SCIENCE. 



[Vol. XVIII. No 452 



SCIENCE: 



A WEEKLY NEWSPAPER OF ALL THE ARTS AND SCIENCES 



PUBLISHED BY 



N. D. C. HODGES. 



47 Lafayette Place, New York. 



Subscriptions.— United States and Canada $3.50 a year. 



Great Britain and Europe 4.50 a year. ' 



Communications will be welcomed from any quarter. Abstracts of scientific 

 papers are solicited, and one hundred copies of the issue containing such will 

 be mailed the author on request in advance. Rejected manuscripts will be 

 returned to the authors only when the requisite amount of postage accom- 

 panies the manuscript. Whatever is intended for insertion must be authenti- 

 cated by the name and address of the writer; not necessarily for publication, 

 but as a guaranty of good faith. We do not hold ourselves responsible for 

 any view or opinions expressed in the communications of our correspondents. 



Attention is called to the "Wants" column. All are invited to use it in 

 soliciting information or seeking new positions. The name and address of 

 applicants should be given in full, so that answers will go direct to them. The 

 " Exchange " column is likewise open. 



THE EVOLUTION OF THE CEEEBELLUM. ^ 



The cerebellum has unquestionably given more trouble to 

 anatomists than almost any other organ, and our present 

 knowledge of its structure seems disproportionate to the labor 

 expended. It is no discredit to the monumental works of 

 Stilling, Meyuert, Purkinje, G-erlach, and Kolliker to admit 

 that scarcely a single tract connecting the cerebellum with 

 other portions of the brain is traced with sufficient detail. 

 Even the external configuration of the cerebellum in lower 

 animals has many lessons for us which may be useful in the 

 interpretation of the human organ. 



The cerebellum is subject to a greater range of variation 

 than any other organ of the brain. From being practically 

 absent, as in amphibia, to preponderating over all other seg- 

 ments of the brain in some fishes, there is every gradation in 

 development. It becomes obvious from a brief study of the 

 relative development of the regions of the encepbalou that 

 the cerebellum does not vary in proportion to the intelli- 

 gence; that, in other words, it cannot be employed as a cri- 

 terion of the position of the animal in the scheme of classiS- 

 cation as can the cerebrum. Although not available for 

 taxonomic purposes, these variations are none the less inter- 

 esting from the clew which they may afford to the functions 

 and laws of development of this and associated organs. 



In the March number of the Journal of Comparative 

 Neurology the writer called attention to the architectural 

 modifications of the cerebellum in reptiles, and the progres- 

 sive evolution of the organ in the several groups, as well as 

 the resemblance of this course of evolution to a peculiar and 

 apparently undescribed law of development of the cerebellum 

 in mammals. 



The cerebellum is peculiarly mobile, considered architec- 

 turally, by reason of its mode of attachment to the axis of 

 the brain. It is morphologically the roof of the fourth ven- 

 tricle. Both before and behind it is connected with the dor- 

 al surface of the brain tube by a velum, or thinned lamina. 



devoid of nervous matter, and extensively folded and com- 

 -bined with vascular sinuses to form a nutritive organ, the 

 plexus choroideus (metaplexus and mesaplexus). The velum 

 posterior extends about the sides of the cerebellum also, so 

 that rigidity is given to that organ only by the several fibre- 

 bundles or peduncles of the cerebellum which connect it 

 laterally with other regions. Thus, however large and 

 heavy it may be, the cerebellum is supported solely by a lat- 

 eral axis entering at the base. There is really nothing to 

 prevent the most extensive rotations or foldings of the body 

 in all directions except laterally. 



The progressive development of this region is nowhere 

 more conveniently illustrated than in the reptiles. Taking 

 the transverse bar which constitutes the cerebellar rudiment 

 in amphibians as a point of departure, we first encounter a 

 leaf like body with the ventricular half of the substance 

 composed of granular material. The tracts are chiefly scat- 

 tered in the dorsal white layer. In the serpents this flap is 

 flexed so as to form a hood-like body. The flexture is due 

 to the so-called pons-flexture of the whole medulla. The . 

 flexture is more pronounced in turtles, and results in a com- 

 plete roof over the fourth ventricle, which may be consider- 

 ably arched. It is obvious that there must be a limit to the 

 development along this line. In higher reptiles, whose mo- 

 tions are more active and require more accurate co-ordina- 

 tion, the increase in size necessary to supply sufficient ner- 

 vous matter renders necessary a complete eversion of the 

 leaf like organ. In the lizards the lamina is folded forward 

 in such a way as to make a double roof over the ventricle, 

 bringing the granular layer, with its neuro-epithelium dor- 

 sad, in the superior layer, while it faces ventrad (toward the 

 ventricle) in the ventral lamina. In the alligators the de- 

 velopment is more extensive, and results in a horizontally 

 placed hollow cone, with the apex directed caudad, and at- 

 tached by the ventral portion of its base to the brain base. 

 The outside of the cone is clothed with epithelium, while the 

 inside is the morphologically eetal surface. Of course, in 

 this description the thin velum which originally connected 

 the edges of the leaf has been disregarded. This eversion of 

 the cerebellum is of the highest importance in preparing us 

 to understand the origin of the cellular elements in the hu 

 man cerebrum. Before alluding to this subject we may pass 

 in review a few illustrations of cerebellar architectonic from 

 other classes of vertebrates. 



In fishes the range of variation is remarkable, in so far 

 that it may render the brains of closely allied genera very 

 dissimilar in appearance. The characteristics of the fish 

 cerebellum, which serves to distinctly separate it from all 

 other classes of mammals, is the development of a second 

 portion of the organ in front of the valve of Vieussens, 

 which is the morphological anterior (cephalad) margin of 

 the cerebellum in other cases. The relatively large amount 

 of cerebellar substance required by active fishes, and the 

 lack of definite walls to the ecephalic cavity, result in curi- 

 ous folds on a large scale and simple plan. The forward 

 fold in front of the valve, which the writer has termed vol- 

 vula, from its purse-like form, often completely fills the 

 cavity of optic lobes, and in some cases (as the black-horse, 

 Cycleptus), actually pries the two halves of the roof or tec- 

 tum of that organ apart, and protrudes upon the dorsal sur- 

 face with only the membranous velum tecti above it. The 

 moon-eye, Hyodon, is the most reptilian of the osseous fishes 

 so far examined, and in this case the cerebellum proper is a 

 simple sac extending caudad: there is no external evidence 

 of a vol vula or of lateral lobes or " bursa." Sections show. 



