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| [[File:Mark_Hill.jpg|90px|left]] This historic 1895 paper by Stroud describes the development of the {{cerebellum}} in human and {{cat}}. | | [[File:Mark_Hill.jpg|90px|left]] This historic 1895 paper by Stroud describes the development of the {{cerebellum}} in human and {{cat}}. | ||
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Stroud BB. The mammalian cerebellum, part 1: The development of the cerebellum in man and the cat. (1895) J Comp. Neurol. 5: 71-118.
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The Mammalian Cerebellum
Part I. The development of the Cerebellum in Man and the Cat
By Bert Brenette Stroud, B.S.
- Part I was presented to the Faculty of Cornell University as a thesis for the degree of Doctor of Science, May i, 1895. The remaining parts (Comparative Anatomy, Histology and Function) will be published as soon as circumstances permit.
Introduction
In 1891 the writer prepared a baccalaureate thesis on the Flocculus. The present investigation was begun as an effort to determine (i) whether the flocculus occurs in all mammals and (2) what are its anatomy, histology and functions. It soon appeared, however, that these questions are part of a more comprehensive problem as to the constitution of the entire cerebellum.
Acknowledgements. — This thesis has been prepared in the Anatomical Laboratory of Cornell University. It is with the greatest pleasure that I acknowledge my indebtedness to Professor B. G. Wilder for permission to examine all of his preparations and for criticism, counsel and encouragement.
I am indebted also to Professor S. H. Gage and Instructor P. A. Fish for advice as to methods, photographing specimens, etc. ; to Mrs. S. H. Gage for suggestions as to drawings and for permission to examine sections of embryo kittens' brains ; and to Dr. H. B. Besemer and other medical friends for valuable material.
List of Brains Examined
Note. — The following list, while incomplete, will indicate approximately the nature of the material upon which the writer has based his conclusions.
The accession numbers are the serial numbers on the Accession Book of the Museum of Vertebrate Zoology of Cornell University.
It is unnecessary to more than mention the brains of the cat, dog, and sheep ; the laboratory is well supplied with them.
Embryo brains of man and the cat are enumerated in the descriptions of plates. A list of the adult human material studied will be given in Part II.
Ac'n
No.
Sex.
Age.
Groups.
SUB-CLASS I. PROTOTHERIA.
Order I. MoNOXRiiMATA.
66
76
Male Male
Echidna aciilcata, spiny anteater. Ornithorhynchus anatinus, duck-bill.
SUB-CLASS II. METATHERIA. Order II. Marsupialia.
3559 712
67
Female Female Male
Fcetal
Dnielphys virginiana, common opossum. Twelve or fifteen specimens were available.
1331
378 108
Male Male
Macroptis giganieus, giant kangaroo. Hypsiprymnus tnoschatus, kangaroo-rat.
SUB-CLASS III. EUTHERIA. Order III. Edentata.
126
3220
Male
Adult Adult
Brad\p7is tridactylus, three-toed sloth. Tatusia scpteincincta, seven-banded armadillo.
Order IV. Sirenia.
844
Male
Adult
Manatiis ainericanus, American manatee. Order V. Cetacea.
^670
Female
Globioceplialiis melas, black-fish, pilot whale. Order V. Ungulata.
2177
Male
Cantelus bactrianus, Bactrian camel.
692 2122
961 1207
776
Male Male
Fcetus at term.
Juv.
Camelus dromedariiis, dromedary. Cervuliis elaphus, red deer. " " " fawn. Cariacus dama, fallow deer. *
2679
Ovis aries, common sheep.
195
Foetus
(h (< C( (t
Stroud, Mammalian Cerebellum.
73
Ai-'n No
2125 2126 2262 3078 2776
337f 2259 2127
3T^ 276
3365
212
3362
3364
2657 2729
St'x.
Male Male
Male
Female
41 1 Female 4S7 Male
3075 Female 269
2906
3212
2938! Female
3370 3352 3353 2907 2960 3357
3367
737
3121
2181
144
3369 2124
2937 1803
175 2919
163
146 151 157
Male
Male Female
P'emale
Male
Female
Male
Male
Male
Male
Male
Male
Female Smo . Male 48hr. Female 22da.
Foetus
Foetus Adult
Adult
Adult
Groteps.
Eqiius caballiis, domestic horse.
Eqitiis asimis, burro. Sus scrofa, domestic pig.
Tapirus malayanus, Malayan tapir. Bos tauriis, domestic cow.
Order VII. Toxodontia.
No specimens.
Order VIII. Hyracoidea.
No specimens.
Order IX. Rodentia.
Arctomys ntonax, woodchuck, marmot.
Castor fider, American beaver. Cynomys ludoviciamts, prairie dog.
Fiber zibcthicus, muskrat.
Lepus cuniculus , rabbit.
Mhs decumamis, brown rat. Mtis musctibis, domestic mouse. Sciurus hudsonius , red squirrel.
Sciuropterns volucella, flying squirrel.
Order X. Proboscidia. Elephas indicus, Indian elephant. Order XL Carnivora, Cams aureiis, Indian jackal. " dingo, dingo. " latrans, coyote, prairie wolf. " fainiliaris, domestic dog.
" " Scotch shepherd.
" •« St. Bernard.
«' " Blood hound,
and several other varieties.
Felis Ico, African lion.
74
Journal of Comparative Neurology.
Ac'n
No.
Sex.
Age.
195
II wk.
156
Male
309
Female
2945
Male
277
Male
304
Male
783
Female
Juv.
263
399
Female
452
Male
752
Female
308
Female
3156
Juv.
770
Male
158
Male
3199
Male
Juv.
2946
Female
3131
Female
Adult
2779
Male
2031
Male
2822
3361
143
Female
3155
Female
1
3156
Female
[- 8da
3157
Female
J
3089
Male
194
196
Female
197
Male
3368
Adult
170
Male
171
Male
152
Female
1-2 yrs
645
Male
517
Female
3356
3093
349
3164
Juv.
^2359
22
3205
571
664
Male
342
Male
2918
Male
3111
Female
Groups.
Fell's leopardus, leopard. Feiis concolor, puma.
(canadensis, Canada lynx. rufits, common lynx. maculata, Texas lynx. Felis pardalis, ocelot.
Felis domestica, domestic cat.
Hyaena striata, hyena.
Vulpes fuhnis, red fox.
Lutra canadensis, otter. Procyon lotor, raccoon.
Mephitis viephitica, skunk. Nasiia rit/a, coaiti-mondi.
Ftitorius vison, mink.
Putoritis domestica, ferret.
" vulgaris, weasel.
Phoca viinlina, seal.
Ursus ame?-icanus^ black bear. " torquatus, Thibet bear.
Order XII. Insectivora.
Blarina brevicauda, mole shrew. Condylura cristata, star-nose mole. Galeopithecus volans, colugo.
Order XIII. Cheiroptera. Alatapha noveboracensis, red bat.
Vespertilio subidatus, common brown bat. Order XIV. Primates.
A. Lemuroidea. Nycticebus tardigradiis, loris. Tarsias spectrum, tarsius.
Chiromys madagascarensis, aye-aye.
B. Anthropoidea. Marmosets.
Hap a le jacch us .
Midas adipus.
Marmoset.
Stroud, Mammalian Cerebellum.
75
No.
Sex.
Grou/s.
Cebid?e.
2911
Female
Cebus apella.
1808
3375 3104
Female
I Ateles melanocheir.
CallUhrix sciurieits. Cercopithicidse.
307 308
Male Male
I Cynocephahis antibis, baboon.
2977 3327 3215
Female Female Female
y Cynocephahis babiiin, common baboon.
2555 3073 3219
Female Female
Cynocephahis sphynx. \ Macacus cynomologics, common macaque.
2S72
Male
" erythraeus.
3072 2998
Male Male
|- Macacus nemtstrmus.
3171
Female
"■
3068 3076 2988 2888
Female
Male Male
|- Macacus rhesus 1
3196 2873 2548 3082
Male Female
Female
Juv.
> Simia satyrus, orang.
265
Female
Juv.
Anthropopithecus troglodytes, chimpanzee.
A. Macroscopic methods.
Having obtained an embryo, the first thing to do is to harden it. If the specimen is quite small simply immerse it in 50 per cent, alcohol, one to three days ; in 6'j per cent, alcohoi, one to three days ; finally in from 75 per cent, to 80 per cent, alcohol, where it may remain indefinitely. The brain may be exposed at leisure.
Dissection of Fresh Specimens. — With older embryos where the cartilaginous skull is thick or has begun to ossify, the brain must be exposed at once or it will not harden well. This is a delicate and tedious operation for the young tissues are so soft that when an opening has been made through the skull, a slight pressure upon almost any part of the head will force a considerable portion of the brain out through the opening, which of course ruins the specimen.
The method employed by the writer has been to wrap some absorbent cotton around the body of the embryo and then place it in a cradle made from a piece of sheet lead bent in this way (^ y The sides are bent together just enough to hold the specimen in place.
The whole is then immersed in a solution of common salt, glycerine and water, or any other suitable liquid of a specific gravity such as will float the embryo. Then with fine forceps, scissors, and needles piece by piece, the skin and skull are removed. The pia should be removed after it has been in the 75 per cent, alcohol for a day or two.
The hardening fluid must not contain glycerine, for it shrinks the tissue; chromium salts make the brains too brittle. The writer prefers plain alcohol for hardening specimens to be used for gross preparations.
Moimtiiig for Museum Specimcjts. — A convenient support for small embryos is made from strips of basswood cut into suitable pieces. These are first boiled in water to which has been added a little caustic potash solution, and then in water containing a small amount of hydrochloric acid to remove all extractive and coloring matter. The blocks are further washed in water, dried, smoothed, coated with glue and finally painted with carbon drawing ink^ and dried.
The blocks are weighted with lead. The specimen may be attached by means of small pins, preferably made of silver, or aluminum.
Figuring. — The writer found great difficulty in obtaining correct outlines of embryonic brains. At first an attempt to photograph them was made. This method, while giving excellent results for large specimens, proved unsatisfactory for small ones. Outlines were made with the camera lucida and details added free hand, the specimen being kept in a vessel of alcohol all the time, whether it was to be photographed (Gage, 12) or drawn with the camera lucida. The object should be well lighted.
- The use of basswood and carbon ink was suggested by Professor S. H. Gage.
B. Microscopic 7iictJiods.
1. Morphology . — Where the specimen is too small to be readily handled, it can be examined much more easily if sectioned than in any other way. However one should have both a gross preparation and sections of the same stage of development whenever possible. A perfect idea can be obtained in no other way. Sections also should be cut in three different planes : {a) Sagittal sections ; (^) Transections ; {c) Frontal sections.
It may not be possible to obtain four specimens of the same stage, but transections and sagittal sections are almost indispensable. Where only one specimen is available, the writer has adopted the following method. First, cut sagittal sections till the meson is just passed, then remove the tissue from the block and fix it on again so as to cut the remainder of the tissue in transections. Figure 32, PI. Ill is drawn from a section which illustrates this procedure.
The fixing, hardening, imbedding, etc., should be the same, whether one wishes the sections for morphology or histology. If for the former, sections may be cut much thicker, from 40 microns to 120 microns, and should be stained only very lightly, the thickness of sections and amount of tissue cut noted so that one can make a wax model, if desired. Sections are cleared and mounted in balsam in the usual manner.
