MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS 349 



Plate XLV, it will be seen that the fissures are identical with those found in the 

 carnivorous brain. Just as in this order we found a tendency to vegetative repe- 

 tition in these fissures, so we find even to a more marked extent the same process 

 taking place in the Ungulata, and indeed in these animals the lines of repetitive 

 fissuration extend out upon the lateral surface of the hemisphere ; and a series 

 may be traced on which, from a single fissure visible on the lateral surface, as in 

 the peccary, Plate XL VI, figs. 4 and 5, we may gradually distinguish as many as four 

 or five distinct fissures. Thus, in the tapir and sheep, Plate XLVIII, fig. 4 and 

 Plate XLVI, fig. 7; two may be seen, in the caribou, giraffe and llama, Plate XLVII, 

 fig. 1, Plate XLVI, fig. 10, and Plate XLVII, fig. 7, three ; whilst in the horse, I 

 have counted as many as four or five. 



Owen has considered each of these fissures as typical, and he has endeavored 

 to point out their homologues on the lateral surface of the carnivorous brain. 

 Evidently the four or five fissures of the horse, taken together as a group, are equiva- 

 lent to the three as found in the caribou, giraffe and llama, to the two of the tapir 

 and sheep, and these in their turn to the single fissure as found in the brain of the 

 peccary. 



When these fissures are compared, therefore, they should be taken together as 

 a group, and we should not expect nor is it possible to determine, from the adult 

 brain alone, what fissures are homologous to each of these separate fissures, in dif- 

 ferent brains. 



A good illustration of the extent to which fissuration may be carried by means 

 of vegetative repetition, without increased complexity, is exemplified in the brain 

 of the dolphin, Delpkinus, Plate XLVIII, figs. 1 and 2, whilst in the manatee, an- 

 other aberrant aquatic mammal, the brain remains entirely smooth, Plate XLVII 

 figs. 10 and 11 and Plate XLVIII, fig. 3. 



MECHANICS OF THE FORMATION OF CEREBRAL FISSURES. 



In order to explain the formation of the fissures I nave adopted a theory which 

 has for its basis the interaction of two sets of forces ; namely, the growth forces of 

 the expanding brain combined with and modified by the resisting forces due to pres- 

 sures produced by the bony environment; and we have further found that in the 

 early stages of development the relations of these forces are of a relatively simple 

 character, producing an arrangement of primary furrows which at certain epochs 

 have definite mathematical relations to each other. Plate XXXV, fig. 4, illustrates a 

 foetal brain in which the fissures are related to each other in multiples of an angle of 

 60°. Thus O 1 and O 2 are inclined to each other at an angle of 120° meeting the calcarine 

 at the same angle anteriorly and being bisected by it posteriorly into angles of 60°, 

 text fig. 2. It will be also noticed in studying many of the sulci, as for instance the 

 one named sulcus triradiatus of the occipital lobe, Plate XXXVIII, fig. 16, that in 

 their early and least disturbed condition they present a triradiate appearance, the 

 rays diverging from a common centre at angles of 120° to each other, or so closely 



