoing pages the steps by which the fissures, and consequent lobes and lobules, of the cerebellum came into existence have been traced in two mammals. It has also been sought to discover the simplest form of mammalian cerebellum, and this having been done, to endeavour to recognise, in the complex as well as in the simpler forms, a likeness to this elementary pattern. Apparently the cerebellum in which the fissures are fewest and the lobes smoothest belongs to the shrew and the smaller bats. In the shrew there are four fissures only ; and of these only one (the second, 4e. IL.) extends through both vermis and hemisphere. The remaining three do not belong to the hemisphere, being confined to the vermis or its immediate neighbourhood.
In following the development of the cerebellum of the rabbit, it was found that this five-lobed and four-fissured stage was reproduced. But in the adult rabbit the number of fissures is increased. In the development of the pig, it appears possible that the five-lobed condition may obtain in its simple form for a time, but it quickly gives place to a much greater complex of fissures. |
In both rabbit and pig fissure IV. was the first to appear, and this in association with the Rautenlippe, which, continuing round the lateral recess of the ventricle, blends with the Rautenlippe of the medulla The association of fissure IV. originally seems beyond doubt. But as development goes on it becomes more and more removed from the edge of the cerebellar lamina, because of the growth of lobe E and the flocculus.
In both rabbit and pig the second fissure to develop is fissure II. which has been recognised by several writers to be of paramount importance, and which is declared by both Stroud and Kuithan to be the first fissure visible in the developing cerebellum.
The next fissures, in point of time of appearance and importance as dividing lines of the cerebellum, are fissure III. and those demarcating the paraflocculus from the rest of the hemisphere. These three are in reality the three elements of. one and the same fissure, which, becoming continuous, they ultimately form.
By the presence of the above mentioned fissures, the cerebellum becomes divided transversely (but not completely as yet) into four unequal portions. (1) À part anterior to fissure IL.; this becoming itself divided later into lobes A and B by fissure I. (2) Lobe C, lying between fissure II. and fissure III. with its lateral elements. (3) Lobe D, to which the parafiocculus belongs. And (4) lobe E, of which the flocculus is an outlying dependency.
Fissure I., separating lobes À and B, appears shortly after fissure IIT. in the rabbit, and somewhere about the same timein the pig. The other fissures, which are formed either at the same time as some of the above (as in the pig), or at a somewhat later date (as in the rabbit), may be considered as of secondary importance, and have no representatives in the simplest type of mammalian cerebellum.
In those adult cerebella which have been examined, there is quite clearly a common pattern running through the whole series. But in many of them there are interwoven into this fundamental pattern subsidiary ornaments, which tend, in a measure at least, to obscure the simplicity of the cerebellum which has been taken as the starting-point. In all the cerebella the five fundamental lobes can be recognised, and their individual peculiarities and tendencies may be summarised as follows :—
Lobe A, in all but the very simplest forms, is divided into two unequal parts by fissure c This fissure is wanting in the shrew and indefinite in the hedgehog, but is constant in all others. Lobule A, in the higher forms consists of three sublobules. In some there are apparently only two of these divisions. It is possible that this complexity of the lobule may be indicated even in the rabbit. Lobule A, is always smaller than lobule A,, and is generally provided with a moderately deep fissure, whose precursor may possibly be shown in the rabbit.
In the higher forms lobe B is divided into two parts, each of which may be again divided. In the rabbit and hedgehog it carries two folia, separated by a moderately deep fissure.
Lobe C consists of three lobules, separated by fissures à and b. Of these two fissures « is held to be much the more important morphologically, because of its earlier appearance in the embryo and its more constant character in the adult. These two fissures apparently develop in a manner peculiarly their own. They both begin in the hemisphere, and grow towards the middle line.
Lobule C, must be considered as standing definitely apart from the rest of lobe C. Its differentiation is early, especially in the pig, and in all the adult animals described, from the squirrel upwards, its individuality is very strongly asserted.
Even in the rabbit there is an attempt at a division of lobe D, but this is not accomplished until the squirrel is reached.