2. Histology proper. — Tissue may be fixed and hardened after any of the approved methods. The writer has used alcohol and potassium dichromate; also 50 per cent, alcohol saturated with common salt two to four days, then alcohols as in A, with good results. Before fixing, the skull must be removed; but great care is needed to avoid tearing the delicate telas and other membranes.
Imbedding. — This must be very carefully and thoroughly done, or loss will be the inevitable result. After dehydrating, the tissue is placed in a mixture of equal parts of strong ether and 95 per cent, alcohol for two days; then into 2 percent, collodion for 2 to 3 days ; then into 6 per cent collodion for 4 to 10 days. This last infiltration with thick collodion is exceedinglyimportant, for upon its thoroughness the usefulness of the specimen depends. It is well to allow the collodion to become quite thick before the final imbedding. When the process has been carefully done it is an easy matter to cut even large sections lo microns thick.
Tissue is imbedded by placing it in a small paper box and filling the box with thick collodion; the box is then placed in a jar of chloroform which hardens the collodion. The object is cleared and cut in the castor-thyme oil mixture : red oil of thyme, 3 parts; castor oil, i part.^
Staining sections. — Nearly all of the usual staining fluids gave good results. Of the hematoxylin stains Gage's, Herrick's, and Mallory's hematoxylin proved highly satisfactory.
Forms selected. — Two forms have been chosen to illustrate the development of the mammalian cerebellum.
1. The Cat, a fairly representative mammal, from which embryos of different ages may easily be obtained.
2. Man whose brain is very highly specialized. An attempt has been made to determine in what the essential differences and agreements consist.
The cat was studied first, because its brain is less modified than that of man.
Plates. — All the figure's have been carefully drawn from nature by the writer from his own dissections. Of the gross preparations each one has been minutely examined by aid of a lens and drawn at least twice ; indeed some have been drawn several times. Detailed explanations will be given in the description of plates.
Those relating to the cat. Plates I and II, were studied first and all except Figs, i, 2, 3, and were 23 drawn from careful measurements. Figs, i, 2, 3, were outlined by the camera lucida and 23 was photographed.
Plate III shows sections of embryo kittens' cerebellums.
ip. A. Fish. A new clearer for collodionized objects. Proceedings of the American Microscopical Society, XV, pp. S6-S9, 1893.
Plate IV shows sections of both kitten and human cerebellums. Plates V to VIII show gross preparations of embryonic human cerebellums.
In the figures no attempt has been made to show parts other than those concerned in the subject of this paper ; but to facilitate orientation the oblongata, the mesencephal, and in a few cases the cerebrum have been added in outline.
Defects. — Material and time to trace every consecutive step in the development of the cerebellum are lacking ; but it is believed that the most important stages have been studied and discussed. The conclusions given are to be regarded as the writer's interpretation of his available material and subject to such modifications as further research may require.
Terminology
A careful study of the cerebellum in the different groups of mammals will convince even the most sceptical that the descriptions found in the present standard works on encephalic anatomy are inadequate for an exact comprehension of this important organ ; i. e., the mammalian cerebellum does not fit the description.
If we take a cerebellum from each of the fourteen orders of mammals, beginning with the monotremes and ending with the primates, at first sight one would say that they are as unlike as are cats from horses. But closer inspection shows that they all agree in certain fundamental characters.
On the other hand it may be said that a given region which is very prominent in the cerebellums of animals belonging to one order may in another order be so reduced and overshadowed by adjacent parts as to be altogether overlooked.
It may not be clear at first but my investigations have convinced me that there is one fundamental plan for the cerebellum running throughout all mammalia, but that this plan is variously elaborated in the different orders.
Therefore the writer ventures to suggest that our present conception of the cerebellum is incomplete.
Perhaps the greatest cause for this misconception lies in the fact that at first it was not thought worth while to study other than human brains. So the human condition was taken for the standard ; and when the brains of the lower mammals came to be studied they were warped into an agreement with the assumed standard condition found in man.
Now this is a serious error. For the human cerebellum represents a highly specialized form. It is, in fact, a morphological monstrosity and can in no sense be taken for the typical mammalian organ. The writer also believes that certain of the morphologically distinct regions have not been recognized, that parts of the same morphological integer have not been considered as such, and that we have as yet no exact standard for comparison.
The writer's idea is that the typical mammalian cerebellum would be a composite drawn from careful comparisons of the cerebellums of all the different orders. Because, —
1. The fundamental plan appears to be the same for all.
2. A given region or feature may be excessively developed in one order and almost obliterated in another order.
Having thus briefly indicated a few reasons why, in the writer's opinion, the present terminology of the cerebellum should be modified, it is only proper to state the opinions of some of the authorities on encephalic anatomy.
More than twenty yeas ago Professor Wilder (34) remarked:
" But the part most enveloped in obscurity, as to its development, its structure, its functions, its size, nay, its very existence, is the cerebellum."
"The development of the brain is treated only as a division of embryology and as such is apt to be overlooked until after the time when it might be most useful in aiding the comprehension of the organ." — Idem (35, 142).
" Usually the adult human brain is the first and only object of examination and is taken as the standard of comparison ; if animal brains are studied at all they are often taken as they come, or as anatomical rarities, not selected in accordance with a principle which might indicate the probable degree of their usefulness. The comprehension of the macroscopic morphology of the brain involves the removal of difficulties varying in kind and degree. These several categories of difficulties should be attacked separately and in the order of (i) their fundamental importance and (2) their simplicity." — Idem (55, 145) A. Van Gehuchten says (32, p. 32): " Les sillons les plus profonds des deux faces du cervelet, ainsi qui les lobules qu' ils delimitent, ontrecu des noms particuliers. Mais nos connaissances de la structure et surtout des fonctions du cervelet sont encore si incompletes que, dans I'etat actuel de la science, cette division et cette homenclature n'ont guere d'importance.
Alfred Schaper (28, p. 493) thinks, from his investigations of the brains of teleosts, that present statements concerning the development of the cerebellum are incorrect and that further investigations should be made.
" In descriptive anatomy an astounding variety of names are applied to the various parts of each lobe of the cerebellum ; it would be an essential gain if at least three fourths of these names could be discarded." — Charles Sedgwick Minot (23, 674). To this the writer would add that, whatever the structure of the cerebellum may be, its terminology must be such that an accurate description of it can be written.
And further, that numerous instances, in standard literature, can be cited where descriptions and figures of brains are incorrect or incomplete, due in some cases, as remarked by B. G. Wilder (35) to,—
[. "Injuries in removal, — by which the appendicular lobes, hypophysis, olfactory bulbs, have been torn off and left in the skull, telas and other delicate parts of the brain have been ruptured " so as to convey the impression of openings which did not exist in nature.
2. Failure of the artist to comprehend morphological details. Anatomical drawings to be useful, ought always to be made from carefully prepared specimens, by some one who has at least a good working knowledge of the general anatomy of the parts concerned.
"It (the cerebellum) must be considered as a compages having throughout, from centre to innermost recess, connection and contiguity, forming such a wicker tissue as to present a labyrinthine and, to the anatomist, completely inexplicable knot, unless nature while living in this intricate abode teaches him to unravel her tissue in the same order in which she herself composed it." — Emanuel Swedenborg, 1744.
"Search after the fundamental lines in the structures of the brain is the present task of the brain anatomist. Once we have accomplished this, it will be easy to understand the complicated conditions present in the more highly organized brain," — Edinger (9, vi).
"The key for understanding the complex brain must be sought in the study of its earliest embryonic stages." — Wilhelm His (16, 348).
The examination of the adult cerebellum gives absolutely no conception of what the organ was like in its earlier development. The only way that it can be comprehended is to "see it grow."
The ectal appearance of the adult organ is that of an oval corrugated mass. The caudal aspect presents an appearance like that of a little hill in a valley between two high mountains. The cephalic aspect is more like that of a long hill in the middle of a plain. The entire surface, plain, hills and mountains, is cut by a perplexing tangle of little ravines and deeper gorges, for the most part parallel, whose beginnings and endings are as bewildering as are the figures of the quadrille to the novice.
A closer inspection shows that there are lobes separated by deep fissures, and that the lobes in turn are cut into a greater or less number of narrow folia by shallower fissures. These lobes have received special names, many of which are artificial and unscientific.
But the study of the adult organ can give no better knowledge than this.
If, however, we begin with early embryos and trace the development of the cerebellum to its adult condition, many of the difficulties disappear.
The writer believes this method to be the soundest one and that it should always be followed if we expect to gain an intelligent knowledge of the brain as a whole or in part.
So far as we can judge from the study of two forms so widely separated as are man and the cat, it seems probable that the general structural plan is the same for all mammals. The question then arises, what is the difference between man's cerebellum and that of other mammals ?
If one had specimens from the whole mammalian series placed before him, it is true the first impression would be that there was little resemblance between them, least of all to man's. A closer study and analysis discloses many features which do appear to be homologous. And I can positively say that so far as the embryos of cat and man are concerned the early stages are apparently identical. The same features and regions are present Both start from the same point; soon, however, modifications appear and the two rapidly diverge from each other. But in spite of all this, embryonic regions, whether they become almost obliterated or hypertrophied beyond all proportion, do persist throughout the life of the individual.
But to return to the question, how does man's cerebellum differ from the others ? I hold that it is simply a case of difference in degree of developnumt.
Intrinsic Terminology. — Unfortunately we possess no intrinsic terminology of the cerebellum. What effort has already been made in that direction is mostly of a superficial and artificial character. The thing needed is a terminology which shall apply equally well to the cerebellum of an opossum, a sheep, or man. Until this has been done, the cerebellums of different mammals can not be easily compared.
The introduction of new terms, while undesirable, is a necessity, because the present terminology is inadequate. There are distinct regions which, it is believed, have never before been recognized. Wherever new terms occur they will be explained. In all possible cases terms already familiar have been retained.
It seems to me that terms applied to parts of the cerebellum ought not to be applied to structures of the cerebrum ; e.g. , the term peduncle, applied to the peduncles of the cerebellum, has also been used in another sense to indicate bundles or tracts of fibers in both the cerebellum and the cerebrum, which latter use can not come under the original meaning of the term. Confusion would be avoided if those structures were designated by terms which should mean what they are, bundles or tracts of fibers. The needs of a progressive and scientific comparative neurology demand terms which are appropriate and not likely to be confounded.
Principles of a Terminology . —
1. Terms should mean something.
2. They should be appropriate.
3. They should be euphonious.
4. They should be mononyms ; /. e., single word terms.
5. They should, when possible, indicate a definite idea of the morphologic or embryologic form or relation of the structures designated.
6. The same term should not be used to designate two different things either in the same brain segment or in different segments.
Morphologically the foldings in the cerebellum are probably homologous with those in the cerebrum and the same statement is true for the fissures. But it does not seem advisable to employ the same term for those structures in two different brain segments. Therefore we shall designate a fissure of the cerebellum by the term sulcus, as proposed by Wilder, and an adjoining ridge by the term /"<?////';//. (137, p. 125.)
hiconsistencies in old terms. — Peduncles. — For the purpose of general and comparative morphology the writer would suggest that it would be best to restrict the term peduncle to the peduncles of the cerebellum, and to regard each peduncle as containing at least three great fiber tracts : viz., (i) the cephalic tract, (2) the ventral, or pontile tract, (3) the caudal tract. These are called by Wilder respectively prepedimcle^ medipeduncUy and postpediincle.
