Paper - The mammalian cerebellum - its lobes and fissures 1 (1904)
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The Mammalian Cerebellum: Its Lobes And Fissures
By O. Charnock Bradley, M.B., F.RS.E., Royal Veterinary College, Edinburgh; Goodsir Fellow, University of Edinburgh. (PLaTES XLVI.-L.).
So far as I am aware, no serious attempt was made to establish the morphology of the mammalian cerebellum upon a rational basis before the year 1895, when Stroud (1) published a paper dealing with the development of that organ in the cat and man. As a result of his investigations, Stroud came to the conclusion that-there is a fundamental plan after which the cerebella of all mammals are formed. He recognised certain very important features which are always present. Among other points he discounted the use of the sulcus horizontalis magnus as the essential line of division of the cerebellum into two parts, and emphasised the importance of a much deeper and absolutely constant fissure, which is equivalent to that found in the human brain separating the culmen from the clivus, and to which he gave the name of furcal sulcus. That part of the cerebellum in front of the sulcus furcalis he called the pre-ramus, and to that part posterior to the sulcus he applied the name of post-ramus. ach of the lateral parts of the postramus he designated pilewm, consisting of a post-pileum and pre-pileum, separated by an inter-pilear sulcus (in the cat), or a peduncular sulcus (in man). In man the pre-pileum is formed by the clivus and its lateral prolongations. In the cat it consists of the clivus, cacumen, and tuber, and their lateral prolongations. It seems a pity that some more uniform division could not have been resolved upon.
A very important fact which was brought out by Stroud’s investigations was the lack of correspondence between the human flocculus and the so-called flocculus of the majority of mammals. It was shown that the flocculus of man is present in the lower animals, but only forms part, and a comparatively insignificant part, of the lobe lying to the side of the cerebellum of the cat. To the major part of this lobe the name of parajlocculus was given. The value of this discovery can hardly be overstated.
Almost contemporaneously with the publication of Stroud’s paper appeared one in which Kuithan (2) traced the development of the cerebellum in the sheep and in man. Here, again, the morphologic importance of the constant deep fissure, called sulcus furcalis by Stroud, was pointed out. From its early appearance during development, Kuithan named it the sulcus primarius cerebelli, From the circumstance that the sulcus primarius is the deepest fissure in the vermis, while what he took to be the sulcus horizontalis magnus is the deepest in the hemisphere, Kuithan proposed to divide the mammalian cerebellum into three lobes, of which these fissures would form the boundaries. The anterior lobe (Vorderlappen) would, by this method of division, consist of lobi centralis and culminis; the middle lobe (Mittellappen) would include lobi clivi and cacuminis; and the posterior lobe (Unterlappen), lobi tuberis pyramidis, uvule, and noduli. This is a variation of the division suggested by Schwalbe (4) in his description of the human cerebellum. Schwalbe also divided the cerebellum into three lobes, but his middle lobe consisted of lobi cacuminis, clivi, and tuberis ; and this recognition of the three parts as constituting one single lobe is supported by embryological evidence, as will be shown later. Kuithan does not appear to- have grasped the significance of the paraflocculus and flocculus, for he simply named the upper part of the paraflocculus of the sheep the vermis lateralis, and made no suggestion regarding its homology. Kuithan further fell into the error of allowing his ideas to be governed by what he knew to be the condition of things in man. His endeavours appear to have been largely in the direction of showing that the cerebellum of the sheep contains the same divisions as does that of man. Although this circumstance marred his publication considerably, the residual value is very great.
In 1897 Ziehen (3) published an elaborate paper on the brain of monotremes and marsupials, but he treated his subject from the purely descriptive side, and did not venture into the field of speculation as to homologies. He did, however, recognise a fissure which he named the sulcus horizontalis magnus, on. the assumption that it corresponded to the fissure bearing that name in man; and he called attention to swlct cerebelli superior, anterior, and posterior, the former of which is obviously the homologue of the sulcus furcalis (Stroud) and the sulcus primarius cerebelli (Kuithan). He further remarked upon the lobus flocculi and its connection with the rest of the cerebellum, but he did not draw any distinction between a paraflocculus and a flocculus. He appeared to fall into error in giving the name of ‘flocculus’ to a projecting part of the lobus flocculi which, doubtless, corresponds to the lobulus petrosus of the paraflocculus.
In 1899 G. Elliot Smith, in his detailed description of the brain of the Edentates (5), struck out an entirely new line inhis manner of dealing with the cerebellum. He recognised the individuality of the lobus flocculi, which is separated from the rest of the cerebellum by a deep limiting fissure, for which he suggested the name of jisswra flocculi (parafloccular sulcus of Stroud). He followed Stroud in dividing the lobus flocculi into two parts, a flocculus and a paraflocculus, the latter being subdivided into upper and lower portions—parafloceulus dorsalis and paraflocculus ventralis: After the lobus flocculi had been subtracted from the cerebellum, Elliot Smith considered the residuum to consist of three azygous lobes, sepdrated from each other by two fissures—a fisswra prima and a fissura secunda. The fissura prima corresponds to Stroud’s sulcus furcalis; the fissura secunda separates lobes apparently homologous with the pyramid and uvula of man. The lobes limited by these fissures were called lobus anticus, lobus centralis, and lobus posticus. The lateral part of the Iobus centralis was named the area crescens, “for it will be found that in the higher mammals it is the exuberant increase of this part of the cerebellum which is the main factor in the evolutionary process.” A point of very considerable importance was clearly stated in this paper—this being the recognition of the connection of the pyramid with the caudal extremity of the ‘upper part of the paraflocculus. It was also stated that the area crescens can be subdivided into three portions, lying the one behind the other. It is perhaps only fair to point out that the term ‘pyramid’ was only used tentatively. “In the meantime,” says the writer, “I shall use the term pyramid with the reservation that the validity of its homology with the similarlynamed part of the cerebellum in man remains to be proved. This point can be satisfactorily settled only by embryological investigation, and neither Kuithan nor Stroud has attempted to solve the problem.”
The large work published in 1899 by Flatau and Jacobsohn (6) on the comparative anatomy of the central nervous system of mammals contains a mass of material descriptive of the cerebellum. These writers, like those previously quoted, admitted that the deep and constant sulcus superior anterior (Ziehen) is the true dividing line by which the cerebellum is separated into two parts. Their descriptions, however, have all the end in view of finding, in the average mammalian cerebellum, representatives of the lobes of the human structure. ' In the early part of 1902 the present writer embodied the results of a somewhat extended investigation, on the development and homologies of the mammalian cerebellar fissures, in a paper which was not published until the beginning of 1903 (7). The investigation had been-carried out on the following lines. The simplest form of cerebellum known to the writer was taken as the starting-point, and the parts of this simple form sought for in the more highly-developed and elaborated cerebella, this method being considered more’ rational than that of assuming that the simpler form must contain, in a simplified condition, the divisions of the more complicated brains. The simplest workable type of cerebellum was found to be one in which a mesial sagittal section shows five lobes separated from each other by four fissures. In order to avoid confusion, no pre-existing names were applied to these lobes and fissures, but the simplest method of designating them— that of letters and figures—was employed. The five lobes were called A, B, C, D, and E, commencing thé enumeration with the most anterior, or rather with the one nearest to the anterior medullary velum. The fissures were similarly designated as L, IL, IIL, and IV. Of the four fissures, II. was found to be constantly the deepest, and therefore corresponds to the sulcus furcalis of Stroud, the sulcus primarius cerebelli of Kuithan, and the fissura prima of Elliot Smith. Embryological evidence was sought in an animal in which the five lobes are comparatively simple, viz. in the rabbit. It was found that the fissures appear in this animal in the following order: IV., IT., III., I. Contemporaneous with the appearance of fissure II. a pair of depressions—one growing inwards from each lateral border of the cerebellum—start to cut off the future paraflocculus from the main mass of the future hemisphere. These two fissures finally join fissure III., thus indicating that the paraflocculus is the most lateral part of lobe D. Lobe E begins its existence in intimate association with what is to become the posterior medullary velum, and the flocculus does the same at the most lateral part of the pontine flexure. For this reason it was held that the fiocculus is an outlying territory of lobe E. The next fissure found in a mesial section was one involving lobe A, and was named c. As a consequence of the presence. of this secondary sulcus, lobe A is subdivided into lobule A,, the more inferior part, and lobule A,, the part above fissure c.
