350 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



approaching these relations as to indicate some quite regular adjustment of the 

 causative forces at work in their production. This tri-radiate type of fissuration can 

 be found cropping out all over the cerebral surface, and in those brains where the 

 lines of furrowing are straight and not interfered with by the action of secondary 

 causes, producing curvilinear deflections and tortuosities, the mathematical relations 

 to an angle of 120° is plainly evident. Indeed, even in those cases where secondary 

 and tertiary disturbing factors come into play, traces can still be determined of the 

 fundamental forces at work and in many cases the nature and relations of the inter- 

 fering elements can be determined. 



Another type of fissuration which is even more widely diffused than the tri-radi- 

 ate type is that to which Wilder has applied the name of zygal. 1 The orbital fissure 

 often presents this arrangement. 2 We might also consider these zygals as formed by 

 two tri-radiate fissures joined by a common stem. On fig. 14, Plate XXXVIII, are 

 shown the orbital fissures of the chimpanzee, indicated by the letters a, b, c, d, e. 

 In fig. 15 of the same plate an anterior view of the frontal region is given and the 

 same tendency to the zygal form of fissuration is quite evident. Often fissures that 

 are primitively simple and continuous lines break up in the course of their develop- 

 ment into a number of distinct and separate or more or less connected portions, 

 which assume the zygal type. Especially is this the case with the mesial occipitc*- 

 frontal. In the lower Simians this is usually a single fissure, but as we advance 

 toward the Anthropomorpha, Plate XXXIX, fig. 3, it becomes quite complex and 

 in Man assumes the appearance as found on Plate XLII, fig. 2, etc. 



A close examination of the structure and mode of arrangement of these tri- 

 radiate and zygal types of fissuration will at once impress the mind with the idea 

 that the fundamental causes at work must have relatively simple relations to each 

 other, and the question arises is it possible to discover the mathematical centres 

 and surfaces together with the relative strength of the forces at work necessary to 

 produce the observed results. Of course such a theory must of necessity be of the 

 most general character, but if the fundamental principles concerned in the formation 

 of these types of fissuration can be determined their application is a question of mere 

 detailed study ; that is, an accurate observation of the action of the modifying causes 

 at work in any special case. 



I believe that such a theory can be formulated by applying the principles 

 deduced by Plateau. 3 



1 On page 4 of his paper, "The Par-occipital Fissure," he defines these fissures as follows: " Zygal 

 fissures (F. zygales), H-shaped fissures, quadradiate fissures — a general name proposed for fissures which, 

 like the par-occipital, present a pair of branches at either end of a connecting bar or yoke (zygon). When 

 the earliest condition of the fissure resembles a U, the rami constituting the sides of the U may be called 

 stipes, and the others rami. To carry out the comparison with letters, the complete or typical condition 

 of a zygal fissure is like two Y's joined by their stems, or, viewed from the side, like an expanded H. 



2 Ecker, Edes transl., 33. 



3 "The Figures of Equilibrium of a Liquid Mass withdrawn from the Action of Gravity." (Sixth 

 Series, containing: 1. Theory of liquid films. 2. Laws of films. 3. Constitution of froth. 4. Generation 

 of films.) 



