STRUCTURAL CULMINATION 955 



mammals. The most coiistaiU of ihcsf compoiU'iits is tlic su|jra-S\ i\ lan 

 lissurc. The mammah'an sulcus usuall\ liomoloirizcd with llu' fissure ot 

 S\ Kius in a]ies and man is, aeeorclinii; to Elliot Smith, nowise homolotious. 

 Ill' calls this subi^rimati' indentation, when it occurs, the ])seud()-Sylvian 

 sulcus. This sulcus may be absent, as m \ iverra and I l\ rax, or its comjjosi- 

 tion max include dillerent (.■h-nuMits in dillerent orders. Thus the jjseudo- 

 Sylvian sulcus of the carnivores may differ from that of tlie unij;ulates, in 

 which latter the fissure often n-sults from the fusion of two infoldings 

 analogous to the eeto-Sylvian sulci ot the carnnores. Concerning these latter 

 lissures, there can be httle doubt but that they are morj^hologicallx unimjjor- 

 tant, and appear h)r the most part as lurrows compensatory to the imagi- 

 nation of the pseudo-Syl\ lan lissure. Li Smith's lissural |)attern of the 

 hypotlu'tical generalized carni\ore this pseudo-S\ Kian element does not 

 appear. The mammalian generalized ])attern includes the supra-S\l\ian, the 

 orbital (pre-Sylvian), coronal, lateral, p()St-S\lvian and cruciati- sulci. 



The Sj'Ivian fissure in lemurs is formed by the merging of the pseudo- 

 Sylvian and supra-Sylvian sulci. \n this complex the supra-Syl\ian element 

 is the most constant in all k'liiurs, just as it is in most mammals. Generally 

 speaking, this stable and constant sulcus l)v its union with tlu' \ariable 

 suprarliinal indentation called the ])seudo-SyIvian sulcus gives an ultimate 

 lixit\ to the latter. The result of this fixation is the jjosterior limb ol the 

 fissure of Sylvius. Li monkt-\s and apes the onl\ portion ol the lissure which 

 develops is this ])osterior limb. The I'ulix formed anterior limbs iivv iu'\er 

 found except in man. Thus the Syl\ ian lissure in its complete lorm occurs 

 only in the human brain \\ hert' it exists as a combination ol the supra- 

 Sylvian (superior limiting) sulcus, the pseudo-Sylvian (inferior limiting) 

 sulcus, and the diagonal (anterior limiting) sulcus. Viewed in this light the 

 formation of the fissure of Sylvius may be understood to represent a 

 consistent exolutional process throughout all gyrencephalie mammals. 