Shortly after the advent of fissure ¢ a pair of sulci become apparent in the lateral (hemisphere) part of lobe C. These were known as fissures @ and b—a being the more posterior. The two halves. of fissure a gradually approach each other in the middle line, and finally fuse, the two portions of fissure 0 doing the same a little later. Lobe C is now subdivided intc three lobules—C,, C,, and C,—the enumeration being from before backwards.
The constancy of these fissures was established by the examination of a number of cerebella differing only. very slightly from that of the rabbit. Very important modifications were found in the cerebellum of the squirrel, which, as a consequence, served as a connecting link between the simpler and the more complicated cerebella. The modifications consisted in the antero-posterior expansion of the hemisphere portion of lobule C,; of the appearance of a fissure (named d@) which divided lobe D into lobule D, and lobule D,; and of an increase in size of the paraflocculus and the arrangement of its folia in two tiers (paraflocculus dorsalis and ventralis of Elliot Smith), the upper of which was found to be connected with lobule D,.
Next, the development of the cerebellum of the pig was traced. This was found to follow the same main lines as that organ in the rabbit, but additional important evidence was discovered. The most striking point, and the one to which most weight was attached, was the evident connection, at one stage of development, of the two portions of the paraflocculus with the two portions of lobe D. Indeed, it was clear that the paraflocculus was divided into two parts by the lateral growth of fissure @. The connection between paraflocculus dorsalis and lobule D, was found to persist into adult life, but that between paraflocculus ventralis and lobule D, disappeared in the later stages of intra-uterine development. It was obvious, however, that not only is the paraflocculus as a whole a part of lobe D, but that the two parts of the paraflocculus belong to the two parts of lobe D, each to each.
From the examination of a number of adult cerebella from various animals, it was found that the disappearance of the connection between lobule D, and the paraflocculus ventralis is the rule; but in one specimen of the horse’s cerebellum, traces of the original connection were found. In many animals the continuity of lobule D, also becomes faint, but on the other hand it may persist, and often does so.
Since the above conclusions were put into writing, Elliot Smith and Louis Bolk have made additions to the literature. The former has made several communications regarding the morphology of the cerebellum in mammals in general. A short paper, published by Elliot Smith at the time my MS. was in the hands of the Editor of the Journal of Anatomy and Physiology, is mainly a condensation of what he has already said in his monograph on the brain of the Edentates (8). In it he adheres to his original statement of the division of the mammalian cerebellum into two floccular lobes and a large inter-floccular mass subdivided into three azygous lobes (anterior, middle, and posterior) by the fissura prima and the fissura secunda. No exception whatever can be taken to the importance attached to the fissura prima. As has been previously stated, it is the furcal sulcus of Stroud and the fissura primarius of Kuithan, and its morphologic status is beyond question. But it appears to me that there are objections to placing the fissura secunda on the same level.
There seems little doubt that it cerresponds to my fissure d, which I have given reasons for considering as secondary in character. It is only an inter-lobular fissure, and owes its importance mainly to the fact that it divides lobe D into portions, linked together at the oral end of the paraflocculus, as Elliot Smith’s diagram and description admit. Further, there does not appear to be sufficient distinction made between flocculus and paraflocculus. That such distinction should be made is indicated by the difference in the development of the two structures. There is, indeed, a danger that the name of “lobus flocculi, as applied to the two parts, may lead the reader into error. It would be better to always speak of them as flocculus and paraflocculus separately and distinctly ; though it must be admitted that their close relation to each other makes a collective name very convenient.
As a consequence of exalting fissura secunda into a sulcus of primary value, the nodulus and uvula (mesial portions of lobe E and lobule D,) are thrown together as constituting the ‘posterior lobe, which is stated to be connected with the flocculus. As has been previously remarked, development appears to pretty clearly indicate that the flocculus belongs to lobe E alone; and no adult cerebellum that I have examined has demonstrated any other connection. This is another reason for clearly distinguishing the flocculus from the paraflocculus.
The ‘anterior lobe’ comes in for but slight attention in this paper; nor is a great deal said of it in the memoir on the Edentate brain, although in a figure illustrating the mesial sagittal section of the cerebellum of Orycteropus (fig. 29) two fissures are marked with the letters ¢ and / which appear to correspond very closely with my fissures I. and c¢ respectively.
Apart from the above-mentioned points, Elliot Smith’s views, up to the time of writing the paper just considered, do not differ very widely from those I have previously stated. But in some
1 To prevent a misunderstanding which may arise out of a criticism by Elliot Smith (9), I may be allowed to state that at the time my first paper was written and submitted to the Editors of this Journal (early in 1902), Professor Elliot Smith had expressed no views on the morphology of the mammalian cerebellum
beyond those contained in his Memoir on the Brain of the Edentates. When my paper was written the different opinions on the subject were conflicting in the later communications our divergence of views is increased. In his latest paper on the cerebellum of mammals in general (10), he supplies a scheme differing in material points from that given in his earlier note (¢f. diagram in (8) and fig. 1 (10)). He states that the suprapyramidal fissure (my fissure III.) may behave in different ways. It may join the parafloccular fissure (the fissure separating the paraflocculus from the bulk of the hemisphere, and named the fissura flocculi in the paper on the Edentates), or it may cut across area C, to join the parapyramidal fissure (a fissure not mentioned previously). Fissure -secunda (@) may become continuous “with the floccular” (IV.) “or paratloccular fissures, or even (as usually happens in the human brain) with a fissure in the paravermis” (in area C of the earlier papers). I can only say that the material (embryological and adult) which I have examined does not lead me to conclude that these fissures are capable of such irregularity. It is now admitted that the uvula and nodule “ become separated the one from the other long before the other lobes and lobules become mapped out.” This fact I have already demonstrated as grounds for considering them as parts of separate lobes, but Elliot Smith still maintains that “they are so small and present so many features in common that it is convenient to group them together as one lobe.” Their different development history, however, I hold as sufficient justification for their separation.
The material I have investigated leads me to further differ from Elliot Smith when he avers that the pyramid “can be shown to be continuous (in most mammals) with the parapyramidal area” (a portion of the lateral part of my lobe C), “including not only its pars dorsalis (biventralis), as is taught in Human Anatomy, but also its pars ventralis (tonsillaris).” The importance attached to the copula pyramidis in his earlier writings is now stated by our author to have been unduly emphasised.
Not only are the fissures of the vermis held as being erratic
extreme, and it was therefore considered that it would be better to attack the problem de novo. To keep the mind of the reader as unbiassed as possible, the literature was only barely touched upon. This explains why Professor Elliot Smith’s paper was not specially alluded to.
in their connections, but the parafloccular fissure in the larger mammals “cuts into the alar part of the middle lobe without becoming confluent with any important fissure.” I am unable to reconcile this statement with what I have seen in the material at’ my disposal.
It will be seen from the above that there has been a considerable modification of the views expressed in the earlier publications, with a consequent complication of the picture of the lobus posticus and a part of the lobus medius (called formerly the lobus centralis),
Elliot Smith’s latest communication deals with the morphology of the human cerebellum along the lines just discussed (11).
Among the papers treating of the mammalian cerebellum which have appeared recently is one by Professor Louis Bolk (14). It claims attention as being the expression of opinion of a wellknown anatomist. Bolk commences by condemning that method of description of a cerebellum which takes the human brain for its type, and points out how such a method is a fount of error. He therefore sets aside the human organ, and the orthodox description of it, referring to it only in the later part of his paper. He states that, with the exception of the monotremes, he has examined the cerebellum in several representatives of all the orders of mammalia. In this communication it is his intention to set forth his idea of the structural type (Bautypus) as exemplified in the cerebellum of Lemur albifrons. He seeks, further, to observe the more important variations in different animals, and finally to answer the question—How does the human cerebellum stand with regard to that of other animals, and in what direction has it specially developed? His present statement is to be regarded as preliminary only. In the course of correspondence, he informs me that he hopes to publish his observations in extenso before long.