Vermis. — The term is both inappropriate, and ambiguous.
But it has taken so firm a root in the literature, that it might be unwise to discard it. It simply designates the mesal part of the cerebellum and is a secondary development.
The vermis as it exists in the adult brain is not present in early embryos.^ See also Plate I, Figs. 8 and 10, Plate V, Fig. 56, Plate VI, Fig. 58.
The writer fails to recognize it in his specimens till the other parts are quite far advanced in development. Compare Figs. 17, 20, 69, and 72. Considering the mode of development, the term fasiigiuni would be more appropriate than vermis.
Hemispheres, Lateral lobes. — The cerebellum is commonly described as consisting of three parts, a middle part, vermis, and two lateral parts, the so-called lateral lobes, or hemispheres. This, it is true, is the apparent condition in man, but we shall show later that it is not the real condition, for the parts which seem to be lateral are so because their excessive development has overshadowed and concealed parts which lie still farther laterad. This is clearly seen in the lower mammals.
The mammalian cerebellum presents at least four distinct regions; viz., one mesal and three lateral regions.
1. The middle part (vermis).
2. The pileum (hemisphere or lateral lobe).
3. Paraflocculus (accessory flocculus).
4. Flocculus.
These regions will be discussed farther on.
Historical
During the first quarter of this century an interest began to be manifested in the development of the cerebellum. The writings of one French and three German investigators upon this subject are recorded.
The first work that I have found mentioned was in 18 12, by los. et Car. Wenzel (33, 256). It is quoted by Burdach (3, 419), who states that in a child three years old the ratio of the cerebellum to the cerebrum was i : 6.
- This was mentioned by Wilder (37, p. 125.)
In 18 14 Ignaz Dollinger (8, 19) stated that at the middle of fetal life the cerebellum is proportionally the smallest, being to the cerebrum as i : 24, but that it rapidly increases till at one month after birth the ratio is 1:17, and in the adult it is i : 6.
In 181 5 Serres (29, 77-107) stated that in the human embryo the cerebellum does not appear until the seventh week of development, and in the fowl until the sixth day of incubation. He also says that the organ is formed in the following manner : two laminse spring from the crura of the cerebellum (Kleinhirnschenkeln) and come to rest against each other at the meson. Later they gradually grow together, after which new lamellae are added both cephalad and caudad, and transverse furrows appear and multiply.
In 18 1 6 Tiedemann (31, 155) says, "At the beginning ot the second month a soft fluid substance occupies the place of the cerebellum. Later in the month two small thin plates arise from each side of the oblongata, along the fourth ventricle, turn inward and rest against each other, but do not unite till later. In the third month they have increased in size and represent the corpora rcstifo}iiiia, called peduncles by Willis and crura by other anatomists."
There follows a period of about forty-one years during which no attention appears to have been given to the subject.
The next reference that I find is a work on the development of the brain by Krishaber (19), published in 1865 ; the next to a description of a fetal brain by Callender (5), in 1870.
1874. His (14, 106) says, "The cerebellum, in most or ders of mammals is divided into a middle part (vermis) and two lateral pieces separated from it by a furrow. The vermis is formed from the mesal roof of the brain tube, i. e. , out of such constituents as were present before the development of the lateral masses (hemispheres)."
1876. Mihalkovics (22, 53) says, "The roof of the epencephal (Hinterhirndecke) forms an expanded lamella which arches over the epicoele. At the right and left it bends over into the epcoeHan floor. Cephalad it is separated from the mesocoelian roof by a constriction, and becoming thinner caudad, merges into the metatela. From the caudal aspect, because of the wedge-like cephalic point of the metatela, this lamina looks hke a pair of lateral plates. This plate may be called Kleinhirnlamella, (lamina cerebelli) because it is the proton (Anlage) of the future cerebellum. The cerebellum is formed from this lamella in the following manner : the cephalic portion of this lamella thickens, while the caudal edge at the transition into the metatela becomes sharpened,"
This thin margin bends ventrad and there results a plaitlike folding in along the caudal edge of the ' Kleinhirnlamella, ' The cephalic, thickened part of this transverse plait (or fold) is the proper proton (Anlage) of the cerebellum. The caudal is the " hinteres Marksegel, ' (velum medullare posticum, v. Tarini) " or what the writer calls kilos. From his own observations the writer can not accept this view as to the development of the cerebellum.
1877. Alix (i) pubhshed a paper on the fetal brain.
1878. Kolliker (20, 537) says, "The cerebellum is developed as a thickening of the cephalic part of the roof of the epencephal (Hinterhirn), which soon assumes the form of a transverse plate. From the lateral aspect it looks as though it bent around the epencephal (Hinterhirn)."
1884. Wilder (35, 179; 37, 125) figures the cerebellum of a human embryo in which the lateral parts are massive, the mesal region is thin and presents a wide groove upon the dorsal surface. It is probably a little older than the embryos shown in Figs. 54 and 56, PI. V.
1890. Hertwig (18, 360) says, "The wall of the fourth brain vesicle undergoes a considerable thickening in all of its parts, and surrounds its cavity in the form of a ring, differentiated into several regions. The floor furnishes the pons. From the lateral walls arise the peduncidi cerebelli adpontem. But it is the roof that grows to an extraordinary extent and gives to the cerebellum its characteristic stamp. At first it appears as a thin transverse ridge which overhangs the thin attenuated roof of the medulla."
1891. Herrick (13, 5-14) describes his investigations upon the brains of rodents and certain reptiles. He obtains results which throw a new light upon the cerebellum and mark an epoch in the study of its histogenesis. He has discovered proliferating regions in the epicoelian roof which apparently are identical with the writer's lateral protons.
Professor Herrick makes a strong plea for the application of the comparative method to the study of neurologic problems. To this argument, the writer joins his most hearty support.
1 89 1. His (15) discusses the development in the human brain of the region from the isthmus to the myel, during the period from the end of the first to the beginning of the third month. He includes the entire region under one segment, the Rautenhirn, but does not deal especially with the cerebellum.
1892. His (16, 373-375) gives a brief description of the development of the cerebellum. He says, "The embryonic epencephal (Hinterhirn) appears as a conical tube of which a portion of the metatela (Rautenfeld) forms a part. Its caudal limit is at the pons flexure; its cephalic, if we first subtract the constituents of the isthmus, is at the isthmus flexure. Between the pons flexure and the isthmus flexure lies the dorsally convex j^knee of the epencephal. The metatela narrows rapidly caudad from the knee, but a narrow extension of it whose edges are nearly parallel extends cephalad. The floor of the metepiccele (fourth ventricle) is formed from the ventral zone (Grundplatte), the cerebellum from the dorsal zone (Fliigelplatte). Its hemispheres are formed in the higher vertebrates from the part of the dorsal zone lying caudad of the knee, the vermis from the part lying cephalad of it."
1893. Schafer (27, Vol. I, Part I, 66) says, "The roof of the fourth ventricle inferiorly becomes greatly thinned and expanded. Superiorly the tube becomes gradually more contracted and the roof thicker. This thickening being the rudiment of the cerebellum and of the valve of Vicusscns. In the human embryo, the cerebellum is seen as early as the second month as a thin plate arching over the cephalic part of the metepicoele (fourth ventricle). From this plate, which enlarges only gradually, is formed the middle lobe; later the lateral lobes grow out at the sides."
" The cerebellum consists of two lateral hemispheres joined together by a median portion called, from its peculiar appearance caused by the transverse furrows or ridges upon it, the worm or vermiform process, .... in mammals it is the first part to be developed and to be marked off into subdivisions." (Idem., Vol. Ill, Part I, p, 69.) I shall show that the vermis is not the first part to be developed and marked off into subdivisions.
1894. Schaper (28, 489) published an account of his exhaustive investigations upon the development of the cerebellum in Teleosts. He confirmed Herrick's discoveries and made further observations of great value. His work, and also that of Professor Herrick, will remain as landmarks in the history of the comparative development of the cerebellum.
Having thus given a brief resume of the opinions of the men who have made a special study of the development of the cerebellum during a period of nearly a century, I proceed to a consideration of the facts which lead me to differ from some of the views hitherto advanced.
The Epencephal
Is the epencephal a distinct brain segment ? This is a question which perhaps can not be settled at once.^ But the writer believes that, as has been stated by Wilder (36, 523), for the purposes of comparative and descriptive neurology it would be a great convenience to regard the epencephal as a distinct brain segment ; hence some evidence will be given in support of that view.
In order to reach a logical conclusion we must first decide what constitutes a brain segment. In the case of the primary vesicles, obviously the tube is dilated in three places and contracted in two places. Then shall v/e say that a bending in of the walls, so as to constrict the caliber of the tube, is sufficient to demarcate a segment ? Why do we not have just as much right to say that the opposite condition, or a bending out and consequent widening of the caliber, demarcates a segment? Or may not also a marked change in the character of the whole or of a part of the walls themselves serve to distinguish one segment from another?
- In this connection, see also Osborn (25, 490).
Now if, after development has progressed for a certain time, we carefully examine the third vesicle, we shall find that, —
A. I. The roof of the cephalic part is composed of substantial nervous parietes.
2. That later a pari of this roof is clothed with a layer of ectocinerea.
3. At a point a little caudad of the caudal border of the substantial parietes of the roof is formed a bend (pons flexure) which involves the entire brain tube. The result of this flexure is the formation of a transverse groove which extends clear across the tube. The sides are also forced out ecto-laterad in the form of a U-like fold which may be compared to the rolling collar which the tailor sometimes puts upon garments. As a result of these changes it is obvious that the caliber of the tube is widened at this point.
4. The substantial parietes are continuous only in the floor of the cavity. Compare Figs. 4, 5, PI. I, and Figs. 49, 50, 53. PI. V.
5. The later growth in this part cephalad of the pons flexure is, ia) dorsad, {p) laterad, {c) ventrad.
B. The part caudad of the flexure is distinguished from the part cephalad by the following characters :
1. The roof is a thin membranous tela and remains so during the life of the individual.
2. It never possesses any ectocinerea.
3. It has no growth dorsad.
4. Its growth is in a ventral and lateral direction only. From these facts it will appear that while these two regions do constitute a single dilatation of brain tube, they are separated, in the embryo at least, by a flexure which involves the entire brain tube. ^ In their essential characters they are as distinct from each other as is the myel from the cerebrum.
Therefore it seems to me that the epencephal constitutes a definitive segment of the brain, having a floor, roof and sides. The cephalic walls and roof are convex ectad, and concave entad. They may be compared to segments of a hollow sphere, as the skin of an orange. They are the dorso-meso-caudal extensions of the floor on each side whose meso-caudal borders are divaricated, but still united by the thinner deckplate. These semicircular plates are the Kleinhirnlamelle of Mihalkovics (22, 53). But it is not probable that anything more than a comparatively narrow strip of their caudal portions develops into the cerebellum. Compare Figs, i, 4, 6, PI. I, and Figs. 24a, PI. Ill, 42, PI. IV.
It would appear that these caudal portions or protons are the corpora restiformia of Tiedemann (31).
The embryonic epencephal may be described as a segment of the brain tube lying next caudad of the mesencephal. It begins at the isthmus and ends at the pons flexure. Considered apart from the remainder of the brain it is a subcylindrical tube whose cephalic boundary is contracted at the isthmus and the caudal extremity is widened and rolls out something like the bell of a brass horn, with this difference, the rim extends only around the sides and roof The floor is continuous with that of the metencephal, but is demarcated from it by the pons flexure.
