. Cunningham's Text-book de l'anatomie. L'anatomie. L'NATUBE DU BEAIN. 515 de la flexion. Si ce mur est solide et résistant, comme la plaque de plancher du tube neural (dans le cas du cancer du col utérin et de l'encéphale des flexions) la courbure n'affecte pas les contours du tube (dans la section) très sensiblement. Mais lorsque la souche est jeté sur la mince plaque de toit pendant l'élaboration de la flexion pontique il n'est pas assez forte pour résister ; il devient tendu et permet les murs latéraux du tube neural à évasement latéralement dans exactement la même manière que se produit lorsqu'un tube de caoutchouc est tordu vers

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. Cunningham's Text-book of anatomy. Anatomy. THE NATUBE OF THE BEAIN. 515 of the flexure. If this wall is strong and resisting, like the floor-plate of the neural tube (in the cases of the cephalic and cervical flexures) the bending does not affect the outline of the tube (in section) very materially. But when the strain is thrown upon the thin roof-plate during the development of the pontine flexure it is not strong enough to resist; it becomes stretched and allows the lateral walls of the neural tube to splay laterally in precisely the same manner as occurs when a rubber tube is bent towards a side which has been split (or weakened) longitudinally (Fig. 457). This mechanical factor determines the curious form assumed by the hind-brain at the end of the first month; and gives its cavity, the fourth ventricle, a lozenge or rhomboid form, when seen from its dorsal aspect through the thin translucent roof. For this reason the hind-brain is known as the rhombencephalon. The rhombencephalon forms at first more than half of the encephalon, and as it expands it appears to become marked off from the rest by a constriction (the isthmus rhombencephali). The development of the pontine flexure subdivides the rhomben- cephalon into two parts, one joined to the spinal medulla, the myelen- cephalon, and the other, joined to the rest of the brain, the metencephalon. In the myelencephalon develop the nuclei of the nerves that regulate the activities of the heart, lungs, and a considerable part of the alimentary canal, and also the receptive nuclei of the nerves of taste. It is known as the medulla oblongata. The insertion of the nervus acusticus in the neighbourhood of the outsplayed lateral angle of the rhombencephalon leads to the pro- found transformation of the meten- cephalon. The nervus acusticus conveys into the hind-brain impulses which are stimulated by movements of fluid in the closed sac developed from the otic vesicle (Fig. 443, p. 501). The truly acoustic function of

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