. An American text-book of obstetrics. For practitioners and students. contact (Fig. 120, b). The junctionof the brain and the spinal cord is marked by the cervical flexure, whichforms an angle of about 90°. A third bend, the metencephalic or frontal flex-ure, appears opposite the primitive cerebellum and the pons, and has its con-vexity directed ventrally or in a manner opposite to the disposition of theother curves (Fig. 120, c). The development of the individual parts of the brain depends largelyupon local thickenings of parts of the walls of the cerebral vesicles, wherebyareas of notable t
. An American text-book of obstetrics. For practitioners and students. contact (Fig. 120, b). The junctionof the brain and the spinal cord is marked by the cervical flexure, whichforms an angle of about 90°. A third bend, the metencephalic or frontal flex-ure, appears opposite the primitive cerebellum and the pons, and has its con-vexity directed ventrally or in a manner opposite to the disposition of theother curves (Fig. 120, c). The development of the individual parts of the brain depends largelyupon local thickenings of parts of the walls of the cerebral vesicles, wherebyareas of notable thickness are produced, as in those which give rise to thecorpus striatum and the optic thalamus; the cleavage of the fore-brain and theingrowth of connective-tissue structures accompanying the growth of theprimitive falx likewise exert a profound influence in shaping the parts around PHYSIOLOGY OF PREGNANCY. 127 the lateral and third ventricles. The appearance of such commissural bandsas the corpus callosum and the fornix still further modifies the adjacent struc-. oi o p/ 0 ol p pf m Fig. 120.—Brains of human embryos from reconstructions by His. A, brain from fifteen day embryo ;B, from three and a half week embryo ; C, from seven and a half week fetus : fb, ib, tub, hb, ab, fore-, inter-,mid- hind-, and after-brain vesicles : o, optic vesicle; ov, otic vesicle; in, infundibulum ; m, mammillaryprocess; pf, pontine flexure; IVv, fourth ventricle; nk, cervical flexure; ol, olfactory lobe; b, basilarartery ; p, pituitary recess. tures. The brain-vesicle undergoing least change is the mid-brain, since itswalls remain uncleft and retain their primary relations to the enclosed canal. B
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