In the higher forms the division is embryonic and early. In the ‘pig, fissure d appears about the same time as fissures III. and ÏI. Particular attention has been called to the development of fissure d because of its forward extension and invasion of the paraflocculus, which is, as a result, divided into two parts, as is the rest of lobe D to which it belongs. Subsequent development may obscure the continuity of the paraflocculus with lobule D,, or, on the other hand, the connection may persist into adult life (eg. in the horse). The connection of the paraflocculus with lobule D, is always lost in the adult, but there may remain slight traces, such as are found in the horse.
The embryonic continuity of lobe E and the flocculus, and their morphologie unity, have already been commented upon. This continuity early disappears, and there is usually no trace of it apart from the posterior medullary velum. But in the horse at least, as has been noted, some evidence may exist even in the adult.
The various fissures and lobes have been distinguished, up to this, by letters and figures only. It would have been easy to employ terms such as those used in human anatomy, but—as Oliver Wendell Holmes has expressed it— words, from occupying for a long time the same place in language, become ‘polarized.” So, in order to trammel the mind as little as possible, it was thought better to avoid those terms which would call up certain fixed and long-rooted conceptions.
The purpose of keeping the judgment as unbiassed as possible being now served, the letters and figures may give place to terms such as are commonly employed. In order to do this, the notion of the plan of the mammalian cerebellum, which has been gained from the descriptions given herein, must be applied to the cerebellum of man. Using the technicalities as employed by Schäfer in Quain's Anatomy, the letters and figures may be transmuted as follows :
There can be little doubt that fissure IT. corresponds to sulous preclivalis, fissure III. to sulcus postpyramidalis, and fissure IV. to sulcus postnodularis. Fissures a and b correspond respectively to sulci horizontalis magnus and postclivalis, and fissure & is equivalent to sulcus prepyramidulis. That sulcus horizontalis magnus should not be employed, as is done in human anatomy, to divide the cerebellum into two primary parts, is evident, and has been pointed out and insisted upon by Stroud. The comparative method clearly shows that sulcus preclivalis (furcal suleus of Stroud) forms the real and fundamental dividing line.
In that part of the cerebellum which falls anterior to fissure IT. (suleus preclivalis), difficulties arise in the use of human anatomical terms. For sulcus postcentralis of the human anatomist corresponds to fissure c; a fissure secondary both in point of time of appearance in the embryo and in morphologie value. In the current descriptions of the human brain, as given in this country, no suleus is mentioned as equivalent to fissure I. The result is that the culmen of human anatomy includes lobe B and lobule A, Lobule A, probably corresponds to the “ascending part of the monticulus” of some German writers (Flatau and Jacobsohn, for instance), but I am not certain that the expression is used for lobule A, alone or always.
The following table shows the parts in the human brain corresponding to the various divisions of the mammalian cerebellum as described in this paper.
Lobus centralis. c. Sulcus postcentralis
I. (Not named by Schäfer)
A. Lobus culminis
A;. A> IT. Sulcus preclivalis
b. Sulcus postclivalis
Lobus cacuminis. Cy } C.
a. Suleus horizontalis magnus__ ’ Lobus tuberis. C..
III. Sulcus postpyramidalis,
Lobus pyramidis. D, d. Sulcus prepyramidalis D. Lobus uvulæ. D.. ] IV. Sulcus postnodularis Lobus noduli. }E.
It will be observed that I have only examined the cerebella .of placental mammals. Lack of suitable material has precluded a first-hand investigation of Monotremes and Marsupials. But, judging from the descriptions and figures given by Ziehen (7), it is clear that the scheme, as elaborated in the foregoing pages, will apply to Marsupials at least. These mammals evidently fall into the group of animals in which the cerebellum follows the simpler type. Whether Monotremes also can be included in this group is not so obvious from the descriptions available. It seems not unlikely that their cerebella belong to a group separate from the rest of the mammalia.
In carrying out the work of this investigation, so much assistance, in the form of material, has been afforded by 80 many persons, that it is impossible to make suitable acknowledgment without going to considerable length. Let it suffice to say, that my debt of gratitude is not to be computed from the extent of the avowal here made. Much of the microscopie work has been done in the Physiological Laboratory of the University of Edinburgh, where, through the courtesy of Professor Schäfer and his assistants, every facility that could be wished for has been afforded.
