40a • ANATOMICAL TECHNOLOGY. 



features are for tlie time ignored, and the brain is viewed simply as a urUa of cavities with 

 parietes of varying thickness, more or less distinctly divisible into walls, floor and roof. 



§ 1055. The fourth proposition is in accordance with the following general aphorism : — 



" In all departments of investigation, it is right to commence with the study of that 

 which is common, simple and regular, and thence to proceed to inquire respecting that 

 which is [complex], unusual and irregular." — Bucknill and Tuke, A. 



The specific idea is admirably expressed in the following passage from a paper in 

 which it is practically carried out : — 



" With man and the other mammals, the cerebrum and cerebellum so far transcend 

 all the other organs of the encephalon, that the parts which in a morphological point of 

 view are of equal value have been frequently overlooked, as forming either integral parts 

 or primary subdivisions. 



" In frogs . . . while no one part takes an excessive development, there is at the same 

 time no one of the fundamental ones either wholly deficient or so far reduced as to deprive 

 the general plan of any of its more important features. The brain is so far reduced in the 

 relative proportion of its different parts, and so far stripped of the ' accessory organs of per- 

 fection,' as to enable the student to obtain with ease a clear conception of the general plan, 

 a conception always so difficult to acquire when studying the brain of mammals or of 

 man." — Wyman, 34, 6. 



§ 1056. The advantages presented by the frog's brain may be categorically stated as 

 follows ; — 



(1) The various parts, while far from equal in size, diflfer much less than in the higher 

 Vertebrates. 



(2) No part is completely hidden by another. 



(3) All lie in the same plane, the organ not presenting the perplexing " cranial flex- 

 ure " (Quain, A, II, 733) of most of the higher Vertebrates. 



(4) The cavities are relatively large. 



(5) The parietes vary little in thickness. 



(6) While all the primary components of the brain are present, there are but few spe- 

 cial additions or modifications to distract attention from the general plan. 



Tet the frog's brain is by no means an ideally perfect type of the vertebrate brain, or 

 wholly adapted for study, for the following reasons : — 



(1) It is undesirably small. Hence the student should select for this purpose the very 

 largest individuals, if possible of the bullfrog, Rana Catesbiana {pipiens of some writers). 



(3) The tissue is very soft. Hence great care is needed, and the organ should usually 

 be hardened. 



(3) The cerebellum is disproportionally small. 



(4) The cavity of the optici presents a projection of the wall which renders a section of 

 the region somewhat puzzling. 



(5) The passages (porta or foramina of Monro) between the mesal and the lateral cavi- 

 ties are undesirably small. 



(6) The cephalic divisions, the Ldbi olfactorii, which are separate in all other Verte- 

 brates, are, in the frog and toad and other anourous Amphibia, not only in contact upon 

 the meson, but there united by somewhat firm connective tissue, constituting a feature 

 which has seriously misled some anatomists, including even Wyman (34, 8, 9). 



(7) The plexuses are nearly or wholly absent. 



In respect to the last four objections, the brain of Menobranchus is preferable to that 

 of the frog. But the animal is less easily obtained, the cerebellum is even smaller, and the 

 optici are so slightly differentiated from the parts caudad and cephalad as to render some- 

 what difficult the recognition of their limits. 