Bolk divides the cerebellum into two lobes—lobus anterior and lobus posterior—by the sulcus primarius cerebelli (Kuithan). He refuses to recognise the division into a vermis and two hemispheres as based upon firm morphologic ground, since he denies the existence of paramedian sulci in the anterior lobe of any cerebellum he has examined, with the possible exception of that of the Cetaceans. The two lobes are different in several respects, but chiefly in their method of growth. The superficial : expansion of the anterior lobe is greatest in the middle line, the growth intensity (Wachstumintensitat) becoming gradually smaller in a lateral direction. Bolk does not subdivide the anterior lobe, at any rate in the present paper. The posterior lobe he divides into two portions—a lobulus simplex and a lobulus complicatus. Of these the former has characteristics which ally it with the lobus anterior. Its folia are arranged transversely, and there are no paramedian sulci. Moreover, its ‘growth centre’ is in the middle line. Sulci paramediani are present in the lobulus complicatus, and divide it into a lobulus medianus posterior and two lobuli laterales posteriores, the former corresponding to the inferior vermis of Human Anatomy.
There are three growth centres in the posterior lobe—one in the middle line and two placed laterally. The presence of these centres, and their influence upon the surface anatomy is emphasised, and held to be of great importance. The transverse expansion energy of the middle growth centre (i.e. of the lobulus medianus posterior) is very limited. The bilateral centres are likewise of small activity in a direction at right angles to their long axis, but in a sagittal direction they are even more active than the median centre. As a consequence, the lobuli laterales posteriores become considerably curved in brains such as that of Lemur albifrons. In simpler brains the sagittal line of growth is not such as to produce distortion. The curvatures of the lobuli laterales posteriores produce the lobulus ansiformis, the lobulus paramedianus, and the formatio vermicularis, The paraflocculus and flocculus constitute the last.
It is difficult to criticise Bolk’s paper adequately, seeing that itis of the nature of a preliminary statement. One will be in a better position to agree or disagree with him when his theses are expanded. As the case stands at present, there are several points which appear debatable. In the first place, the recognition of two lobes only in the cerebellum may be simple as a method of division of the whole organ, but it is one which may be considered inconvenient and insufficient for purposes of description. This is made evident in Bolk’s own paper. He finds it necessary to divide the posterior lobe into two parts (lobuli simplex and complicatus). A vermis may not always be very conspicuous in the anterior. lobe, indeed in many instances it is impossible to distinguish a vermis and two lateral hemispheres in this region; but a vermis is always sufficiently obvious in the posterior region. And it cannot be denied that a division into a vermis and two hemispheres is a convenience, if not a necessity, for purposes of description of the surface anatomy. of the cerebellum. Though the paramedian sulci may be shallow anteriorly, still, in the majority of animals, their presence can be affirmed. Bolk’s own figure of the cerebellum of the Lemur (fig. 1) shows this. At the same time it should not be forgotten that the paramedian sulci are quite secondary objects, as their late appearance shows.
In his consideration of the lobulus complicatus, Bolk appears to pay too much attention to the adult condition, and relegates the simple transverse arrangement of the fissures to too secondary a place, though he certainly does not overlook it. Indeed, he mentions the fact that originally each part of the posterior lobe consisted of two lateral portions and a middle connecting link. But, unless the embryonic condition be employed as a basis for the division of the adult organ, there is great danger of misconception when adult cerebella are considered.
As an explanation of the mode of production of the complex arrangement of the lobules of the posterior lobe, Bolk’s arguments are ingenious and to the point. But the fact that the complexity is secondary must not be lost sight of. In another paper on the cerebellum of some New World monkeys (13), speaking of the posterior lobe, Bolk says that the circumstance that in Hapale and Midas the lateral parts are joined to the corresponding middle portions by means of low ridges, leads one to suppose that in the posterior as in the anterior lobe the sulci originally ran uninterruptedly from one border of the cerebellum to the other. Therefore, he maintains, one must consider that the genesis of the sulci paramediani is secondary, and of quite another nature from that of the interlamellar fissures. On this point no one who has examined the brain in embryos will express dissent.
In his second paper Bolk considers the cerebella of Hapale, Midas, Chrysothrix, Mycetes, Cebus, and Ateles, which he arranges in this order, as being the one in which the gradual growth of complexity of the lobes among the New World monkeys is best indicated. The whole paper is practically the application of his division of the cerebellum, as previously explained, to a series of concrete examples.
In the large work on Human Anatomy at present appearing under the editorship of Professor Karl von Bardeleben, Ziehen (15) describes the anatomy of the cerebellum at considerable length. He divides the cerebellum of man into eight lobula cerebelli, as shown below :—
1. Lingula cerebelli.
Sulcus postlingualis. 2. Lobulus centralis.
8. postcentralis (cerebelli). . 3. Monticulus.
a. Culmen. S. superior anterior. b. Declive. S. superior posterior. 4. Folium vermis. S. horizontalis. . Tuber vermis.
8. inferior posterior.
S. inferior anterior. 7. Uvula.
S. preeuvularis. 8. Nodule.
It is interesting to note that, though he recognises the importance of the sulci superior anterior, Ziehen still considers culmen and declive to be parts of one lobulus cerebelli.
A very elaborate description of the lobes and lobules follows ; after which comes a section devoted to the comparative anatomy of the cerebellum. Here sundry new names are suggested. The term lobulus impendens is used as it was in the monograph on the brain of monotremes and marsupials ; that is, it is applied to the declive and tuber vermis in those forms in which these overhang the culmen, eg. in Rodentia. Ziehen’s lobulus eructformis is found in the hemisphere, and corresponds “ without doubt to the lobuli semilunares superior and inferior (of ‘man).” It is in the form of a worm-like complex in the carnivora, The term lobulus palpiformis is used to indicate a finger-like lobule lying anterior to 1. eruciformis, and is evident in Ungulata. Under the name of tabulata is included the flocculus and paraflocculus, at any rate in some animals, and it is suggested that it may correspond, in part at least, to the ‘tonsil.’,
In those animals in which the lobules of the vermis are apparently fewer than in man (¢g. the rodents), the lobes in the posterior part of the cerebellum are considered by Ziehen to be either tuber vermis and pyramid (including uvula and nodule), or, a8 appears more likely, pyramid and uvula (including nodule). Facts have been adduced in an earlier part of this paper for traversing the supposition that the nodule and uvula are combined in any cerebellum.
Ziehen is doubtful as to the homology of the small lobule found below the lobulus petrosus in rodents. This has been shown to be the flocculus.
There are other points to which attention might be directed, but the above will suffice to show that Ziehen’s contribution to the literature, while painstaking and detailed, would have been even more valuable if it had had an embryological basis, and had the brain of man been used as a goal, and not as a startingpoint.
Still more recently Ziehen has given a description of the brain of Tarsius, Nycticebus, and Galeopithecus (17). In an illustration showing the appearance of a mesial sagittal section of the cerebellum of Tarsius spectrum (fig. 5), five lobes are indicated; these being a lobulus centralis, culmen, lobulus impendens, pyramis, and a lobule stated to represent the combined uvula and nodule. This last lobe is perfectly simple, but the ‘ pyramis’ is divided into two nearly equal parts by a moderately deep fissure.
In the following pages is contained an expansion of the writer’s previous research. Some additional embryonic material has been investigated, and the examination of adult cerebella has been continued. The same methods as those used in the earlier work have been followed. The brains of embryos have been carefully exposed and sketched in outline, all the fissures visible to the naked eye, or by means of an ordinary pocket lens, being introduced in their proper relative positions in the sketch. Then the whole cerebellum, or rather more than half of it when its size was considerable, was embedded in paraffin and serial sagittal sections cut. These sections have been worked through, outline drawings being made wherever alterations in the fissures were encountered. In this way the first outline sketch of the surface has been checked. The smaller adult cerebella have been treated in the same way ; and in the larger organs microscopic sections have been made when the naked-eye examination left any doubt as to the connections of the lobules or the continuity of the fissures.
