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.This is due, in part, to the growth of the hemicerebral masses which enclose them,but in part also to the appearance of specialstructures which project into the cavities.As the hemispheric vesicle of the mammalian brain arches backover the diencephalon and mesencephalon, the portion that lies behind the Sylvian fossa takes a downwardturn (Figg.36, 42, p.108, 112).The result is that the paracele possesses two branches, or cornua, as they aretermed: a precornu, bounded on the outside by the arched wall of the hemisphere, and a medicornu (cornuinferius) whose extremity is drawn out to a point.The growth of the hemispheric vesicle over the caudex isaccompanied throughout its progress (as we have already seen: p.114 above) by a parallel growth of the aula(foramen of Monro), the original means of communication between prosocele and diacele.As the aula, then,curves over the caudex, at first posteriorly and then ventrally, what was originally its dorsal extremitycoincides with the pointed end of the medicornu.The part of the aperture that now lies in the anterior wall ofthe medicornu forms a fissure (the hippocampal fissure, of which more presently), which is occluded by avascular plexus from the pia (fh Fig.56).In fine, therefore, the primitive aula remains open at beginning andend, but is closed over its middle portion by myelinated fibres.These belong to the fornix and callosum,structures which we shall discuss in the following section.In the brain of the primates (the apes and man), the conformation of the paraceles undergoes yet anotherchange, due to the large development of the occipital portion of the hemispheres.The outer wall of eachparacele pushes vigorously backwards before it takes the curve downwards, so that the cavity itself isCHAPTER IV.Morphological Development of the Central Organs 76 Principles of Physiological Psychologyprolonged in the same direction.We thus have a postcornu (cp Fig.51, p.123) in addition to the precornu andmedicornu.The backward growth of the prosencephalon stops, as it were, with a jerk, to continue forwardsand downwards.This fact is attested both by the outward appearance of the occipital region, and by the shapeof the postcornu, which is drawn out into an even finer point than the medicornu.In the apes, the postcornu issmaller than it is in man; in other mammals with strongly developed hemispheres, as e.g.the cetacea, it is nomore than a trace or rudiment of what it is later to become.(b) Fornix and Commissural SystemAt the anterior extremity of the primitive aula, the two hemispheres grow together in the middle line.Theresulting strip of alba is termed the terma (lamina terminalis: bd Fig.43, p.113).The backward curvature ofthe hemispheres round the transverse axis of the diencephalon naturally brings with it a correspondingcurvature of the terma.Its most ventral and anterior extremity becomes a band of cross-fibres, connecting thetwo hemispheres, and known as the precommissure (k Fig.43).In its further course it divides into two lateralhalves running longitudinally from before backwards, on either side of the median fissure.We find the firstbeginnings of these longitudinal fibre-tracts in the birds, but they do not attain to any high degree ofdevelopment until we reach the mammals, where they constitute the fornix.Closely approximated anteriorly,the columns of the fornix diverge as they pass backwards.The myelinated fibres of their anterior extremityextend ventrally to the brain-base, where they stand in connexion with the alba of the albicantia (Fig.53, p.127).The fibres of their posterior extremity are distributed in man and the apes into two bundles, the smallerof which comes to lie upon the inner wall of the postcornu, and the larger upon the inner wall of themedicornu of the paracele.The projection thus occasioned in the wall of the postcornu is termed the calcar(pes hippocampi minor), that in the medicornu, the hippocampus (pes hippocampi major: Fig.55)
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