:NTERNAL STRUCTURE or MEDULLA OBLONGATA AND PONS. 553 



plexuses, which are situated at the three corners of the epithelial roof of the ventricle, 

 oval or elliptical perforations develop in 

 the roof at about the fifth month of foetal 

 life. These are known as the apertura 

 medialis ventriculi quart! (O.T. foramen of 

 Magendie), which opens between the clavae 

 on the posterior surface and the aperturae 

 laterales upon the anterior faces of the 

 lateral recesses (Fig. 527), behind the in- 

 sertion of the glossopharyngeal nerve on 

 each side. Through each of these lateral 

 openings the great swollen cauliflower-like 

 extremity of the chorioid plexus becomes 

 extruded from the ventricle. The inferior 

 extremities of the two plexuses lying side 

 by side present an analogous relationship 

 to the apertura medialis, but they are 

 exceedingly attenuated and the epithelial 

 lamella from which they spring becomes 

 dragged backwards into contact with the 

 cerebellum (Fig. 477), so that, when seen 

 from below, the apertura medialis is a great 

 funnel-shaped tube leading into the fourth 

 ventricle and the chorioid plexuses look 

 like two delicate vascular fringes on the 

 cerebellum. 



These three apertures are the only 

 means provided for the escape of the fluid 

 contained in the ventricles of the central 

 nervous system. The fluid is poured into 

 a space, enclosed by the arachnoid, which 

 is called the subarachnoid space. 



As a result of the pontine flexure the IG< 

 side walls of the neural tube in the 

 neighbourhood of the bend fall away the 

 one from the other and eventually come 

 to be placed in the same transverse plane, 



B 



483. TRANSVERSE SECTIONS ACROSS THE 

 MEDULLA OBLONGATA IN TWO HUMAN EMBRYOS, 

 REPRESENTING DIFFERENT STAGES IN THE EX- 

 PANSION OF THE ROOF AND THE FALLING LATER- 

 ALLY OF THE SIDE WALLS. (From His, slightly 

 modified.) 



Cerebrum 





one with the other and 

 also with the floor-plate. 

 At the time this process 

 is in operation (see Fig. 

 483) the alar and basal 

 laminse are particularly 

 well defined, and the 

 limiting sulci are ac- 

 centuated by the bending 

 of the side wall; but 

 this sharp distinction is 

 soon lost as the result of 

 the great expansion of 

 the basal lamina (Fig. 

 485). This is due not 

 only to growth of its 

 intrinsic elements, but 

 even more to its in- 



. THE BRAIN OF AN EMBRYO OF ELEVEN WEEKS, viewed from vasion by large numbers 



behind. The epithelial roof of the fourth ventricle has been removed. O f neuroblasts which 

 At this stage the cerebellum is in the form of a simple band or plate f f fV 1 



which arches over the posterior aspect of the anterior part of the cavity mi g rare 



of the hind-brain. (From His.) into the basal lamina. 



Mesencephalon 



Early cerebellum 



Cavity of 

 fourth ventricle 



Lateral recess 



Medulla oblongata 