Sheep Embryos,—Although the development of the cerebellar fissures of the sheep has already been described by Kuithan (2), there are still certain points upon which he is silent which appear to require study. It is a matter for regret that as full a series of sheep embryos as have been already described of the pig could not be procured. But the breaks in the series can be filled, partially at least, by the material described by Kuithan.
68 mm. embryo (Pl. XLVI. fig. 1).—At this stage the cerebellum of the sheep presents much the same appearance to the naked eye as does that of the pig of a somewhat similar size (44 days, 64 mm.). It is very difficult to make out any fissures without the aid of a lens, and even by such assistance fissure II. is not conspicuous. Microscopic sections, however, remove any doubt as to its existence; and they further reveal the presence of two shallow depressions which turn out to be the beginnings of fissures IV. and ¢. Kuithan’s figure of a section of the cerebellar lamina of a 5 cm. embryo gives a very similar picture to that presented by the present specimen. On examining a series of sections beginning in the middle line, all three fissures are soon found to disappear, fissure II. being the last to do so, as was to be expected. In sections of the lateral part of the cerebellum there is a faint fissure, which may possibly be the initiation of a; subsequent development lending support to this supposition.
87 mm. embryo (Pl. XLVI. figs. 2 and 3).—There is already the indication ofa vermis and two hemispheres. Fissure II. is easily visible on the surface, and extends for a considerable distance in a slightly curved direction across the cerebellum. Posterior to fissure II. two depressions can be detected with the unaided eye when the cerebellum is undisturbed, and there is a third hidden between this structure and the medulla. These three sulci are IV., d, and IIL, the last being very shallow. Fissures IV. and d cross the whole of what may be termed the vermis, and appear to be invading the hemisphere. Fissure III. does not extend so far laterally. Sections show unmistakably that fissure IV. is continued to the very lateral boundary of the cerebellum, where it curves forwards and downwards to circumscribe the anlage of the flocculus. This arrangement is one which has been met with in both the rabbit and pig. There are clear indications of the separation of the paraflocculus; that part which now represents it standing out prominently from the side of the cerebellum, limited by a fissure which curves round its anterior end. The fissure, tentatively named a@ in the previous embryo, is now sufficiently deep to be visible by means of a pocket lens. It is in the form of a slight indentation of that rounded mass of the lateral part of the cerebellum which lies between fissure IT. and the future paraflocculus (fig. 2). Fissures I. and ¢ have appeared in front of, or rather below, fissure II. The greater importance of I. is shown by its being a little deeper than c, and extending considerably farther laterally.
100 mm. embryo (Pl. XLVI. figs. 4, 5 and 6).—-In the posterior part of the cerebellum, matters have not advanced much since the 87 mm. stage. Fissures IV., d, and IIT. are deeper, but little more can be said for them. With the exception of IV., they do not extend far into the hemisphere. The paraflocculus is rather more distinctly defined than in the previous specimen. Fissure @ is much the same as it was before. Posterior to fissure II., therefore, development is now slow. But, as one expects from what has been found in other animals, the part in front of II. has become very active in its development. Fissures I. and ¢ are easily detected. Their depth, as shown in sections, is greater than it was in the 87 mm. embryo, I. still extending farther in a lateral direction than c.
114 mm. embryo (Pl. XLVI. figs. 7, 8 and 9).—Great progress has been made, but more especially in the more anterior region. Fissure II. now clearly reaches the margin of the cerebellum, and the margin in this district is well defined. Fissures I. and ¢ are very obvious ;. and sections show that lobule A, is complicated by subsidiary (intralobular) fissures. The posterior part of the cerebellum is also entering upon a period of activity. Fissures III. and d are not much longer (relatively) than before, but they are certainly deeper. Fissure a is now complete; that is, it rums across the entire cerebellum. Since the last stage a fissure 6 has made its appearance, having begun, as far as one can judge, in the middle line. The flocculus and paraflocculus are very distinct, but the latter is still a simple rounded projection from the side of the cerebellum ; that is, it has not as yet begun to divide into dorsal and ventral portions. It is fortunate that a stage such as is illustrated by this embryo was procurable, for here all the fissures are firmly sketched in, the details only being wanting. It is further fortunate because the next older specimen is one in which many, if not most, of the details are added.
163 mm. embryo (Pls. XLVI. and XLVII. figs. 10 and 11).—Viewed: from the front, this cerebellum presents features strikingly like those found in the adult. The foliation is abundant, and the main fissures are deep. It is upon the posterior part that attention will be more particularly bestowed ; because here, though development has proceeded a considerable distance towards the adult condition, the more primitive characters of the lobes are not yet lost, their original transverse disposition being still preserved. Lobule C, contains two folia in the vermis, but these are reduced to one in the hemisphere. Lobule C, has become quite characteristic. In the vermis it is composed of two folia, but in the paramedian sulcus the intralobular fissure disappears, and the lobule becomes constricted. In the hemisphere there is a notable indication of that expansion in a sagittal direction, with which one is now familiar: this part of the lobule has three folia.
Lobule C, is, in the main, very like C,. It has two folia in the’ vermis: there is a constriction in the region of the paramedian sulcus: there is an expansion in the hemisphere. Lobule D, has also certain features of great value. It has two folia in the vermis, which are reduced to one in the paramedian sulcus ; and then the lobule curving under the lower border of lobule C, becomes directly and beyond question continuous with the dorsal paraflocculus, the connecting ridge being transversely foliated. Lobule D, is large, but cannot be said to pass outside the limits of the vermis.
Lobe E in this embryo, and in those younger, is relatively large. If serial sections are examined, it is found to extend very considerably beyond the vermis. It disappears for a short time, and then reappears as the flocculus, which is also relatively large in sheep embryos.
The paraflocculus is now divided into dorsal and ventral limbs, but how the division has been brought about is not clear. Unfortunately, Kuithan is silent upon this point; for, as has been previously said, he did not recognise the true nature of this portion of the cerebellum. At this period the paraflocculus has advanced so far towards the adult condition as to be beset with transverse folia, especially in its dorsal portion.
Although the adult cerebellum of the sheep has been described in the former paper, it may not be out of place to revert to it here. Concerning the anterior part of it there is nothing to be said, because the adult features are recognisable at an early period in the embryo. It is to the region behind fissure IT. that attention is asked. Lobule C, does not undergo any radical changes in the later stages of development, but lobules C, and C, alter very markedly. In the 163 mm. embryo, it has been seen that lobule C, is constricted in the paramedian sulcus, and is of greater antero-posterior diameter in the hemisphere than in the vermis. These features are exaggerated in the adult. The vermis grows so largely in a sagittal direction that distortion is produced. The same remarks apply to lobule C,, except that the constrictions are even better marked, while the distortion in the vermis is little or nothing. These peculiarities in the development of lobe C will be more fully considered in connection with calf embryos, for in the ox the sagittal expansion is much greater, sa far as the vermis is concerned, than in the sheep.
Lobules D, and D, retain their embryonic features, a foliated ridge joining the former to the dorsal paraflocculus. Lobe E in the adult, as in the embryo, is relatively well developed; but, unlike the condition in the embryo, it does not pass the boundaries of the vermis.
The paraflocculus and flocculus of the adult call for no remark, as the 163 mm. embryo prepares one for their appearance in the future.
Calf Embryos.—Three calf embryos have been obtained which, because of their stages of development, add greatly to the evidence in support of some of the statements previously made.