At the isthmus, the tube is constricted to about one half the diameter of the mesencephal ; it gradually widens caudad. Thus there results a subcylindrical funnel-shaped tube composed of substantial nervous substance, whose roof and sides are continuous with the metatela.
The tela is joined to the substantial nervous substance around its entire circumference ; between the substantial parietes and the tela there is a thinner, transitional margin which gradually merges into the tela. The term kilos is proposed to designate it. If the writer understands His correctly it corresponds to the Rautenhppe. See kl., Fig. 28, PI. Ill,
The appearance of the early epencephal is well shown in Figs. I, 3, 4, 5, 6, PI. I, and in Figs. 50, 51, 55, PI. V. In Figs. 1-3, PI. I, the pons flexure is just beginning to appear. Its further development causes a greater separation of the mesocaudal border of the epencephal till it comes to lie in the position shown in Figs. 4, 5, 6 and 7, PI. I. These specimens also show the dorso-cephalic rotation of the two lateral protons of the cerebellum; compare also Figs. 31, 35, 37, PI. Ill, and Figs. 50, 51, 55, PI. V.
In order to understand the changes which follow we must consider those which have preceded.
1. The brain tube was once an open groove.
2. The lateral walls of this groove have extended dorsomesad and fused along the meson. Now we would naturally expect that the sides of the tube would be the thicker and contain potential proliferating elements. Does not this idea explain why the Deckplatte is so thin ?
As the pons flexure is developed, the kilos and a part of the substantial wall are forced ectad and in this manner the lateral U-bend is formed. The ectal opening between the two arms of the U is closed by a portion of the original metatela which is thus folded off, and later becomes the dorso-caudal wall of the parepicoele. See Figs, i, 3, 5, 12, PI. I, Fig. 60, PI. VI, and Fig. 65, PI. VII.
Dorsad of the pons flexure, a fold of the metatela sinks down into the cavity, bloodvessels follow and form a plexus (choroid plexus of the older writers) which extends clear across the brain tube. The place where the fold occurs forms the line of attachment for the plexus ; it encircles the dorsal part of the brain tube like a girdle, hence the name cestus is proposed for it.
From the foregoing it appears that that portion of the brain tube which lies between the isthmus and the pons flexure presents peculiar characters, such as are not presented by the parts lying next cephalad or caudad of it.
The question which now demands attention is, How much of the roof of this segment is involved in the development of the cerebellum ?
The adult condition may here be considered. Compare Fig. 24, PI. Ill with Figs. 42, 43, 44, PI. IV. In the adult brain we have as mesal landmarks,
1. The mesencephal and isthmus.
2. The kilos. Between the structures lie,
a. The valvula.
b. The cerebellum.
At the constriction of the isthmus, there is a thickening in the roof which increases laterad. It is in fact "a transverse plate inclined at a wide angle to the axis of the oblongata." See Fig. 24, PI. III. Now the universal statement in all textbooks on embryology is that this transverse plate is the cerebellum. It is true that it does bear a resemblance to the amphibian cerebellum. But if this plate is the cerebellum, ivhere is the valvjcla f What also is the meaning of the two lateral thickenings, in the caudal part of the roof, which do not extend across the meson. See the section shown in Fig. 24 «, PI. Ill, and also gross preparations shown in Figs. 4, 6, PI. I, 52, 54, 55, PI. V. Again what is the meaning of the condition shown in Figs. 8 and 10, PI. I, and Fig. 58, PI. VI, where these lateral protons have fused upon the meson ?
The only explanation which the writer, at present, has to offer is this : that the thickening at the isthmus is really less than it appears to be. For, suppose that as a result of the dilatation of the mesencephal its walls become thinner. The effect would be to make the appearance of an isthmus thickening greater than it really is.
The writer believes that the results of his own observations upon the brains of both man and the cat go to show that this so-called transverse plate is not the cerebellum, and that in early embryos the cerebellum does not appear in a mesal section.
The Cerebellum
Definition. — The cerebellum, in its broadest sense, may be defined as that part of the epicoelian roof which is clothed with ectocinerea.
In its early stages it consists of two fusiform protons which lie in the latero-caudal convexity of the roof of the epencephal just cephalad of the kilos and are twisted so as to conform with the general outline of the walls. In the stage shown in Figs, 6 and 7, each extends from near the floor successively cephalad, dorsad, caudad, mesad, and again cephalad and ends at a notch in the roof situated at about one fourth the distance from the meson to the lateral border and forms the cephalic arm of the U-bend. In Fig. 5, a younger embryo, the proton and adjacent parietes are folded over ecto-cephalad. Compare Figs. 7, 29, 32, 50, 51, 52, 53.
The further development appears to be a process of proliferation, by which these lateral protons are extended dorsad, cephalad, and mesad. The mesal extension meets its opposite at the meson and fuses therewith. From now on we have to deal with a single structure instead of with two. Reference to Figs, 42 and 43, PI. IV, shows that the dorsal extension at the base is not as wide as might be expected, but that the organ widens as it extends dorsad. This is still more apparent in human embryos. The result of this method of growth is the spreading, umbrella-like appearance so characteristic of the adult organ.
The transition from the condition shown in Figs. 4, 5, PI. I, and in Figs. 52, 53, PI. V, to that shown in Figs, 8, 9, PI, I, and 58, 59, PI. VI, is very surprising. The explanation which seems most reasonable to me, is that during the process of growth the cerebellum, still thin at the meson, and quite flexible, becomes rotated caudo-ventrad, or folded upon itself; and in this way the ripa is brought from the position it occupies in Fig. 52 to that shown in Fig. 58. It seems to me that the specimens shown in Figs. 35, 36, 37, PI. Ill, in Figs. 42, 43, PI, IV, and in Figs. 56, 57, PI. V, support this view.
His ([5, 22) has a different idea. He says, " Die Oberflache des Kleinhirns verwachst mit dem sie beriihrenden Theile der Deckplatte, an Stelle der primaren vor dem Cerebellum befindlichen Taenia, entsteht eine secundare vom hintern Randesich ablosende und die sackartige Hiille welche das Kleinhirn noch im Beginn des 3 Monats bessen ist spater spurlos verschwunden."
The writer fails to obtain any such idea from his own specimens.
Main Divisions of the Cerebellum. — The dorsal aspect of comparatively late embryos presents one mesal and three pairs of lateral regions, viz :
1. The vermis, azygous.
2. The pilea, paired.
3. The paraflocculi, paired ; in the lower mammals, each is divided into supraflocculus and mediflocculus.
4. The flocculi, paired.
Of these the vermis is the last to appear, as might be expected from the method of development.
The Pilea. — The regions which form the pilea apparently are the first to be formed. Compare Figs. 8, 10, 56, 58. In the adult cat they form quite prominent masses but receive their highest development in man, where they form the chief mass of the entire cerebellum.
Their development is briefly as follows: Beginning with the stage shown in Figs. 8 and 9, there are two lateral sub-ovate masses, joined at the meson by a thin plate of substantial parietes ; compare with Figs. 2, 4, 6, 52 and 58. On the lateral aspect there is a slight depression which evidently foreshadows the division into pileum and paraflocculus. The successive changes are more readily traced in the cat because the enormous human pileum obscures adjacent parts.
The first distinct demarcation is into pileum and paraflocculus by the parafloccular sulcus. This shows clearly in Fig. ii. At a later stage, shown in Figs. 12, 13, and best in 14, a new sulcus has appeared which divides the pileum into two regions, prepileum and postpileum. The postpileum is very small in Fig. 13, but it rapidly increases till it fully equals, if it does not exceed, the prepileum ; see Fig. 17. After this stage the prepileum grows rapidly till in the adult it is very large and well developed, while the postpileum appears quite insignificant (Fig. 23). Contrast with the condition in man where the postpileum forms the chief mass of the postramus. See Figs. 70-75.
From the great size of the pileum, its amount of cortex, and its intimate connection with the myel caudad and the cerebrum cephalad, it would seem that it must be in some way correlated with man's superiority or erect attitude.
In the lower mammals, the prepileum is the larger.
The paraflocathts. — A general idea of the paraflocculus of the lower mammalia can be obtained from a study of the cat's cerebellum. It is a large, richly foliated structure lying between the prepileum and the flocculus and apparently of great functional value. It is divided into two lobes — the supraflocculus dorsad, and the mediflocculus ventrad — by the deep interfloccular sulcus. The supraflocculus is connected with the postpileum by substantial nervous substance. This connection is obscured in the adult by the superficial development of the adjacent parts.
In man, on the contrary, it is variable, small, and would seem to be a degenerated structure of comparatively unimportant functions. In him it is foliated, but I do not feel certain that there is a proper division into supra- and mediflocculus; it appears to be larger in the embryo than in the adult.
The same connection with the postpileum appears to exist in human embryos, but in the adult it is even more obscure than in the cat. Compare Figs. 17, 19, 22, 23, 58, 60, 71, 75.
The mesal connection of the mediflocculus with the postvermis is clearly shown by the ridge of nervous matter, the vermian tract, an enormous bundle of fibers ; it is not readily recognized in the adult, but appears in Figs. 17, 19, 65, 71.
In general it may be said that the adult cerebellum in both man and the cat presents such a plump and compact superficial development that many of the embryonic features are recognized with difficulty.
TJie flocaihts. — In both man and the cat the flocculus proper is foliated and would appear to have about the same morphological significance. It arises from the caudo-lateral part of the roof, between the paraflocculus and kilos; Figs. 17, 18, 58, 60, 61, show its early form and the method of its development. In man, the lateral portions of both flocculus and paraflocculus form the chief mass of these structures. They are compressed and forced laterad by the enormous growth of the postpileum which finally overshadows and dwarfs them. Their mesal extensions are submerged by the growth of adjacent parts ; they apparently exist only as bundles of fibers running to the postvermis, imbedded in the foundation mass of alba. Compare Figs. 65, 70.
The rami. — If we retain the old idea of comparing the cerebellum to a tree, the mesal aspect shows that it consists of two enormous branches — preramus and postramus. The postramus is much the larger.
The preramus. — It appears that at an early stage of development a deep transverse cleft — the furcal sulcus — appears. It divides the cerebellar mass into two unequal portions. The preramus, the cephalic, is the smaller and forms the cephalo-mesal portion of the organ, which lies cephalad of the furcal sulcus. See Figs. 64, 66, 6j, 74. It is relatively much smaller in the lower mammals than in man. The preramus includes the regions recognized as, (i) lingula, (2) cephalic lobe, (3) central lobe, (4) culmen, and their lateral extensions.
The postramus. — This includes the remaining and by far the greatest mass of the cerebellum. It comprises the following mesal parts and the lateral masses connected with them, viz., (i) the clivus, (2) the cacumen, (3) the tuber, (4) the pyramis, (5) the uvula, (6) the nodulus.
In man the clivus and its lateral prolongations constitute the prepileum. In the cat, the clivus, the cacumen, and the tuber with their lateral extensions constitute the prepileum. In man the peduncular sulcus separates the the pre- from the postpileum ; but the peduncular sulcus of man does not correspond to the interpilear sulcus of the cat.
For an idea of the extent and mass of the prepileum in man consult Figs. 6^/ and 66, or the part lying cephalad of the peduncular sulcus.
