1176 



EMBRYOLOGY. 



ventricle), formed by the remains of the first vesicle and the whole of the second 

 vesicle, into one. 



The third vesicle (mesencepJialon) is converted into a narrow channel, the iter 

 a tertio ad quartum ventriculum (Fig. 725, c). 



The fourth vesicle (epencephalon) becomes widened out, and assumes a trian- 

 gular form, with its apex directed forward, and situated at the original point of 

 constriction where the third vesicle joins the 

 fourth. It is at the same time flattened from 

 above downward, and constitutes the anterior 

 half of the fourth ventricle (Fig. 725, D). 



The fifth vesicle (metencephalon) undergoes 

 the same change in form as the fourth, becom- 

 ing triangular in shape and flattened from above 

 downward, but with this difference, that the apex 

 of the triangle is directed backward, and is 

 continuous with the portion of the medullary 



v 



FIG. 725. Plan showing the mode of 

 formation of the ventricles of the brain 

 and the central canal of the spinal cord. 

 (After Gerrish.) A. Prosencephalon. B. 

 Thalameiicephalon. c. Mesencephalon. 

 D. Epencephalon. E. Metencephalon. F. 

 Central canal of cord. G. Lateral ventri- 

 cle. H. Foramen of Monro. 



FIG. 724. Section of the medulla in the cervical region, at six 

 weeks. Magnified 50 diameters. 1. Central canal. 2. Its epithe- 

 lium. 3. Anterior gray matter. 4. Posterior gray matter. 5. An- 

 terior commissure. 6." Posterior portion of the canal, closed by 

 the epithelium only. 7. Anterior column. 8. Lateral column. 

 9. Posterior column. 10. Anterior roots. 11. Posterior roots. 



canal which goes to form the central canal of 

 the spinal cord (Fig. 725, E). The base is 

 directed forward and is continuous with the base 

 of the triangular space formed by the fourth 

 vesicle ; the constriction between the two ves- 

 icles having disappeared, the two spaces freely 

 communicate, and together form a rhomboidal 

 cavity which is the fourth ventricle. 



These vesicles do not remain in the same 

 plane, but certain definite flexures take place, 

 which result in an alteration of the position of the vesicles to one another. 

 The first of these flexures (cephalic) is opposite the base of the middle vesicle, 

 which becomes sharply bent on itself over the end of the notochord. This 

 has the effect of causing the mid-brain to become the most prominent part of the 

 encephalon on the convexity of the curve (Fig. 723). A second flexure (pontal), 

 with its curve in the opposite direction, takes place in the epencephalon, and is 

 very abrupt. A third but less marked flexure (nuchal) takes place in the meten- 

 cephalon at its junction with the cord. The first of these curves or flexures 

 remains permanent, but the second and third almost entirely disappear in the 

 further development of the brain. 



The manner in which the different parts of the encephalon and cord are 

 formed from the walls of this greatly altered medullary canal must now be consid- 

 ered, and it will be convenient first of all to study the development of the spinal cord. 