138 mm. embryo (Pl. XLVII. figs, 12, 13 and 14).—The cerebellum of this embryo has already become divided into a vermis and hemispheres by shallow paramedian sulci. All the main fissures are present in the vermis, and many of them extend all the way across the cerebellum. As was found in the embryos of the other animals, the anterior portion had proceeded farther in development than the posterior. Fissure II. can readily be detected on the surface; and when sections are made, it is seen to be of great depth, and with foliated sides. Fissures I. and c can also be clearly detined when the surface is inspected. Lobe A is comparatively simple, but lobe B is divided into two parts by a fairly deep fissure which runs across the central part of the cerebellum. In lobe C there is very unmistakable evidence of a fissure a, which runs a sinuous course across both vermis and hemisphere. It is interesting to note that. fissure 6 is also complete, and, as seen in sections, very much deeper than a. Unfortunately, this embryo is too old to exhibit the method of commencement of fissures a and /. Fissures III. and d are present, but are almost entirely confined to the vermis. There are shallow sulci marking off the paraflocculus, but the more anterior is scarcely to be made out except in microscopic sections. It is important to notice that the paraflocculus is being divided into upper and lower portions by a depression which is deepest anteriorly (fig. 13), and which, therefore, has presumably begun its development in front, and is now growing backwards. The position of the future flocculus is marked by a comparatively slight bulging at the extreme lateral part of the pontine flexure. Lobe E is small in the middle line, but becomes a little larger laterally. Its appearance suggests that peculiarity of development from the border of the cerebellar lamina which has been observed in the rabbit and pig.
150 mm. embryo (Pl. XLVII. figs. 15 and 16).—Little need be said of the condition of the parts in front of fissure II, Fissures ¢ and I. are both of considerable depth, and numerous intralobular fissures have developed. Lobe C is now completely divided into its three lobules. Fissure 6 has retreated from fissure IJ. to some extent, and is of much greater depth than a, as was found in the previous embryo, Fissure III. is complete, and, along with a, bounds a very characteristic lobule C,, which shows the sagittal expansion which was found in other animals. Fissure « is more extensive than in the 138 mm, embryo, but, even now, does not reach very far into the hemisphere. It is still some little distance short of joining the fissure which has been previously described as dividing the paraflocculus into dorsal and ventral limbs. Indeed, it appears as though the latter fissure is growing towards d rather than that d is extending much laterally. The dorsal paraflocculus is showing signs of developing folia. Fissure IV. is of considerable depth and very sharply defines lobe D, with its lateral parts, from lobe E and its connection with the flocculus.
175 mm, embryo (Pl. XLVII. figs. 17, 18, 19 and 20).—This specimen shows a fairly great advance in development. There is now no difficulty in distinguishing a vermis and two lateral hemispheres in all parts of the cerebellum, with the possible exception of the most anterior. The anterior surface is well foliated, and its main fissures are of good depth. But even at this stage the lateral limits of lobes ‘A and B are indefinite (fig. 19). This is, no doubt, to be associated with the remarkably small lateral expansion of these lobes in the adult (fig. 21). Lobules C, and C,, on the other hand, have very clear lateral boundaries. Again, it is to be noticed that fissure & is relatively very deep, and comparatively close to fissure II. Lobule €, is consequently narrow in a sagittal direction. Lobule C, is well developed in the vermis and in the hemisphere, and is beginning to show a decided constriction in the region of the paramedian sulcus. The vermis portion of lobule C, remains small, and still consists of only one folium. Its lateral segments have increased in a sagittal direction, and have become foliated. As a consequence of the lateral growth of lobule C, (which now extends as far out as the paraflocculus), lobule C, appears dwarfed in its transverse diameter. Fissure d is of good depth in the vermis, but rapidly becomes shallow in the paramedian sulcus. Though there is no direct evidence to be obtained from the present material, there can be little doubt, arguing from analogy, that it will ultimately become continuous with the fissure present in the earliest embryo, and which now divides the paraflocculus into two very distinct portions. The characteristic growth of lobule C, has resulted in a very clear definition of the paraflocculus. The dorsal paraflocculus carries two separate fissures ; the ventral paraflocculus is still perfectly simple. Both the mesial part of lobe E and also the flocculus are small, this being in remarkable contrast to what was found in the sheep.
It is very interesting to compare this cerebellum with that of the adult animal (Pls. XLVII. and XLVIIL figs. 21, 22 and 23). Lobes A and B apparently do not alter in the later stages of development, but the lobes behind fissure II. undergo a noteworthy alteration in form. In lobule C, the change is merely one of increase of size, with a resulting backward displacement of fissure 6. In lobule C, there is a great increase in size in the hemisphere, accompanied by an alteration in the direction of the intralobular fissures, and so considerable a growth in the vermis that this part of the lobule is bent upon itself, -and even then occupies so much room that the vermis segment of lobule C, is crowded over to one side (figs. 22 and 23). The middle portion of fissure a ran in an almost perfectly sagittal direction in the specimen from which the figures were drawn. The three portions of lobule C, also expand in an antero-posterior direction. As just stated, this lobule is crowded out of the vermis, and therefore comes to lie in what was once the paramedian sulcus. Those portions of the lobule which forms parts of the hemisphere are connected with the vermis by very much constricted but still quite obvious bridges of cortical substance. Lobe D shares in this antero-posterior development, and consequently its lobules present a sinuous appearance in the vermis, their folia being thrown out of the transverse direction completely. The connection between the dorsal paraflocculus and lobule D, is. maintained in the adult state, but is, to a certain extent, obscured by the growth of lobule C;. The paraflocculus in the adult animal is rather smaller than is usually found in Ungulates, and the arrangement in dorsal and ventral tiers is not so clear as in the embryos described. It is evident that lobe E must develop greatly after the stage illustrated by the 175 mm. embryo; for in the adult it is large and richly foliated, usually extending as far backwards as. to be visible when the cerebellum is examined from behind (fig. 23). The flocculus also increases markedly in size.
The cerebellum of the ox affords a very good illustration of Bolk’s. ‘growth-centres,’ The ‘centre’ in the vermis expands in a sagittal but not in a transverse direction. This expansion is so great that the whole vermis behind fissure ) is thrown into a series of billowing coils, Bolk’s bilateral ‘growth-centres’ are also illustrated, but not. in so striking a manner as that in the vermis.
Horse Embryo 18 weeks old (Pl. XLVIIL figs. 24, 25 and 26). I have only been able to get one embryo which shows the method of development of the fissures in the equine cerebellum, but this happens to be of a stage to which great interest is attached. There is no difficulty in distinguishing a vermis and two hemispheres, even in the most anterior region. Fissures c and I. are well developed, the latter being only second to fissure II. in point of depth (fig. 26). No better specimen could be desired to support the contention that fissure I. is of fundamental importance. This fissure evidently continues of great depth from the time of the 18 weeks embryo onwards, for in the adult it is only slightly shallower than fissure II. Lobule A, is becoming divided into its permanent three subdivisions. There is a fairly deep indentation of the surface of lobule A,, which no doubt corresponds to the deeper of the two intralobular fissures met with in the adult. Lobe B is relatively as extensive as in the adult, and has already become subdivided. Fissure II. is as pronounced as one would expect it to be at so advanced a period. Lobe C is comparatively large, and consists of three expanded portions, separated by faint constrictions in the paramedian sulci. In it are two depressions, which may possibly correspond to fissures a and 6. Fissure } (?) crosses the entire cerebellum, and the element of doubt which attaches to its identity is not so great as that concerning fissure a (?). The questionable fissure @ is present as a shallow depression in the vermis, but does not cross the paramedian sulcus. There is a fissure running from the border of each hemisphere towards fissure a (?), but not joining it. If this is indeed fissure a—and older material is needed to remove all dubiety—then it either develops somewhat differently in the horse from what has been found in the embryos of the pig and sheep, or else a phase of its growth has been caught in this specimen which has been missed in the others. The former view seems the more probably correct.