The postpileum comprises the remaining five lobes of the postvermis and their lateral prolongations. For the extent and relative mass of these in man consult Figs. 64, 66 and 67, and in the cat Figs. 20, 23 and 44.
Summary
The following points are believed to be new or little known.
1. Some ideas about the epencephal as a definitive brain segment.
2. The mammalian cerebellum is developed from the caudal part of the epicoelian roof.
3. The definition of the cerebellum as "that part of the epicoelian roof which is clothed v/ith ectocinerea. "
4. The existence of paired notches in the caudal part of the epicoelian roof. See Fig. 6.
5 An explanation of the shifting of the ripa ventrad.
6. The non-appearance of the cerebellum on a mesal section of the early embryo brain.
7. That the lateral parts are formed first, although, as stated by previous observers, they do not receive their adult foliation till after the mesal sulci have appeared.
8. That mesal sulci appear before the vermis, as a distinct lobe, is formed.
g. The late appearance of the vermis.
10. A new topographic terminology for the cerebellum.
11. The recognition of new sulci :
a. The floccular sulci, paired at first, but fused at the meson later.
b. The parafloccular sulci, paired, but not fusing.
c. The interfloccular sulci, paired, but not fusing.
d. The interpilear sulci, paired, but not fusing. These are best seen in the embryo.
1 2. The first sulci appear on the lateral parts, not on the vermis, as has been previously stated.
13. The existence of the interpileum marking the mesal limit of the interpilear sulcus.
14. The non-correspondence of the peduncular sulcus in man with the interpilear sulcus of the cat.
15. The recognition of a new lobe, the cephalic lobe, in the prevermis.
16. The division of the postramus into two regions, preand postpileum, apparently different in man and the cat.
17. The recognition of the flocculus and paraflocculus as distinct regions, common to all mammals.
18. The division of the paraflocculus in the lower mammals into two parts, supra- and mediflocculus.
19. The connection of the supra flocculus with the postpileum and of the mediflocculus with the postvermis.
20. The recognition of the vermian tract, a distinct ridge in some of the lower mammals, a bundle of fibers imbedded in the foundation alba, in some of the higher.
21. The difference in size and apparent relative functional importance of the paraflocculus in man and the cat.
22. The cestus.
23. The kilos.
24. The development of the auditory eminence.
25. The multiple character of the auditory nerve in its development.
26. The lateral U-bend.
27. The caudal fold.
28. The structure of the metepiplexus.
29. The caudal extension of the mesencephal.
30. The presence of transitory foldings in the roof of the mesencephal.
31. The caudal tip of the mesencephal is bifurcated in the cat, pointed in man.
32. The homology of the different parts of the cerebellum in man and the cat.
33. In the cat, the pyramis is rudimentary.
34. In man the cephalic part of the cerebellum becomes foliated before the caudal part.
35. The identity of the kilos and postvelum.
New Tenns Defined.
Cestits. — Lat. eestns, a girdle, Gr. kestos, stitched, embroidered. The band-Hke Hne of attachment of the metepiplexus. It surrounds the dorsal part of the brain tube like a girdle.
Interpilciim. — \.2i\.. inter, between, 2iX\d p ileum, a cap. In the embryo kitten, a slight elevation between the interpilear and the uvular sulci. Figs. 15 and [6.
Kilos. — Gr. cheilos, a lip, rim. The thin zone of nervous substance which forms the transition between the substantial parietes and the metatela.
PileiLvi. — Lat. pileiun, a cap. A lobe of the cerebellum lying between the vermis and the paraflocculus. Its relation to the peduncle is like that of a cap.
Pugnus. — Lat. pugnus, a fist. An especial development of the mediflocculus inclosed in a cell in the petrous bone, in Rodents, some Carnivora, Chiromys, monkeys, and some other mammals. Named from its resemblance to a fist. [The "appendicular lobe " of the older anatomists.]
Cornell University, Ithaca, N. Y., May i, 1895.
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31. 1816. Eiedemann, Frederic. Anatomic und Bildungsgeschichte des Gehirns ira Fotus des Menschen. Niirnberg.
32. 1893. Van Gehuchten, A. Le systeme nerveux de I'homme. Lierre.
33. 1812. Wenzel, Ios. et Car. De penitiori structura cerebri hominis et brutorum. Tiibi?tg.
34. 1872. Wilder, B. G. On the cerebellum of the lower vertebrates. The Cornell Era, Apr. 12.
35. 18S4. Wilder, B. G. The Cartwright Lectures on the methods of studying the brain. New York Aledtcal Journal, Feb.
36. 1884. Wilder, B. G. Do the cerebellum and oblongata represent two encephalic segments or only one ? Proc. American Assoc, for Advancement of Science, XXXIII, p. 523, September.
37. 1889. Wilder, B. G. Gross anatomy of the brain. Reference Handbook of the Medical Sciences, New York, Vol. VIII.
38. 1877. Zuckerkandl, E. Beitrag zur Morphologie des Gehirns. Zeitschr. f. Anat. u. Entwicklungsgscht., Leipzig, ii, 442-450. I, PI.
Description of Plates
Note. — The length for embryos is that from the top of the head to the vent. The magnification of figures is given in the explanations, except for Plates III and IV, where a line of given length indicates the enlargement.
The preparation of specimens has been after the method given in the introduction.
Heavy black lines represent either (a) the cut edge of endyma, or {b) the cut or torn edge of a tela consisting of the endyma and pia conjoined.
The occasional disregard of the rules formulated by Wilder (Handbook, IX, 120, 56) was due sometimes to inadvertence, sometimes to the condition of the specimen, the right side being either more perfectly preserved or alone available.
The number of a given specimen refers to the accession number of the Museum Vertebrate Zoology of Cornell University.
Abbreviations
cac. — cacumen.
cr. — crus.
cbl. — cerebellum,
cb. — cerebrum.
cdl. fd. — caudal fold.
ch. pi. — choroid plexus.
cl, — clivus.
crnl.flx. — cranial flexure.
est — cestus.
cul, — culmen.
dent. — dentatum.
d. fd. — dorsal fold.
Dien. — diencephal.
dpi. — Deckplatte of His.
Em. ail. — auditory eminence.
epcl. — epiccele.
Epen. — epencephal.
epnd. — ependyma.
vittl. — metatela.
mylcl. — myeloccele.
71. — nodulus.
jtk.Jlx. — neck flexure.
72od. s. — nodular sulcus.
olv. — oliva.
obi. — oblongata.
parepcl. — parepiccele, lateral recess of the cerebellum.
paraflc. — paraflocculus. paraflc. s. — parafloccular sulcus.
plm. — pileum.
pdcl. — peduncle. postplin. — postpileum.
preplni. — prepileum. P. or pn. — pons.
pym. — pyramis. py. — pyramid.
eptl. — epitela.
yff.— flocculus.
Jlc. s. — floccular sulcus.
/rcl, s. — furcal sulcus.
g'm. — gemina.
ist/t. — islhmus.
kl. — kilos.
/. ct. — central lobe.
Ing. — lingula.
/. £/-(5(j'.— lateral U-like bend.
mediflc, — medi flocculus.
mesen. — mesencephal.
meten. — metencephal.
mscl. — mesocoele.
nitcl. — metaccele.
7ntpr. — metapore.
pvl. — post velum.
pvm. — post vermis,
p. fix. — pontile flexure.
plx. — plexus.
rf.pl. — roof plate.
rp. — ripa.
Rg. au. — auditory region.
supraflc. — supraflocculus .
St. — stem.
si. cl — saddle cleft.
//.—tela.
trans, s. — transitory sulci.
uv. — uvula.
vl. — velum.
vm. tr. — vermian tract.
vlv. — valvula.
Plate I
Fig. I. Left lateral aspect of the caudal three brain segments and myel of an embryo pig 16 mm. long, No. 3337 (x 7.5).
The specimen had been preserved in alcohol ; the cephalic two-thirds of the cerebrum had been sliced off" by a previous inquirer. Shows,
a. The large subspherical mesencephal.
b. The contracted isthmus which has only about one half the diameter of the mesencephal.
c. The convex flaring walls and roof of the epencephal ; the roof arches dorso-cephalad.
d. The expanded metatela (roof or deckplate) and its lateral extension caused by the pontile flexure. (This extension forms the caudal wall of the adult parepicoele. See also Fig. 3.)
e. The cranial flexure (head bend), saddle cleft, bend of the pons, and a concavity in the metatela which later becomes a fold, the dorsal fold, Fig. 5.
f. The left lateral proton or rudiment of the cerebellum, cbl.
Fig. 2. Dorsal aspect of the same specimen. Shows,
a. The ovoid appearance of the mesencephal at this period.
b. The constricted isthmus.
c. The elongated shield-shaped metatela and its lateral extensions which later form a part of the paracoelian wall.
d. The two lateral protons of the cerebellum. Compare Figs. 24 and 24 a, PI. in.
e. The roof of the epencephal. Its extent is shown better in Figs. I and 3.
Fig. 2' Oblique view of the same specimen. Shows the same features as Figs, I and 2, but gives a better idea of the isthmus and the cephalic termination of the metatela.
Figs. 4 and ^. Epencephal of an embryo kitten i8 mm. long, No. 3338 (x 15). Fig. 4, dorsal aspect, shows,
a. General form of the epencephal, constricted at the isthmus, widened caudad.
b. The roof-plate (or deckplate) which being thinner than the lateral parts of the roof has been forced dorsad in the form of a longitudinal fold as the brain hardened and contracted slightly.
c. The lateral U-like bend of the substantial parietes which projects laterad from the brain tube and has resulted from the folding of the tube upon itself at the pontile flexure. It appears to have been formed as a result of the flexion of the tube and the concomitant growth of the parts concerned, so that there results a U-shaped bend which projects laterad like an ear from the brain tube. Fig. j shows the lateral aspect of the bend.
d. The two lateral protons of the cerebellum which have not yet fused at the meson. It is interesting to note that these two structures have rotated so that they stand up dorsad at nearly right angles to the brain tube. See Figs. 29, 30, 31, PI. III.
e. The caudal fold which is formed as a result of the lateral U-bend, cdl. fd. The lateral wall is doubled upon itself, so that a fold is projected entad
and in this manner forms the floor of the parepiccele, and apparently develops into the auditory eminence of the adult. I have indicated this structure in the earlier embryos by the term auditory region. See Figs. 8, 10, 12, 13, PI. I, Fig. 35, PI. Ill, and Figs. 52, 53, 54, PI. V.
f. The relations of the tela which forms the roof of the metacoele and the dorso-caudal wall of the parepicceles. The tela has been torn away on the left side so as to expose the cavities and the ental surface of the cerebellum. The heavy black line represents the ripa, the torn edges of the tela. The membranes are shown entire on the right side.
Fig. 5. Shows the lateral aspect of the same embryo. The mesencephalic roof is seen to extend both cephalad and caudad. The caudal extension rapidly increases, so that in embryos a little older, it reaches almost to the caudal border of the epencephal. The dorsal fold of the metatela which dips down deep into the metaccele is also clearly shown. Compare Figs, 31 and TjT,, PI. Ill, mtplx.
