We have iu)\v to consider that hack ward extension of the calcarine tissure known as the posterior 

 calcarine (Cunningham) and by Elliot Smith burdened with the name sulcus occipital is intrastriatus 

 mrsialis. As others have indicated, it is a fissural unit of minor morphological importance, and 

 unstable. In Man and in the Anthropoid Ape both its walls and the free surface of the 

 bounding gyri are covered by visual cortex, and it is the rule for the fissure to be fused with the 

 'stem.' 1 Its homologue lower down the animal scale seems to be, as Elliot Smith tells us, a 

 small isolated tissure which has received the same name and is seen to advantage in certain 

 Carnivore and Ungulate brains. In Canis the sulcus is plainly seen and undoubtedly lies surrounded 

 by visual cortex, so fulfilling one postulate. Next it appears that when in course of cerebral 

 expansion the complex assumes a horizontal position, this previously isolated element fuses with 

 the hinder extremity of the "true calcarine,'' that is, with the end formerly uppermost. Per contm, 

 even if we are at fault in supposing that the posterior calcarine tissure of the lower orders 

 acquires permanence in this manner, it is easy to believe, as an alternative, that the "true 

 calcarine" sends a branch backwards to develop into the posterior calcarine (homiu-is), because in 

 many of the lower mammalian brains exhibiting the calcarine-intercalary combination one notices 

 a distinct backward offshoot, a tendency to bifurcate, at the point where the two main con- 

 stituents unite. 



It remains for us to determine the homologue of the third and last constituent of the calcarine 

 complex of Homo, the flssura extrema of Seitz, a short vertical tissure running behind and at 

 right angles to the posterior calcarine, to which though at times isolated it is usually joined, a 

 tissure the walls of which are invariably covered by visual cortex, and one which forms an 

 absolutely reliable anatomical guide to the posterior boundary of the primary sight area. In an 

 earlier chapter (p. 121) I maintained that in the Anthropoid, as the result of altered mechanical 

 conditions, this constituent finds itself dislocated outwards to become the Simian external calcarine 

 fissure, or sulcus intrastriatus lateralis (Elliot Smith) ; to this honiology I still adhere, and 

 I need not repeat my reasons. But what the homologue is in lower animals is more difficult 

 to decide. A clue is offered by the subsequent history of an outside fissure, the lateral. Elliot 

 Smith, whose unequalled experience in the comparative anatomy of the brain entitles his opinion 

 to profound respect, maintains that the lateral sulcus is converted into the intraparietal sulcus 

 of Man; and if I read his meaning correctly, in further detail, the lateral sulcus proper and the 

 postlateral sulcus as seen in Felis and Canis are respectively interchangeable with the ramus 

 horizontalis and the ramus occipitalis transversus of the intraparietal fissure of Homo. Accepting 

 these homologies as true, it follows that the antecedent of the fissura extrema vel calcarina externa, 

 if present, must lie somewhere between the lateral and " true calcarine " fissures, but on searching 

 the brains of Felis and Canis for a fissure in this situation, with an undetermined homology, the only 

 one we can find is that recognised by the name marginal or suprasplenial, and the question is, will 

 this fulfil our requisition 1 I believe it will. The fissure is a well-defined one in the two animals 

 just mentioned ; it seems to be represented with a considerable degree of constancy in other 

 natural orders; it is situated in the midst of cortex having visual characters; and although in 

 Carnivorae, etc., it lies towards the upper margin of the hemisphere, it is still placed, as in 

 Homo, more or less at right angles to the " true calcarine " fissure, a relation we would expect 

 it to maintain as the occipital lobe tilted backwards. On these grounds, therefore, I submit that 

 the sulcus suprasplenial is and the fissura extrema of Seitz are homologous. 



Here attention must be directed to the arrangement of the sulci and the disposition of the 

 cortex in Sus. In figures of this animal's brain which I have seen one is led to suppose 

 that there is no suprasplenial sulcus, but on the other hand a well-defined lateral sulcus is 

 indicated, placed, however, remarkably close to the margin of the hemisphere, and, moreover, 

 isolated. Now, on referring to my drawings of the visual area in the pig, it will be seen that 



c. 36 



