The development of the human body; a manual of human embryology . cations ofthe walls. The cavity of the third vesicle persists in theadult as the fourth ventricle, traversing all the subdivisionsof the vesicle; that of the second, increasing but little inheight and breadth, constitutes the iter; while that of thefirst vesicle is continued into the cerebral hemispheres toform the lateral ventricles, the remainder of it constitutingthe third ventricle, which includes the cavity of the medianportion of the telencephalon as well as the entire cavity ofthe diencephalon. 406 THE DEVELOPMENT OF THE
The development of the human body; a manual of human embryology . cations ofthe walls. The cavity of the third vesicle persists in theadult as the fourth ventricle, traversing all the subdivisionsof the vesicle; that of the second, increasing but little inheight and breadth, constitutes the iter; while that of thefirst vesicle is continued into the cerebral hemispheres toform the lateral ventricles, the remainder of it constitutingthe third ventricle, which includes the cavity of the medianportion of the telencephalon as well as the entire cavity ofthe diencephalon. 406 THE DEVELOPMENT OF THE HUMAN BODY. During the differentiation of the various divisions ofthe brain certain flexures appear in the roof and floor, andto a certain extent correspond with those already de-scribed as occurring in the embryo. The first of theseflexures to appear occurs in the region of the mid-brain,the first vesicle being bent ventrally until it comes to lieat practically a right angle with the axis of the may be termed the mid-brain flexure (Fig. 216, mj). Fig. 217- -Mediax Longitudinal Section of the Brain of an Embryoof the Third Month.—(His.) and corresponds with the head-bend of the embryo. Thesecond flexure occurs in the region of the medulla oblon-gata and is known as the neck flexure (Fig. 216, nf); itcorresponds with the similarly named bend of the embryoand is produced by a bending ventrally of the entire head,so that the axis of the mid-brain comes to lie almost atright angles with that of the medulla and that of the firstvesicle parallel with it. Finally, a third flexure occurs inthe region of the metencephalon and is entirely peculiar THE MYELENCEPHALON. 407 to the nervous system; it consists of a bending ventrallyof the floor of the hind-brain, the roof of this portion of thebrain not being affected by it, and it may consequentlybe known as the pons flexure. In the later development the pons flexure practicallydisappears, owing to the development in this re
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