Figs. 6 and y. Embryo kitten 23 mm. long. No. 3339 (x 15). Shows the same general features as, Figs. 4 and 5, and in addition,
a. The still greater caudal extension of the mesencephalic roof.
b. The increased size of the two protons of the cerebellum which have not yet fused at the meson. A peculiar notch or fold marks their mesal extremity. This notch is, perhaps, an artifact due to the shrinking of the brain during hardening. The caudal (ental) surfaces of the cerebellum are not shaded to represent them more clearly, but it must be remembered that they are covered by the tela and are therefore within the proper brain cavity [i. e., they are entocoelian surfaces). For the sake of clearness, the kilos and the ripa or the line where the kilos ends and the metatela begins have not been shown. The ripa is approximately just caudad of the place where the shading ends. The line caudad of the shading represents the line of the attachment of the plexus to the tela. It extends around the lateral and dorsal sides of the brain tube like a girdle, therefore the term cestus [est.) is proposed to indicate it. This is a very interesting specimen and apparently presents a condition identical with that shown in the human embryos, Figs. 52 to 55, PI. V.
Fig. 7. Shows the left lateral aspect of the same specimen ; here the auditory region is quite large, the cestus shows clearly and the plexus may be dimly seen through the thin semi-transparent tela.
Figs. S and g. Embryo kitten 40 mm. long (x 11), prepared by Dr. P. A. Fish. Shows,
a. The caudal extension and bifid extremity of the mesencephal. This appearance is very characteristic.
b. The lateral protons of the cerebellum have now fused at the meson. It seems probable that the change has been brought about in the following manner. Each lateral proton has grown by an increase in thickness and has gradually extended itself by a sort of proliferation cephalad, dorsad, and mesad till at last it has met the proton of the opposite side of the brain tube at the meson and has fused with it. Thus the deckplate has been obliterated. From now on we have to deal with a single structure instead of with two. The lateral portions, however, have a much greater mass than the middle part which is still thin. This relative proportion continues for some time. The vermis of the adult cerebellum is comparatively late in its development. The remaining parts are self explanatory.
Fig. g. Shows the left lateral aspect of the same specimen. There is a small elevation at the cephalo-lateral extremity which shows that the prepileum has begun to be differentiated, but the entire region of the pre- and postpileum is indicated under the general term pileiim. A shallow furrow marks the floccular sulcus. The other parts are self explanatory.
Figs. 10 and II. Embryo kitten 36 mm. long, No. 3340 (x 15). Fig. 10 shows the dorsal aspect of the cerebellum which has now begun to be differentiated into regions which persist through life. It appears that the earlier changes begin laterad and proceed mesad. From an examination of the surface the mesal part appears to be still quite thin. At this stage at least two of the primary furrows have appeared. In the lateral regions two elevations separated by a shallow furrow — the parafloccular sulcus — ^have appeared. These elevations constitute the pileum and the paraflocculus, shown in profile in Fig. 10. Fig. II shows a face view. The floccular sulcus extends transversely clear across the cerebellum. Dorsad of it there is a slight narrow depression which probably represents the later nodular sulcus. At this stage there is a dorsocaudal extension of the cerebellum ; at the meson there is a distinct notch, barely perceptible in Fig. 8. It shows very clearly in Fig. 13. Fig. lo also shows the kilos, cestus, tela, and plexus quite clearly.
Fig. II. Shows the first real demarcation of the cerebellum into distinct regions, the pileum and the paraflocculus. A little later the pileum is divided into two regions, prepileum and postpileum, by a new sulcus the interpilear sulcus. See Fig. 15, PI. II. Fig. 11 also shows the floccular sulcus, the kilos, the parepicoele, and the plexus which shows through the tela. During the dissection the tela was torn away from the kilos. Its lines of attachment is indicated by the heavy black line, rp. The auditory region shows a marked increase in size. At this stage the paraflocculus is larger than the prepileum. The figure does not show the cerebellar peduncle which is just beginning to form. See Fig. 14.
Fig. 12. Shows the dorso-caudal aspect of the cerebellum of an embryo kitten 53 mm. long (x 7-5) • This is practically the same condition as that shown by a human embryo, Fig. 60, pi. VI. The raesal part of the cerebellum is still quite thin, and four distinct furrows are present. The nodular sulcus is unmistakable and in addition to the parafloccular sulcus, a new furrow, the interpilear sulcus, has appeared. There is a corresponding sulcus upon the other side of the cerebellum. These sulci do not cross the meson and apparently do not become continuous with any of the mesal sulci which appear later. The interpilear sulcus divides the pileum into two regions which appear to be distinct through life. The parepicoele and caudal fold are shown very clearly. The specimen is turned at such an angle that the caudal projection of the cerebellum does not show. Tela and plexus have been removed. The heavy black line indicates the ripa.
Figs . jj and i^. Cerebellum from an embryo kitten about 55 mm. long, No. 3341 (x 7.5). Shows a later stage of development ; two new furrows are present — the furcal sulcus and the uvular sulcus. I have not been able to obtain specimens to show which one appe.ars first. But the adult condition indicates that it is the furcal. See Fig. 44, PI. IV. The furcal sulcus divides the mesal portion of the cerebellum transversely into two unequal parts. For reasons to be stated elsewhere we shall call all oi the organ which lies cephalad of the %w\zv.s, prerannis, and all that lies caudad postrannts. In addition to the features just noted the figure shows essentially the same parts as Fig. 12.
Fig. 14. Left lateral aspect of the same specimen. Shows very clearly the caudal extension of the mesen. Compare Figs. I, 5, 7, 9. The peduncle of the cerebellum is now recognizable The specimen also shows the relation of the interpilear sulcus to the parafloccular sulcus and the relation of the prepileum to the postpileum ; the former now has a volume greater than that of the paraflocculus. The kilos, ripa, and plexus show plainly.
Plate II
Figs, i^ and 16. Cerebellum of an embryo kitten 63 mm. long, No. 3342, (x 7.5). Shows practically the same features as Figs. 13 and 14, PI. I. In addition to these there appears a slight elevation surrounded by a very shallow groove,' the interpileum. (This term was suggested by Professor Wilder.) The structure forms an interruption between the interpilear and the uvular sulci. At present I am unable to explain its significance, but shall discuss it in a future paper, as soon as enough material is available. The mesal shading indicates a depression extending cephalo-caudad.
Fig. j6. Left lateral aspect of the same specimen.
I The lines surrounding the interpileum in Fig. 15 should not have been quite so heaTy, They represent only a very shallow groove, not a sulcus.
Figs. 77, iS, ig. Cerebellum of an embryo kitten 77 mm. long, No. 3443
(x7.5) Fig, ij. Shows the dorsal aspect. The organ presents numerous folia and begins to assume distinctly adult characters. At this stage the vermis may be recognized but it still presents a mesal depression. Upon each side there is a rather wide depression which becomes the vallecula of the adult. The brain was somewhat distorted in hardening ; this is an advantage because it allows the left flocculus and paraflocculus to be seen from the dorsal aspect. A depression is seen in the paraflocculus, the interfloccular sulcus. It divides the paraflocculus into two parts, of which at Professor Wilder's suggestion, I have called the dorsal part, supraflocculus, and the ventral part, mediflocculus.
The flocculus proper is not diff"erentiated as early as the paraflocculus. This is the youngest specimen that shows it clearly. The early embryos show an apparent connection between the supraflocculus and the postpileum. The same condition exists in the adult but is obscured by the great development of the adjacent parts. An attempt will be made in Part II to trace the fiber relations of these two regions. A connection also exists between the floccular region (flocculus and paraflocculus) and the postvermis. I get this idea from dissections of adult cerebellums. This connection is indicated by the term vermian tract.
A considerable portion of the preramus and its middle part, the prevermis, is seen, also the relations of the furcal sulcus and the peduncular sulcus, next caudad. The entire mass from the peduncular sulcus caudad to the uvular sulcus represents the cacumen and the tuber. The cacumen in the cat is relatively much larger than in man. The remaining part of the postvermis, shown in the figure, represents the uvula. The pyramis is rudimentary in the cat. See Fig. 44, PI. IV, The floccular and nodular sulci do not appear in this aspect.
Fig. 18. Right lateral aspect of the same specimen ; the right aspect was drawn because the left was distorted. In additition to the general features it shows the lateral U-bend and its relation to the parepicoele, the auditory region, and the auditory nerve.
Fig. ig. Shows an oblique view of the same specimen (x ii-5). Shows the relation of the supraflocculus to the postpileum, also that of the flocculus and paraflocculus to postvermis, by means of the vermian tract. The other parts need no explanation.
Figs. 20,21, 22. Cerebellum of an embryo kitten 93 mm. long, No, 3344, (x about 6.7). This specimen shows all the features of the adult organ but in a. much more simple condition. The individual regions do not show that crowding together and distortion which in the adult organ are so puzzling. From now on the growth appears to be an elevation of the general surface, not including the primary sulci.
The vermis presents no mesal depression, but instead a mesal elevation. All the mesal parts present in the adult may now be recognized. The postpileum has undergone an enormous change, both in relative size and in general appearance. Its long axis has changed to a latero-cephalic direction; it has also been crowded dorso-caudo-mesad so that it has come to lie up against the postvermis, and now is much thicker than wide. The nodulis is concealed by the uvula.
The entire mass of the prepileum has increased so that now it is greater than that of the postpileum. It has also expanded and the caudal part has extended both laterad and caudad. The supra- and mediflocculus have become foliated, but more folia appear at a later stage; compare Fig. 23, while no folia have as yet appeared upon th6 flocculus.
Fig. 2J. Lateral aspect of the same specimen. Shows clearly the preramus and its vermis.
Fig. 22. Oblique view of the same specimen intended primarily to show the relation of the floccular region to the postvermis by way of the vermian tract and also that of the postpileum to the supraflocculus.
Fig. 2j. Left lateral aspect of an adult cat's cerebellum, No. 153 (x 2.3). Shows the compact and pump appearance of the adult organ. It looks as though the skull had been too small for it. The great size of the vermis is very marked. Compare Figs. 15, 17, 20, 22. The increase in size of the prepileum over that of the postpileum is astonishing. Compare Figs, il and 14, PI. I, and Figs. 16, 17, 18, PI. II. Note also the comparative size of the flocculus and of the medi- and supraflocculus, and its relation to them.
The flocculus is really larger than it appears to be here, being partially concealed by the mass lying dorsad ot it. In addition, note the relation of the parepicoele to the auditory eminence, the flocculus, and the plexus. The heavy black line represents the torn edge of pia and endyma, the ripa. A portion of the tela was removed to expose the cavity. Compare Figs. 4, 5, 11, 12, 14, PI. I, and Figs. 18, 19, 20, 22, PI. II.
Plate III
Figs. 24 and 24a. Sagittal sections through the mesencephal, the epencephal, the metencephal and a part of the myel of an embryo pig 16 mm. long, the same age as Figs, i, 2, 3, PI. I. The sections were cut a little obliquely and the mesal aspect of Fig. 24 is reconstructed from several sections.
Fig. 24 shows a very rudimentary condition ofithe parts, also the following distinct features, viz.,
a. The comparatively thick floor of the brain tube ; it becomes thinner in the floor of the diacrele.
b. The roof is thin at the meson but thickens laterad.
c. The thin metatela and thicker roof of the epencephal and mesencephal. The change from tela to epicoelian roof is through a region of transition, the kilos, which consists essentially in a decreased thickness of the parietes. This is more abrupt and clearly defined in older embryos.
d. The constriction at the isthmus. At this point there is a greater thickening in the roof which increases laterad. At present I am unable to explain this condition ; would suggest that, as a result of the dilation of the mesecephal and a possible contraction at the isthmus, the difference in thickness has been brought about. I shall discuss this subject as soon enough as material is available. Compare Figs, i, 2, 3, PI. I, and Fig. 24a, PI. Ill, which is a sagittal section at some distance from the meson. The proton of the cerebellum is marked cbl. ; it does not appear in a mesal section. The other parts are self explanatory.
