Aids to embryology / by J.S. Baxter.

  • James Sinclair Baxter
Date:
1948
Catalogue details

Licence: In copyright

Credit: Aids to embryology / by J.S. Baxter. Source: Wellcome Collection.

    63/204 page 51
    During its early growth the future brain region of the neural tube becomes bent upon itself at three places (see Fig. 12). These are the three primary brain flexures. The first appears in the region of the mid-brain during the fourth week and is called the mid-brain flexure. As a result of this flexure, the first primary brain vesicle takes up a position at right angles to the axis of the second brain vesicle. Shortly after this (fifth week) the second or cervical flexure occurs in the region of the future medulla. It is caused by ventral bending of the whole head region of the embryo, so that the axis of the mid- brain forms almost a right angle with the medulla., and the axis of the first brain vesicle takes up a position almost parallel with the latter. The third, or pontine flexuie, occurs in the region of the meten- cephalon during the sixth week and is the reverse direction to the other two, being a ventral bending of the floor of the hind brain. The first two brain flexures are the expression of changes taking place during the formation of the head end of the embryo, but the third flexure (pon- tine) is peculiar to the brain itself, and does not influence the external form of the head. The pon- tine flexure is not apparent in the adult brain because of the development of the transverse fibres of the pons, but the mid-brain and cervical flexures persist, although reduced in acuteness. The Myelencephalon.—When the pontine flexure occurs it causes the roof plate of both the myelen- cephalon and metencephalon to become thinned out and expanded laterally. Thus the alar and basal laminae lie in the ventral part of the myelencephalon. The cells of the basal lamina become segregated in several groups. Most medially are the cells of the hypoglossal nerve. More laterally are cells (branchio- motor) which form the nucleus ambiguus, lateral to
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