Fissure III., though not of any great depth, is complete. Lobe D presents the most interesting feature in this cerebellum. Those portions of it which will ultimately become paraflocculi form very prominent projections from the sides of the cerebellum. They are as yet perfectly simple, but it is evident that fissure d is gradually growing into them, in the manner in which it develops in the pig. From the material examined, we may conclude that the dorsal and ventral portions of the paraflocculus are separated off from the primarily single mass in at least two different ways. On the one hand, fissure @ may extend laterally, as in the pig and horse. On the other hand, an independent fissure may appear in the most lateral part of this region of the hemisphere, and grow towards fissure d, with which it finally fuses. The latter method is found in the ox, and possibly also in the sheep.
Concerning lobe E, one need only remark that it is fairly large in the vermis, and that the flocculus is as yet small and inconspicuous. An examination of the adult cerebellum shows that, though there is some distortion produced by the growth in a sagittal direction of lobules C, and C, in the vermis, this is not nearly so marked as in the ox, nor does it involve lobe D, The connection between lobule D, and the dorsal paraflocculus is always distinct in the adult.
Didelphis azure (Pls. XLVIIT. and XLIX. figs. 27, 28, 29 and 30).—Opportunity has been afforded me of examining the cerebellum of this marsupial in a perfectly satisfactory condition, and the examination has been productive of several points of interest. The freshness of the material when it came into my hands (the animal was actually living when it reached me) allowed me to obtain sections in the best possible state.
A first glance at the cerebellum of Didelphis shows one an organ very similar to that found in some of the rodents. There is the same comparative simplicity, and the same prominent lobulus petrosus. In lobe A no paramedian sulci are present. A fissure ¢ cuts off a simple undivided lobule A,. Lobule A, is larger than the preceding, and is subdivided intv two parts, each of which has two folia on the surface. Fissure II. is deep, transverse in direction in the vermis, curving sharply downwards and forwards on reaching the paramedian sulcus, and running in a sagittal direction in the sulcus. Lobe B, as the result of the course of fissure II., presents a four-sided area on the surface. This lobe is relatively larger than it is in most animals, and is subdivided by a fissure running almost parallel with II. Lobe C is a veritable lobulus impendens, in the sense in which the term is used by Ziehen, It is very much greater in its anteroposterior diameter in the hemispheres than in the vermis. Whether a fissure } is present is open to question, but there can be little doubt of the actuality of fissure a, which, running a curved course, much as it does in the rabbit, cuts off a distinct lobule C,. This lobule consists of two folia in the vermis, but is reduced to one in the hemisphere. At first sight it appeared that lobule C,; was continuous with the dorsal paraflocculus, but this was found not to be the case. Fissure III. passes for a considerable distance into the substance of the cerebellum, and runs a course similar to that of fissure a. Lobe D is subdivided by a fissure d, which sections show to be even deeper than III. Lobule D, has one folium only, lobule D, hastwo. Fissure IV. and lobe E do not offer anything of interest, so far as a naked-eye examination is concerned.
It was considered advisable to make perfectly sure of the arrangement and connections of lobule Cz, lobe D and lobe E. To this end serial sections of one-half of the cerebellum were very carefully worked through, and drawings made wherever crucial points were encountered. The following facts were noted. In the vermis these three parts of the cerebellum were found to consist of six folia, which may be numbered as follows:—1 and 2 belong to lobule C,; 3 belongs to lobule D,; 4 and 5 are parts of lobule D,; and 6 is lobe E. In following the various folia in a lateral direction, the following was observed. Jfolia 1 and 2 blended with each other, 4 and 5 doing the same. That is, lobules C, and D, were reduced to one folium each. Then folium 3 gradually became less and less prominent, and finally disappeared. The next change was the termination of the single folium produced by the fusion of 4 and 5, this giving place to a wide sulcus, the bottom of which was formed of white matter. Folium 6 was next greatly reduced in size, and came to consist of a small ridge of grey matter. Now grey matter began to appear at the bottom of the sulcus left after folia 4 and 5 had come toanend. This was found to become the paraflocculus. At the same time folium 6 began to increase in size. The combined folia 1 and 2 diminished rapidly, and finally, after becoming detached from the rest of the hemisphere, ended. Folium 6 was ultimately traced into the flocculus.
To sum up. Lobule C, ends at the border of the hemisphere. The whole of lobe D soon comes to an end when the limits of the vermis are passed. Lobe E, however, is directly continuous, as a grey ridge, with the flocculus. This last fact adds considerably to the strength of the assertion that the flocculus is a part of lobe E, and of nothing else. The paraflocculus begins to appear in the depths of the sulcus left by the disappearance of lobe D. This might lead one to suspect the original connection of these two parts, even if one had no other evidence.
To the naked eye, the paraflocculus consists of several folia and carries a very distinct lobulus petrosus which is closely invested by bone. The flocculus consists of one folium, and its identity is only rendered clear by the aid of microscopic sections. It expands to a certain extent as it passes forwards. | Pteropus medius and Xantharpyia (Rousettus) collaris—The cerebella of these two fruit-eating bats have been examined, and are found to be very similar to that of Pleropus poliocephalus, which is described in my former paper. In discussing the cerebellum of P. poliocephalus, I said that “there is apparently no fissure c.” The new material leads me to think that this is possibly not the case. It is quite likely that the fissure marked I. in the figures should be called c, and that the fissure two folia distant above it should be indicated as deeper and labelled I.
Herpestes Mungo (Pl. XLIX. figs. 31, 32, 33 and 34).—In some respects the cerebellum of the mongoose departs from the average type as found among the carnivora. Its anterior portion presents nothing very remarkable, there being the usual fissures and lobules, disposed in the customary manner. If we take the deepest and most continuous fissure in lobe C to be fissure }, then lobule C, is remarkably small. It consists of three folia in the vermis, and these become two in the left hemisphere and only one in the right. There can be little doubt regarding the presence of a fissure a in the hemisphere, where it is in a position usual in the carnivora, but it cannot be traced with certainty into the vermis. Lobule C, is of moderate size in the vermis, but it becomes very narrow at the paramedian sulcus. Lobule C, has very typical carnivore characters in the hemisphere. In this region it is developed into a row of folia, running practically vertically, and reaching the medulla. Lobule D, is rather small, and both it and lobule D, are limited to the vermis. There is nothing remarkable about lobe E.
The paraflocculus of the mongoose differs to a certain extent from what may be considered as the type. A superficial examination would lead to the conclusion that there is but one row of folia, but on looking more closely into it two rows are disclosed. The ventral limb, however, is small, and extends backwards for only a short distance, tailing away in the interval between the flocculus and the dorsal paraflocculus. A lobulus petrosus, of some size, is carried by the ventral paraflocculus ; but instead of springing from its most posterior end, it is attached auteriorly.
The flocculus is rather larger, relatively, than is mostly found in the carnivora. Its anterior end is broad, and composed of two or three folia. These fuse to form a single grey ridge, which passes inwards to become continuous with lobe E in the vermis. The connection of this ridge with the medullary velum is very intimate.
Procyon lotor (Pls. XLIX. and L. figs. 35, 36, 37 and 38).—The cerebellum of the racoon resembles, in a general way, that organ in any other moderately large carnivore. A detailed description of all its parts is not called for, since the figures in illustration of this cerebellum speak for themselves. But there are some points to which it is deemed desirable to direct attention. Fissures c and ] and lobe A are very like those features in other cerebella. The large size of lobe B, however, is remarkable ; as is also its almost entire inclusion in the vermis, because of the curved boundary formed by fissure II. Lobe C is of great interest. The identity of fissue 6 is uncertain. If the fissure marked 6 in the figures is really homologous with the boundary between lobules C, and C, in other animals, then one at once notices its asymmetrical arrangement. Lobule C, shows in a marked degree the characteristic sagittal expansion in the hemisphere, with the result that its folia run in several different directions. Fissure a@ is very decided in the hemisphere, but when traced mesially it becomes lost in the depths of fissure III. From this it follows that one cannot distinguish with certainty a lobule C, in the vermis, though sections show a fissure which may at one time have been part of a. Lobule C, in the hemisphere is comparatively small, consisting of a row of some half dozen folia, against which the dorsal paraflocculus abuts.