Figs. 2^, 26, 2y, 28 are oblique transections of the brain of an embryo kitten about 12 mm. long and show a very early condition, a little farther advanced however than Figs. 24 and 24a. The sections are as nearly transections as I could cut them ; but every one knows how difficult it is to section so small an embryo exactly at right angles to the meson.
Fig. 23. Section through the isthmus. The left side includes a little of the mesencephal. It also shows the thickening of the wall at the isthmus and the general arrangement of the brain at this point.
Fig. 26. A transection through the epencephal cephalad of the cerebellum.
Fig. 27. A transection through the cerebellum ; includes the cephalic part of the lateral U-bend. Compare Figs. 5 and 7, PI. I. This stage of development is of the highest morphological interest because it shows, •
a. That the lateral parts of the caudal portion of the epiccelian roof are thick, while the mesal part is thin at the deckplate.
b. The lateral pprts are clothed with cinerea which does not extend across the meson (compare Figs. 4 and 6, PI. I); this proves conclusively that the cerebellum is developed from the two lateral areas, or protons, which by a kind of proliferation gradually extend mesad till they form a continuous thickened mass.
The dorsal projecticm at the meson is an artifact due to collapse of the brain during the process of hardening and infiltrating with collodion. The linelaterad of the U-bend represents the tela which forms the lateral wall of the parepicoele.
Ftg. 2S. A transection of the same embryo through the mesencephal, showing the general character of the metatela and kilos.
Figs. 2g, JO, ji, jij, are sagittal sections of the epencephal of an embryo kitten 18 mm. long.
Fig. 29 is a section near the lateral extremity of the lateral U-bend and shows,
a. The parepicrele.
b. The beginning of the plexus.
c. The extreme lateral end of the cerebellum and the auditory region,
d. The relation of the kilos and tela to the cerebellum at this extreme lateral point. Compare Figs. 4, 5, 6, PI. I, also Figs. 50, 53, and 54, PI, V.
Fig. JO is a section a few microns farther mesad. It shows essentially the same conditions as Fig. 29.
Fig. 31. Section still farther mesad. Shows,
a. The dorsal rotation of the cerebellum to a position nearly at right angles with the brain axis.
b. The kilos both at the caudal (here dorsal) edge of the cerebellum and around the edge of the metencephal.
c. The metaplexus.
The condition here is apparently comparable to that found in certain reptiles, Herrick (13, 7). The black line, endyma, is a continuation of the cells forming the plexus.
Fig. jj. Shows the mesal aspect of the same specimen. Note the general arrangement of the parts, and the great reduction of the roof at the meson. Compare Figs 4 and 6, PI. I.
Fig. ^2. A transection through the lateral U-bend of the same specimen. After the meson had been passed, the tissue was removed from the block and its position changed so as to cut transections of the remainder. It shows the relation of the bend to the brain tube and that of the tela to the bend.
Figs. ^4 and ^j. Sagittal sections through the cerebellum etc., of an embryo kitten 23 mm. long.
Figs . jb and 27 • Sagittal sections through the cerebellum of an embryo kitten a little older than the previous one. It had been partly dissected by a previous observer, so that the exact length could not be determined. This shows a condition which would seem to indicate that the cerebellum, after having rotated dorso-cephalad, again rotates caudo-ventrad. The condition found in still older embryos would appear to support this view. See Figs. 42 and 43, PI. IV.
His (15, 22) holds a different view. He says that the "tela grows fast to the apposed ental surface of the cerebellum." But it seems tome that the facts disprove this statement. See discussion on p. 94 of text.
Figs. J S and J g shovf transections through the cerebellum of an embryo kitten 45 mm. long.
Fig. j8. Section through the caudal part of the cerebellum. It shows,
a. The tips of the overlying mesencephal. Compare with Figs. 6 and 8, PI. I.
l>. The structure of the cerebellum as a substantial mass of alba clothed with ectocinerea, forming a part of the roof of the epicoele ; it is the roof at this particular point. Compare with Figs. 10 and 11, PI. I ; also with Figs.
27, 33, 35. 37, PI- III c. The epicoele and epiplexus, the metacoele and metaplexus. In this figure the two plexuses have no apparent connection, but they are really continuous. The apparent independence is due to the fact that they do not lie in the same plane. The same is true for the epicoele and metacoele; the section is made caudad of their point of junction.
d. The section also shows the kilos, the cestus, the floccular and parafloccular sulci, the paraflocculus and prepileum.
Fig. jg. Transection of the same specimen farther cephalad. The section is slightly oblique ; the right side passes through the peduncle, the left is just caudad of it. It shows,
a. The epicoeles and the parepicoeles, at the left the connection between the two lateral U-bends.
b. The epiplexus.
c. The mesencephalon with its cavity, mesocoele ; sections farther cephalad show the junction of the epicoele with the metacoele.
d. The thin roof of cephalic part of the epicoele (valvula) and a part of the cephalic extension of the cerebellum, which later becomes the preramus.
e. The cephalo-lateral extensions of the cerebellum (prepileum) clothed with cinerea, while just mesad two rounded elevations appear, the cephalic bundles, or prepeduncles.
Figs. 40 and 41. Sagittal sections through the cerebellum of an embryo kitten 93 mm. long. Compare with Pigs. 17 and 20, PI. II. Shows a section through the U-bend including,
a. Supraflocculus ) r- .■ . .. c ..\, a i
, A< i-xi 1 y Constituents of the paraflocculus.
b. Meditlocculus / ^
c. Flocculus.
d. Kilos, plexus, tela, parepiccele and cestus.
e. Auditory region.
Fig. 41. A section a little farther mesad includes the tip of the prepileum.
PLATE IV.
Fig. 42. Mesal aspect of the cerebellum of an embryo kitten 45 mm. long. Shows the condition of the parts at this early stage. The ental surface of the cerebellum is now concave instead of convex. Compare Figs. 31, 35, 36, 37, PI. Ill ; also Figs. 43, 44, PI. IV. It certainly looks as though the cerebellum had rotated caudo-ventrad. This seems to me the simplest and most logical explanation of the change in the position of the ripa from that shown in Figs. 31, 34, 35, PI. Ill, to the position shown in this figure. Note the relative thickness of the valvula and metatela, also the position of the plexus, and the floccular and uvular sulci.
Fig. 4:^. Mesal aspect of the cerebellum of an embryo kitten 55 mm. long. Shows essentially the same features as Fig. 42, and in addition the furcal sulcus which divides the cerebellum into two great divisions, the preramus and postramus. The former presents three sulci while the latter shows only one, the uvular sulcus. Contrast with Fig. 42, where the only one present is a very slight depression, the uvular sulcus. Note also the small volume of the cerebellum as compared with that of the oblongata in all stages up to this period.
Fig. 44. Mesal aspect of the cerebellum of an adult cat, No. 753 (x about 4). Shows the general structure of the mesal part of the cat's cerebellum which is quite like the condition found in man. The most striking feature, on first sight, in the arbor, whose treelike branching is so characteristic of sagittal sections through the organ. It consists of a short thick trunk supported upon two lateral roots, or peduncles, and of two enormous branches, between which is the furcal sulcus. This sulcus forms a natural division and separates the cerebellum into two great regions. The cephalic is the preramus, the caudal is the postramus ; the latter forms the greater portion of the cerebellum.
As for the mesal part, or vermis, so much of it as is a part of the preramus is the prevermis, the remainder is the postvermis. In previous works all of the vermis cephalad of the peduncular sulcus has been called prevermis, but if we are to be governed by natural divisions, the prevermis ends at the furcal sulcus.
Each ramus is divided into smaller branches or lobes practically the same as in man where they have been given specific names. The same terms are applied to the regions which the writer considers homologous in the cat. It is interesting to note that apparently the pyramis is rudimentary in the cat, and that there is an additional lobe in the cephalic part of the prevermis. The writer suggests for it the term cephalic lobe. This subject will be further discussed in part II. It would have been desirable to show stages between those given in figures 43 and 44, but suitable material was not available.
Figs, 4^, 46, 4J. Sagittal sections of the cerebellum of a human embryo, No. 2862, about 10 cm. long ; poorly preserved, but shows well the relation of the pileum to the paraflocculus, flocculus, and kilos.
^'^S- 47- Shows the parafloccular sulcus, the floccular sulcus, and vermian tract.
Fig. 48. Section farther mesad than Figs. 45, 46, 47. It cuts the peduncle and shows the pontile and caudal bundles of fibers which stream up into the pileum ; also the corrugations due to the developing sulci, the dentatum, the floccular sulcus, the kilos, the lateral U-bend enclosing the parepicoele, and its relation to the peduncle.
Plate V
Figs, 4g, JO, J/, Brain of a human embryo 22 mm. long. No. 2652 (x about 6.7).
Fig. 4g. Shows a slightly oblique view of the dorsal aspect, also the right side of the prosencephal foreshortened. Can hardly be understood without reference to Figs. 50 and 51. The mesencephal shows several curious transitory sulci which disappear later. The form and extent of the metatela are very apparent as is also the in-sinking collocated with the bend of the pons. This is indicated by the darker transverse shading and is marked est. The cestus seems to be formed here. A part of the tela was destroyed in the dissection; this exposed the metepicoele and also shows the early parepicoele. Other features shown are,
a. The dorso-cephalic rotation of the cerebellum, better seen in Figs. 50 and 51.
b. Fhe relation of the tela to the kilos around its entire circumference.
c. The lateral U-bend.
Fig, JO. Right lateral aspect. Shows,
a. The position and extent of the isthmus.
b. The deckplate.
c. The fold in the metatela.
d. The cerebellum, the kilos, and ripa.
e. The lateral U-bend and parepiccele, /, The bend of the pons.
g. The mesencephalic roof which is beginning to extend both cephalad and caudad.
h. The head bend and saddle cleft.
i. The bending ventrad of the kilos along the metaccele.
Fig. ji. Shows the same features and also the rotation of the cerebellum dorso-cephalad. Compare Figs, i, 2, 3, Plate I.
Figs. J2 and jj. Human embryo, No. 3205, 41 mm. long, (x about 6.7). Shows the same features as Figs. 49, 50, 51, No. 2652, except the tela. The parts are larger, the caudal fold and lateral U-bend are better developed.
Fig. ^4. Transection through the epencephal of a human embryo 39 mm. long, No. 2139, (x 6.7). Shows very clearly,
a. The thin deckplate not yet obliterated.
b. The cerebellum as two thickened masses forming the lateral parts of the roof.
c. The epicrele and its lateral extensions the parepicceles. (/. The kilos.
e. The tela and epiplexus.
f. The caudal fold.
g. The transverse furrow in the metacoelian floor caused by the bend of the pons.
Fig. jj. Mesal aspect of the epencephal of a human embryo, No. 2, 22 mm. long, (x about 6.7). ShoM's the cerebellum as a fusiform mass lying in the lateral part of the roof and twisted somewhat so as to conform with the general shape of the epencephal at this period of development. The other features need no further explanation.
