FORMATION OF FISSURES. 497 



1344. Problems Connected with the Cerebral Fissures. 



The study of the fissures of a single brain is comparatively uninter- 

 esting and unprofitable unless three general questions are consid- 

 ered : 

 i 



(1) What relations do the fissures bear to the ental structures? 



(2) What is the fissural pattern in the cat ? 



(3) How do the fissures of the cat compare with those of man and other mammals ? 



1345. Formation of Fissures. At birth the cat's hemispheres present fewer and 

 shallower fissures than in the adult. Presumably they were entirely smooth at an earlier 

 period, as is the case in all other mammals which have been examined. 



The hypocampal fissure represents an involution of the entire thickness of the parietes, 

 the hypocampa ( 1243) being the reverse elevation. The callosal fissure, and perhaps 

 some of the others already enumerated ( 1343), are formed in some peculiar way. 



So far as the other and ordinary fissures are concerned, although sometimes described 

 as depressions, it is probable that they are to be regarded as lines of less elevation as com- 

 pared with the intervening folds. More extended and accurate observations are needed 

 upon this matter. For the formation of the cruciate fissure, see 1359. 



1346. Structural Relations of Certain Fissures. So far as appears from sections 

 of the cat's brain at any period after birth, only eight of the cerebral fissures have any 

 intimate or constant relation to structural features, viz. , the callosal, fimbrial, hypocampal, 

 olfactoria, postradical, prceradical, rhinalis and postrhinalis. These have all been men- 

 tioned in connection with the parts with which they appear to be correlated. 



1347. In man two other fissures, which do not exist in the cat, are related to ental 

 structures : the calcarine to the calcar ( 1194) and the collateral to the Eminentia collat- 

 eralis at the place of departure of the postcornu from the medicornu. 



A few of the fissures will now be mentioned separately. 



1348. F. callosalis, F. cl., the callosal fissure. Fig. 117, 122,125; PI. II, Fig. 3; 

 PI. Ill, Fig. 18 ; PL IV, Fig. 17, 20. 



This coincides with the dorsal border of the callosum, curves about the splenium to 

 join the hypocampal (Fig. 125) and about the genu to be continuous with the F. prceradi- 

 calis when the latter is distinct. 



1349. F. fimbriae, F. fmb., the fimbrial fissure. Fig. 121, 125; PI. IV, Fig. 14, 17; 

 1172. 



A distinct and apparently constant depressed line between the fasciola and the fimbria, 

 thus coinciding with the margin of the cinerea. It is not a true cortical fissure, and 

 perhaps should not be enumerated with the rest. 



1350. F. hypocampae, F. hmp., hypocampal fissure. Fig. 121, 125 ; PI. IV, Fig. 14, 

 17 ; 1172. 



Syn. F. hippocampi, hippocampal fissure. 



This is not the deepest of the cerebral fissures, but it is one of the longest, and is per- 

 haps the most coristant of all among the Mammalia, being present when the hemispheres 

 are otherwise smooth or indented only by the rhinalis and postrhinalis. 



It extends from near the tip of the Lobulus hypocampm to the xplenium, where it is 



continuous with the F. callosalis. In the larger part of its course it presents a decided 



caudal convexity, forming nearly the half of a circle and coinciding in general with the 



medicornu and with the hypocampa, of which it is the depression in reverse. Near the 



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