284 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



I divide the fissures of the cerebral surface into five classes as follows: — 



1. Primary or fundamental fissures. 



2. Secondary fissures. 



3. Sulci (following Pansch) which are usually either vegetative repetitions of 



the secondary fissures as a whole, or they are due to the separation of 

 these fissures into a number of similar parts. They are the lesser cortical 

 fissures or furrows, constant in the adult brains of the higher Primates, 

 including Man. 



4. Sulculi (Pansch), inconstant or little sulci or furrows, having no apparent 



regular location or relationship to the convolutions ; they are usually 

 vegetative repetitions of the sulci. 



5. Rami, constant branches of fissures or sulci, etc. 



PRIMARY FISSURES. 



These fissures constitute, according to the morphological view I have to offer, 

 the typical fundamental fissures of the hemispheres in the Primates, and I will 

 attempt to show that all the remaining fissures bear precise and symmetrical rela- 

 tions to them. 



The fundamental or plan fissures are, as has been pointed out, few in number 

 and simple in character, bear precise and symmetrical relations to the structure of 

 the hemisphere, and are produced by a purely mechanical action taking place 

 between the growing brain and the expanding skull. In the early stages of de- 

 velopment the differential forces produced are simple in their relations as indi- 

 cated by the precise and mathematical regularity of the direction and position 

 of the temporary radiating furrows. As development proceeds these relations 

 become more and more complicated ; so much so that when the secondary fissures 

 commence to make their appearance the primitive simplicity has entirely disap- 

 peared and the directions of the lines of least and greatest resistence to brain growth 

 have not only changed but they commence to assume a more or less tortuous aspect. 

 These conditions and the accompanying results will be pointed out as we proceed. 

 Plate XXXV, figs. 5 and 6, show the human foetal brain at the middle or toward 

 the end of the fifth month. With the exception of the Sylvian fissure it is entirely 

 smooth on the lateral aspect; whilst on the median we see well developed the fis- 

 sura tri-radiatus with its various branches as given in the diagram, fig. 3. 



Having now pointed out the general relations and connections of these funda- 

 mental or plan fissures in the developing human brain, it becomes necessary to in- 

 quire into their constancy throughout the Primates. I have found that the evi- 

 dence, as derived from these sources, is not only in harmony with the results to which 

 I had been led by a study of the foetal human brain, but seems almost sufficient of 

 itself to demonstrate that the fissures pointed out above as fundamental and estab- 

 lishing the true morphological type or plan, have in every respect the significance 

 and importance which has been ascribed to them. I think it will also serve to 

 show that many of the fissures on which more attention has been bestowed and 



