474 PHYSIOLOGICAL 8ERIl-:>. 



mencing) than that of the Anthropoid Apes. Of the latter the brain 

 of the Gorilla approaches the human condition much more nearly than 

 does that, either of the Chimpanzee or Orang. It must be remembered, 

 however, that the " occipital " and " insular " regions exhibit an 

 extraordinary amount of variation in each of the Simiidae ; thr 

 average condition of these two changing areas is much nearer the 

 human type in the Gorilla than in either of the other great Apes. 



Of the other sulci of the human brain (besides those already dis- 

 cussed) the only ones which can be called " old " in the phylogenetic 

 sense are the orbital and possibly the inferior frontal sulci. 



The orbital sulcus is probably one of the most primitive furrows in 

 the neopallium, if not the earliest. It is the only sulcus found in 

 the most generalised mammals, Erinaceus and Peramdes. It is a very 

 constant and precocious sulcus in all the Carnivora, Ungulata, Edentata, 

 Cetacea, and many Rodents and Galeopiihecus. Most writers call it 

 " presylvian " in all these non-Primate orders, but there can be little 

 doubt as to its homology with the orbital sulcus, although, so far as I 

 am aware, such an interpretation has never hitherto been suggested. 

 Hut it would be strange if this (the most widespread and constant) 

 sulcus of the neopallium should not be represented, in the Primates, and 

 there is no other furrow of sufficient constancy in the pararhinal region 

 to represent the presylvian sulcus of other mammals. If moreover we 

 compare such brains as those of Dolichotis (Rodent), Oaleopithecus 

 (Insectivore), Bradypus (Edentate), and Phascolonn/s (Marsupial) with 

 the Lemur's, it is clear that the " presylvian " sulcus of the former IMM 

 bo represented in the Prosimia) only by the orbital or the fron to-orbital 

 sulcus. Of those the former is not only by far the more constant of 

 the two sulci, but it is also that which occupies the same position and 

 relationship to the rhinal fissure as the " presylvian." A comparison 

 of (ralayo and Dolichotis shows this. 



If again wo compare the behaviour of the orbital sulcus in the 

 larger Ungulates (e. y., the Camel, Horse, and Ox) and Carnivores 

 (. </., the Seals), wo shall find that as the hemisphere increases in 

 magnitude (and more especially if at the same time it becomes more 

 microsmatic) the " presylvian " sulcus becomes relegated to a position 

 alongside the anterior rhinal fissure exactly analogous to that occupied 

 by the orbital sulcus in the Gorilla's brain. In Man the simple linear 

 orbital sulcus becomes complicated by numerous side branches so as to 

 form triradiate, H-shaped or other patterns ; but if a largo number of 

 human brains be examined, the orbital sulcus will be found to consist 

 in a very considerable proportion of these cases of a single deep linear 

 sulcus, the apparent branches of which are mere shallow furrows of 