The remainder of the lobes do not depart to any very great extent from the customary arrangement. But it should be noted that lobule D, is continued into the hemisphere as two folia, one of which, the upper, is the larger, and touches the dorsal paraflocculus. Lobule D, is confined to the vermis. The paraflocculus is composed of the usual dorsal and ventral portions. The dorsal paraflocculus is much the larger, and extends backwards to a considerable amount. So far as this specimen is concerned, one might easily commit the mistake of supposing that the dorsal paraflocculus is a continuation of lobule C,. In short, there is an appearance typical of the lobulus ansiformis of Bolk. So far as the naked-eye examination. goes, there is no connection of lobule D, with the dorsal paraflocculus, though we feel fairly safe in assuming that such a connection was originally present, and that the coiled row of folia, which Bolk calls the lobulus ansiformis, is of secondary development. The ventral paraflocculus is small, and does not reach so posterior a level as does its dorsal companion. The flocculus, as is often the case in the carnivora, is small, and consists of one folium only ; it is mainly hidden by the paraflocculus, between which and the roots of the 7th and 8th cranial nerves it lies. Its attachment to the lateral part of the posterior medullary velum is a very intimate one.
Viverra civetta (Pl. L. figs. 39, 40, 41 and 42).—In this cerebellum there is certainly no paramedian sulcus anterior to fissure II. ; therefore it affords an excellent example in support of Bolk’s refusal to recognise a vermis. ‘The fissures in this region being numerous, and all of approximately the same depth, it is impossible to decide with certainty upon fissure I. and c¢. Still a fissure I. can be tentatively fixed upon from an observation of the arrangement of the rays of white matter in a mesial section. Lobes A and B have a relatively great antero-posterior diameter, which no doubt makes up for their comparatively small transverse width. This exuberance of sagittal development causes fissure IT. to be placed far back in the vermis.
Lobe C is very singular, Fissures 6 and a cannot be distinguished, nor can the different lobules be determined upon from the disposition of the folia. The most anterior part of the lobe certainly resembles lobule C, as we are accustomed to see it, but its boundaries are not clear. It is, however, in the posterior part of the lobe that the most noteworthy features are found. There is the usual narrowing of the lobe at the paramedian sulcus, and the consequent reduction of the intralobular fissures. But these fissures are remarkable for the manner in which they join together just at the paramedian sulcus. In the hemisphere the fissures mostly end without joining each other,
Tn lobe D the paramedian sulci are much shallower than is often found to be the case in the cerebellum of a carnivore of this size. Lobule D, has three folia in the vermis, but in the hemisphere the lowest folium is very slender and soon fades away. Between lobes C and D, and close to the paramedian sulcus, is an uncertain area on each side (dotted in fig. 41). It is connected, quite distinctly, with both lobe C and lobule D,, and it is impossible to decide to which it really belongs, though its positio: makes one think that it represents lobule C,. Whether this is an accidental variation or the usual arrangement, can only be decided from an examination of other specimens. It may, however, be at once stated that Viverra malaccensis does not offer a similar enigma (fig. 43).
There is nothing peculiar in lobule 1),._ It belongs exclusively to the vermis. Lobe E is fairly large, and is interesting because it shows a condition only occasionally met with. A grey lamina extends for some distance outwards from it, running under lobule D,. This lamina is attached to the medullary velum, and reaches nearly to the posterior end of the flocculus.
The paraflocculus is very commonplace. It is composed of the usual two limbs, the ventral of which carries a lobulus petrosus at its posterior end. The flocculus is mainly concealed. At its anterior end it consists of two folia, one above the other. The upper folium is small; the lower reaches as far back as the posterior end of the paraflocculus, and is then continued by a very narrow ridge to the grey lamina or folium projecting from lobe E. This should be remembered in connection with what has already been seen microscopically in Didelphis and macroscopically in Herpestes, and with what is yet to be remarked in Cebus.
Viverra malaccensis (Pl. L. fig. 43).—This merits special attention on account of the points in which it differs from the cerebellum of V. civetta. Lobes A and B are the same in both animals, and there is the same difficulty experienced in determining which are the primary fissures anterior to II. It is in lobe C that the chief differences reside. One feels again the same hesitation in deciding upon a fissure b, but a can be more readily recognised. Its position in the hemisphere is clear, as it forms the upper boundary of a very typical lobule C, in this area, and it can be traced, though not very easily, across the vermis. Lobule C, shows that bending upon itself which has been frequently seen, and the radiation of its folia from a deep sulcus which has arisen secondarily as the result of the bending. The lateral extent of lobule C, is inconsiderable. In the vermis it consists of a vertical row of some four transverse folia; in the paramedian sulcus it is narrow. There is no uncertain area such as was met with in V. civetta, As in the larger animal, there is no difficulty in observing the connection of the three parts of lobule D,. There is no continuation of either lobule D, or lobe E beyond the vermis.
The paraflocculus is like that of V. civetta, and the flocculus is small. There is no attempt at continuity of grey matter between the flocculus and the central part of lobe EK. Ot
Phocu vitulina (Pl. L, figs. 44, 45 and 46).—At first sight the cerebellum of the seal appears an inextricable tangle of fissures and coiling lobes. Beside it the cerebellum of man appears comparatively simple, for in man the fissures do run transversely in the main, but in the seal their course is in all directions. A closer examination shows that the labyrinth occurs in the hemispheres, the vermis being almost perfectly straightforward.
Taking the organ as a whole, it may be described as decidedly flattened from above to below. This is perhaps best indicated by actual measurements of the vermis of a cerebellum after being preserved in a solution of formalin, followed by alcohol. In the particular specimen from which the accompanying figures were made the vermis measured 37 mm. from before backwards, and 23 mm. from above to below. These figures are approximately equal to those given by Flatau and Jacobsohn (6), who state the dimensions of the vermis as being 4°0cm. by 2°4cm. This vertical flattening is even more pronounced in the hemispheres, which slope somewhat rapidly downwards and outwards from the vermis (fig. 45). The hemispheres, further, have a greater antero-posterior diameter than the vermis, since they project farther forwards (fig. 44). There is a deep and moderately wide notch in the anterior border (‘incisura cerebelli anterior’), but no corresponding notch behind. The vermis appears narrow, but this appearance is doubtless fictitious, and produced by the great expansion of the hemispheres. Anterior to fissure II. there are really no paramedian sulci, but posterior to this fissure these sulci are deep. If we except fissures a and b, which are rather doubtful, there is little difficulty in recognising the various customary fissures, lobes, and lobules in the vermis. There is certainly a deep fissure in lobe C, which one feels inclined to call a, but its identity is lost as soon as it reaches the paramedian sulcus. There is also a moderately deep fissure anterior to a, but it is by no means certain that it is fissure 0.
Unlike the vermis, the hemispheres present many instead of few difficulties. Lobes A and B are comparatively simple, the latter forming the bottom of the notch already mentioned as existing in the anterior border of the cerebellum. Lobule C, is only slightly more difficult to recognise than A and B. But the remaining lobules give rise to much hesitation, as they are confused and deep fissures are many. It is well-nigh impossible to follow the fissures of the vermis into the hemisphere ; indeed, in some it is quite impossible to do so with any degree of certainty. The following statements, therefore, are put forth as appearing to me possibilities and probabilities, not as matters of certitude. It is difficult to see how we can be positive regarding the homologies in this particular region of the seal’s cerebellum until someone has been fortunate enough to get an embryo or embryos of an age when the fissures and lobes are simple.