Figs. j6 and ^y. Cerebellum of a human embryo. No. 2926, 60 mm, long (x about 6.7).
Fig. j6. Dorsal aspect, shows a very interesting stage in the development of the cerebellum. It looks as though a ventro-caudal rotation from the position shown in Figs. 50, 51, 52, had taken place. Compare Figs. 31, 35, 36, 37, PI. Ill, and Figs. 42, 43, PI. IV. I have thus far failed to obtain specimens between 41 and 60 mm. in length. The examination of such specimens would probably show whether such rotation does or does not occur. The specimen also shows,
a. The caudal projection of the mesencephalic roof. It differs from the conditions seen in the cat where the mesencephal is notched or heart shaped; here the reverse is the case.
b. The floccular sulcus which begins in the lateral region.
c. The kilos, the tela, the cestus, and a mesal transitory sulcus in the mesencephalon.
Fig. 57. Shows the left lateral aspect and practically the same features as 56 ; also,
a. The peduncle of the cerebellum.
b. The pontile flexure.
c. The crus and the overlapping caudal margin of the cerebrum.
PLATE VI.
Figs, jc? and ^g. Cerebellum of a human embryo. No. 3348, 80 mm. long (x about 6.7).
P^S- 5^- Shows the dorsal aspect,
a. The mesal part is still depressed and thin ; no sulci have as yet appeared upon it.
b. The lateral parts, pilea (so-called lateral lobes), are large subspherical masses. Their increase in size from now on is quite rapid.
c. The parafloccnlar sulci, one on each side, have appeared and the floccular sulcus now extends clear across the cerebellum.
d. The paraflocculus is more obvious than the flocculus ; compare Figs. 10-14, PI- I e. The tela, kilos, cestus, lateral Ubend, auditory region, caudal fold, etc., are quite obvious.
/. A part of the metatela has been removed to expose the metacoele. g. The caudal extension of the mesencephal. Fig. jg. Left lateral aspect of the same specimen.
Figs. 60 and 61. Human embryo, No. 3209, 90 mm. long (x 6.7). Fig. 60. Shows the dorsal aspect. A few new sulci have made their appearance,
a. The furcal sulcus.
b. The uvular sulcus.
c. The nodular sulcus.
The tela and plexus have been removed ; this exposes the cavities and other features which otherwise could not be clearly shown; viz:
1. The metaccele.
2. The epicrele and paracoeles continuous with the metacoele.
3. The lateral U-bend.
4. The caudal fold.
5. The auditory region.
6. The mesal groove in the floor of the cavities.
7. The preramus ; all of the organ caudad of the furcal sulcus constitutes the postramus.
Fig. 61. Shows the left lateral aspect of the same specimen and the beginning of a small sulcus, which appears to be the peduncular sulcus. It divides the pileum into two general regions, the prepileum the smaller of the two, and the postpileum, which forms the greater part of the so-called lateral lobes of the cerebellum.
Figs. 62 and 6j. Cerebellum of a human embryo, male. No. 3347, 95 mm. long (x 6.7). The growth of the mesal part is gaining upon that of the two pilea. But there is a question whether or not the vermis is yet present. No new sulci have appeared upon the dorso-caudal aspect but the ones already there have a greater depth. The left parepicoele is represented as though a part of the tela had been removed (it was badly torn in the dissection) to show the structure of the plexus. It may properly be compared to a series of bunches of grapes suspended from a strap-like support, the cestus. The metapore was -artificially enlarged during the dissection.
Fig. 6j. Shows the left lateral aspect of the same specimen. A few (two are shown) new sulci have begun to form upon the cephalic aspect of the cerebellum. An opening into the parepicoele, the parepiplexus, and the torn edge of the tela, are also shown.
Plate VII
Fig. 64. Mesal aspect of the cerebellum of a male human embryo. No 2084 (x 6.7), age and history unknown. Shows the division by the furcal sulcus into preramus and postramus ; also the beginning of the regions found in a mesal aspect of the adult organ. Compare with Fig. 67, also fetal, although adult features exist. Note the relative difference in size of the preramus in the two figures 64 and 67. I am in doubt about the part marked lingula in Fig. 64. This subject will be discussed further in Part II.
Figs. 6j, 66, 67. Cerebellum of a human embryo, No 2279, size, age, and sex unknown (x 4). The specimen had been medisected and the left half partly dissected by a previous observer.
Fig. 6j. The dorso-caudal aspect of the cerebellum is foliated, but less than in the adult. The following features are shown,
a. The form and divisions of tbe caudal three lobes of the postvermis and their relation to the pilea and floculus.
i>. The enormous development of the pilea and the consequent covering up of the flocculus and paraflocculus.
c. No attempt has been made to describe the different lobes of the pilea since that subject and comparisons with other mammals can be more profitably discussed in part II.
(/. The metepicoele (" fourth ventricle ") and its relations to the parepicoele.
e. The relation of the parepiccele to the flocculus and kilos; the latter forms what is commonly known as \}!\& postvelum,Jiinteres Marksegel, valvula semilunaris, etc.
f. The relation of the plexus to the parepiccele.
g. The existence of the flocculus and paraflocculus as distinct regions of the cerebellum, demarcated by sulci. These features are concealed by the overhanging pileum upon the right side. The pileum has been dissected away from the left side. The fiber relations of the flocculus are very peculiar; a large bundle goes ventrad into the pons. These relations will be discussed in part II.
h. The continuity of the uvula with the tonsilla.
Fig. 66. Right lateral aspect. The left half had been previously dissected. Shows the division of the cerebellum into two unequal parts, the preramus the smaller and cephalic, and the postramus, the caudal part. The latter forms the chief mass of the organ. This is again divided by the peduncular sulcus into prepileum and postpileum ; compare Fig. 67. The prepileum and preramus become furrowed earlier than the postpileum. Even in this specimen the sulci of the postpileum have not yet reached its lateral border. Compare Figs. 69 and 70, PI. VIII. The figure also shows a lateral view of the paraflocculus, flocculus, the fifth and eighth nerves, the peduncle, and the pons.
The human paraflocculus becomes more or less foliated; indeed so far as my observations have gone it is quite variable ; but I do not feel certain that there is the division into supraflocculus and mediflocculus, which is so general in the lower mammals.
Fig. 67. Mesal aspect. Shows the same features that are to be recognized in the adult organ, but in a condition to be more easily understood. The most obvious features are,
a. The tree-like appearance of the cut surface.
b. The approximately rectangular mass of alba which forms the trunk of the tree; its roots, i.e:, the peduncles, do not appear in a mesal section.
c. The two great branches, the preramus and postramus, which divide into smaller branches. Beginning with the preramus, from the lower part of the trunk, there arise,
1. The lingula.
2. In the cat and in most human cerebellums, a branch which is absent from this specimen, the cephalic lobe.
3. The central lobe forms a third branch.
4. The remainder of the preramus forms the culmen. Inspection shows this to contain four quite large branches. These four lobes constitute the prevermis.
The postramus contains six lobes ; viz.,
1. Arising from the caudal base of the trunk is the nodulus.
2. The uvula, divided into three smaller branches.
3. The pyramis, divided into two branches.
4. The tuber.
5. The cacumen, on the meson, a single twig-like branch.
6. The clivus, branched to form two twigs. The mesal parts of these form the postvermis. These lobes are separated by deep sulci, each one named from the lobe which lies next caudad of it.
d. The relative size of the pons and oblongata at the meson.
1. Prepontile recess.
2. Postpontile recess.
Fig-. 68. Dorsal aspect of an embryo human cerebellum (x about 5.5). The figure has been drawn from two specimens, the cephalic part from No. 3161, human embryo, 15.5 cm. long, female; the caudal part from A. S27, human, male, 16.5 cm. long. Shows the general appearance of the developing sulci.
I am not yet ready to discuss the disposition of the sulci, but it is interesting to note,
a. That the two sides areunsymmetrical and that some of the sulci overlap i.e., while most of the sulci seem to rise in the mesal region, one at least arises in the lateral region and extends mesad.
b. The right side of the cacumen is furrowed in a manner different from the left side ; this will be further discussed in Part II, Compare Fig. 65, PI. Ill and Figs. 59, 71, 72, 73, PI. VIII.
The position of this figure upon the plate is undesirable but unavoidable.
Plate VIII
Figs. 6<p, 70, Cerebellum of a human embryo, No. 2947, male, 15 cm. long (x 6.7). The preramus and prepileum are foliated, but the postpileum on each side is smooth.
Fig. 6g. Dorsal aspect. It shows,
a. General regions.
b. The mesal beginning of the vermian lobes and the sulci which demarcate them.
c. The postpil^um upon each side is still smooth.
Fig. 70. Left lateral aspect. Shows the lateral aspect of the same features as Fig. 69 and,
a. The paraflocculus and its sulcus.
d. The flocculus and its sulcus.
c. The kilos and parepicoele.
d. The auditory region and eighth nerve.
e. The peduncle of the cerebellum.
Fig. 71, Caudal aspect of the cerebellum of a human embryo, No. 2947, male, 15 cm. long (x 6.7). The relation of the paraflocculus has been added from No. A. 827, human embryo, male, 16.5 cm. long. The caudal part of the left postpileum has been sliced off so as to show the relation of the flocculus and paraflocculus ; compare Fig. 65, PI. VII. Shows the beginning of the general regions and structures to be found in the adult organ.
Figs. 72, 7j, 74. The cerebellum of a child at term. No. 2961, female, (X2.3).
Fig. 72. Dorsal aspect. Shows the division of the organ into lobes and folia. Compare with Figs. 68, 69, PI. VII.
P^S- 73- Shows the caudal aspect. Compare with Figs. 65, 69, 71. Note, that the flocculus and paraflocculus are almost entirely concealed by the postpileum and parepiplexus.
Fig. 74. Shows the cephalic aspect. Compare with the preceeding figures. Note,
a. The flocculus.
b. The eighth nerve.
c. The peduncle.
d. The pons.
e. The prevermis (culmen, central lobe, cephalic lobe); thelingula does not appear.
f. The crura.
These three figures show the condition so characteristic of the adult organ, i. e., the division into lobes by deep sulci and the subdivision of the lobes themselves into narrow folia by the shallower crevices or rimulse.^ The probable use of this profuse foliation is to give a greater amount of potential cinerea. The structure will be further discussed in part II.
^^^- 75- Caudal aspect of an adult human cerebellum. No. 31 17, male, (x about I).
1. The term rimula was proposed by Wilder, 36, 125.
Preparation. — The dorsal third of the organ was sliced off. The projecting parts of the postpilea were crowded dorso-cephalad and kept in this position till hardened by means of pledgets of absorbent cotton. Alcohol was the hardening agent employed. The result has been to disclose features which are not seen without special preparation. Points illustrated are,
a. The massive, hypertrophied pilea.
b. The pons and the peduncles of the cerebellum.
c. The caudal part of the postvermis, part of pyramis, uvula, nodulus, and vallecula.
d. The kilos, [valvula semihinarts, postvelum, hinteres Marksegel.)
e. The metatela and plexus.
f. The cestus.
g. The metapore.
h. The eighth nerve.
i. The oblongata.
j. The flocculus and paraflocculus.
Cite this page: Hill, M.A. (2024, April 23) Embryology Paper - The Mammalian Cerebellum part 1 (1895). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_Mammalian_Cerebellum_part_1_(1895)
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