(7) Zienen, Tu, ‘Das Centralnervensystem der Monotremen und Marsupialier. Thiel L. Macroscopische Anatomie,” Jenai’sche Dentkschriften, vi., 1897.
EXPLANATION OF FIGURES.
Fig. pa. Pig embryo, 40 days, 52 mm. Posterior view. x 2.
Fig. 55 » 40 days, 52 mm. Left lateral view. x 2.
Fig. 56. » 40 days, 52 mm. Mesial sagittal section.
Fig. 57. 5 44 days, 64 mm. Posterior view. x 2,
Fig. 58. 5 44 days, 64 mm. Anterior view. x 2.
Fig. 59. » 44 days, 64 mm. Mesial sagittal section.
Fig. 60. » 48 days, 80 mm. Posterior view. x 2.
Fig. 61. 5 48 days, 80 mm. Anterior view. x 2.
Fig. 62. » 48 days, 80 mm. Mesial sagittal section.
Fig. 63. 5 51 days, 88 mm. Posterior view. x 2.
Fig. 64. 5» 51 days, 88 mm. Anterior view, x 2. 240 Fig.
Fig. 65. Pig embryo, 51 days, 88 mm. Mesial sagittal section.
Fig. 66. 5 55 days, 100 mm. Mesial sagittal section.
Fig. 67. » 59 days, 118 mm. Superior posterior view.
Fig. 68. 5» 59 days, 118 mm. Anterior view. x 2.
Fig. 69. » 59 days, 118 mm. Mesial sagittal section.
Fig. 70, » 65 days, 132 mm. Posterior view. x 2.
Fig. 71. » 65 days, 132 mm. Superior view. x 2
Fig. 72. » 65 days, 132 mm. Anterior view. x 2.
Fig. 73. » 65 days, 132 mm. Left lateral view. x 2.
Fig. 74. » 65 days, 132 mm. Mesial sagittal section.
Fig. 75. » 70 days, 150 mm. Superior view. x 2.
Fig. 76. » 165 mm. Posterior view. x 2.
Fig. 77. mn 165 mm. Superior view. x 2.
Fig. 78. » 165 mm. Left lateral view. x 2.
Fig. 79. 5» 165 mm. Anterior view. x 2.
Fig. 80. » 165 mm. Mesial sagittal section.
Fig. 81. Pig, adult. Anterior surface. x 1.
Fig. 82. ,, 5 Superior view. x 1.
Fig. 63. » Posterior view. x 1.
Fig. 84. ,, 5 Mesial sagittal section. x I.
Fig. 85. Mustela furo. Anterior surface. x 2
Fig. 86. » Superior view. x 2.
Fig. 87. 5 Posterior view. x 2.
Fig. 88. » Mesial sagittal section,
Fig. 89. Meles taxus. Anterior surface. x 1
Fig. 90. 5 Superior view. x 1.
Fig. 91. » Posterior view. x 1.
Fig. 92. Mesial sagittal section.
Fig. 93. Canis familiaris. Anterior surface. x 1.
Fig. 94. » Superior view. x 1.
Fig. 95. » Inferior surface. x 1.
Fig. 96. » Mesial sagittal section.
Fig. 97. Canis vulpes. Superior view. x 1.
Fig. 98. » Posterior view. x 1.
Fig. 99. » Mesial sagittal section.
Fig. 100. Cat. Superior view. x 1.
Fig. 101. ,, Posterior view. x 1.
Fig. 102. ,, Mesial sagittal section. x 1.
Fig. 103. Ovis aries. Anterior view. x 1.
Fig. 104. » Posterior view. x 1.
Fig. 105. Goat. Mesial sagittal section. x 1.
Fig. 106. Bos taurus. Mesial sagittal section. x 4.
Fig. 107. Equus caballus. Anterior superior view. x à.
Fig. 108. » Posterior view. x à.
Fig. 109. » Mesial sagittal section. x à.
Cite this page: Hill, M.A. (2021, March 8) Embryology Paper - Development and homology of the mammalian cerebellar fissures 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Development_and_homology_of_the_mammalian_cerebellar_fissures_2
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