It appears feasible to suppose that the large lobules, which project forwards and form the lateral boundaries of the notch already spoken THE MAMMALIAN CEREBELLUM: ITS LOBES AND FISSURES. 473
of, are lobules C,. They occur as double rows of folia, connected anteriorly ; and one can easily imagine that the originally few and transverse folia have increased so enormously in number that the lobule has been bent upon itself. This would demonstrate a much exaggerated activity in a sagittal direction of Bolk’s lateral ‘growthcentre.’ If the above supposition be correct, then the lobule which is placed posterior to the large double row of folia is lobule C,. One can speak a little more confidently of lobule D,. It consists of an extremely well-developed vermis portion and two rows of transverse folia in each hemisphere, the three parts being moderately well connected together, z.e. the paramedian sulci are not so deep here as in some other places. Lobule D, and lobe E do not extend into the hemisphere. At first sight the paraflocculus appears to be too large to correspond to the paraflocculus of other animals, but a close examination of it leaves one little option. The doubt that the whole of it is the homologue of the paraflocculus in the rest of the mammalia is small, The dorsal paraflocculus is much larger than the ventral, and consists of a number of radiating masses of folia. From a point a little in front of the middle of its lower surface springs a lobulus petrosus. The ventral paraflocculus lies along the side of the medulla oblongata, immediately behind the pons. The flocculus cannot be seen until the ventral paraflocculus has been removed, It is moderately large, and is composed of a row of folia.
The above account differs in some points from that given by Flatau and Jacobsohn, but it is the one which seems best fitted to bring the cerebellum of the seal into line with that of other mammals.
(1) Stroud BB. The mammalian cerebellum, part 1: The development of the cerebellum in man and the cat. (1895) J Comp. Neurol. 5: 71-118.
(2) Kurruay, W., “Die Entwickelung des Kleinhirns bei Saugetieren,” Miinchener Medic. Abhandl., vii. Reihe, 6 Heft, 1895.
(3) Ziggen, Tu., “Das Centralnervensystem der Monotremen und Marsupialier,” Theil i, Semon’s Zoologische Forschungreisen in Australien und dem malayischen Archipel. Jenai’sche Denkschriften, vi, 1897.
(4) ScuwaxzeE, Lehrbuch der Neurologie, Erlangen, 1881.
(5) Extrior Situ, G., ‘The Brain in the Edentates,” Trans. Linn. Soc. Lond., Zoology, 2nd series, vol. vii. part 7, p. 277, 1899.
(6) Fuatav, E., und Jaconsoun, L., Handbuch der Anatomie und vergleichenden Anatomie des Centralnervensystems der Sdugetiere, I. Makroskopischer Teil, Berlin, 1899.
(7) Bradley OC. On the Development and Homology of the Mammalian Cerebellar Fissures: Part I. (1903) J Anat. Physiol. 37: 112-30. PMID 17232548 Bradley OC. Development and Homology of the Mammalian Cerebellar Fissures: Part II. (1903) J Anat Physiol 37: 221-240.13. PMID 17232554
(8) Exxior Smiru, G., “On the Primary Subdivision of the Mammalian Cerebellum,” Journ. Anat. and Phys. vol. xxxvi. p. 381, 1902.
(9) Ibid., ‘Notes on the Morphology of the Cerebellum,” Journ. Anat, and Phys., vol. xxxvii. p. 327, 1903.
(10) Zé¢d., ‘Further Observations on the Natural Mode of Subdivision of the Mammalian Cerebellum,” Anat. Anz., Bd. xxiii., p. 368, 1903.
(11) Ibid., “The Morphology of the Human Cerebellum,” fev. Neur. and Psychiatry, vol. i. No. 10, p. 629, Oct. 1903.
(12) Zbid., “On the Morphology of the Brain in the Mammalia with Special Reference to that of the Lemurs, recent and extinct,” Trans. Linn. Soc. Lond., Zoology, 2nd series, vol. viii. part 10, p. 319, Febry. 1903.
(13) Boux, Louis, “ Beitrage zur Affen-anatomia, IV. Das Kleinhirn der Neuweltaffen,” Morph. Jahrb., Bd. xxxi. Heft 1, p. 44, 1902.
(14) Ibed., “Hauptziige der vergleichenden Anatomie des Cerebellum der Siiugetiere mit besonderer Beriicksichtigung des menschlichen Kleinhirnes,” Monatsch. 7. Psych. u. Neurol., Bd. 12, Heft. 5, p. 432, 1902.
(15) Zizuzn, Tu., ‘‘Makroscopische und Mikroscopische Anatomie des Gehirns,” Handbuch der Anatomie des Menschen, herausgegeben von Prof. Dr Karl von Bardeleben, 10 Lief., Jena, 1903.
(16) Stroup, B. B., “The Morphology of the Ape Cerebellum,” Assoc, Amer. Anatomists, Dec. 28-30, 1897, p. 107.
(17) Zensen, TH., “ Ueber den Bau des Gehirns bei den Halbaffen und bei Galeopithecus,” Anat. Anz., Bd. 21, p. 505, 1903.
Explanation of the Figures In Plates XLVI-L
|Fig. 1. sheep embryo, 68 mm. Mesial sagittal section.
Fig. 2 sheep embryo 87 mm. Posterior view. x 2.
Fig. 3 sheep embryo 87 mm. Mesial sagittal section.
Fig. 4. sheep embryo 100 mm. Superior-posterior view. x 2.
Fig. 5. sheep embryo 100 mm. Anterior view. x 2.
Fig. 6 sheep embryo 100 mm. Mesial sagittal section.
Fig. 7 sheep embryo 114 mm, Superior-posterior view. x 2.
Fig. 8. sheep embryo 114mm. Mesial sagittal section.
Fig. 9. sheep embryo 114mm. Anterior view. x 2.
Fig. 10. sheep embryo 163 mm. Superior-posterior view. x 2.
|Fig. 11. sheep embryo 163 mm. Mesial sagittal section.
Fig. 12. calf embryo, 138 mm. Superior-posterior view. x 2.
Fig. 13. calf embryo 138 mm. Left lateral view. x 2.
Fig. 14. calf embryo 138 mm. Mesial sagittal section.
Fig. 15. calf embryo 150 mm. Postero-superior view. x 2.
Fig. 16. calf embryo 150 mm. Left lateral view. x 2.
Fig. 17. calf embryo 175 mm. Superior view. x 2.
Fig. 18. calf embryo 175 mm. Posterior view. x 2.
Fig. 19. calf embryo 175 mm. Anterior view. x 2.
Fig. 20. calf embryo 175 mm. Medial sagittal view. x 2.
Fig. 21. Bos taurus. Anterior view. x 4.
|Fig. 22. Bos taurus. Superior view. x 4.
Fig. 23. Bos taurus. Posterior view. x 4.
Fig. 24. horse embryo, 18 weeks old. Posterior view. x 2.
Fig. 25. horse embryo, 18 weeks old. Anterior view.
Fig. 26. horse embryo, 18 weeks old. Mesial sagittal section. The section has been cut slightly obliquely ; the anterior part of the figure therefore shows the fissures, etc., a little more to the right than the posterior part.
Fig. 27. Didelphis azare. Superior view. x 2.
Fig. 28. Didelphis azare. Anterior view. x 2.
Fig. 29. Didelphis azare. Postero-lateral view. x 2.
Fig. 30. Didelphis azare. Mesial sagittal section.
|Fig. 30. Didelphis azare. Mesial sagittal section.
Fig. 31. Herpestes mungo. Anterior view. x 2.
Fig. 32. Herpestes mungo. Superior view. x2.
Fig. 33. Herpestes mungo. Posterior view. x 2.
Fig. 34. Herpestes mungo. Mesial sagittal section.
Fig. 35. Raccoon Procyon lotor. Anterior view. x1.
Fig. 36. Raccoon Procyon lotor. Superior view. x1,
Fig. 37. Raccoon Procyon lotor. Posterior view. x1.
|Fig. 38. Raccoon Procyon lotor. Mesial sagittal section.’
Fig. 39. civet Viverra civetta. Anterior view. x 1.
Fig. 40. civet Viverra civetta. Superior view. x1.
Fig. 41. civet Viverra civetta. Posterior view. x1. The dotted areas are connected with both lobe C and lobule D,
Fig. 42. civet Viverra civetta.Mesial sagittal section.
Fig. 43. civet Viverra malaccensis. Posterior view. x 1.
Fig. 44. seal Phoca vitulina. Superior view. x 4.
Fig. 45. seal Phoca vitulina. Posterior view. x 4.
Fig. 46. seal Phoca vitulina. Inferior view. x }.
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