1460 A MANUAL OF ANATOMY 



The parts of the encephalon, formed from the foregoing subdivisions, are 

 as follows : 



Telencephalon. Thalamencephalon, or Diencephalon. 



Cerebral hemispheres. Posterior part of third ventricle. 



Lateral ventricles. Optic thalami. 



Anterior part of third ventricle. Pineal body. 



Foramina of Monro. Interpeduncular structures. 



Olfactory lobes. Pituitary body. 



Optic nerve and retina. 



Mesencephalon. Hetencephalon. Myelencephalon. 



Corpora quadrigemina. Cerebellum. Medulla oblongata or bulb. 



Crura cerebri. Pons Varolii. Bulbar part of the fourth 



Aqueduct of Sylvius. Pontine part of the fourth ventricle, 

 ventricle. 



Metamorphoses of the Secondary Cerebral Vesicles. 



Myelencephalon. — This is the fifth secondary cerebral vesicle, being the 

 caudal subdivision of the rhombencephalon or posterior primary cerebral 

 vesicle. Its walls give rise to the medulla oblongata or bulb, and its cavity 

 forms the bulbar part of the fourth ventricle. The lateral walls of the myelen- 

 cephalon correspond for the most part with those of the spinal portion of the 

 neural tube, each consisting of three zones — ependymal, mantle, and mar- 

 ginal, from within outwards. Moreover, the lateral walls are connected 

 by two plates — dorsal or roof-plate, and ventral or floor-plate. Each lateral 

 wall is composed of a ventral or basal lamina and a dorsal or alar lamina, 

 these being separated by a longitudinal groove, called the sulcus limitans, as 

 in the spinal portion of the neural tube. 



In the myelencephalon certain changes are effected. 



1 . The roof -plate becomes much widened. 



2. The basal and alar laminae of each lateral wall, which primarily occupy 

 an almost vertical plane, spread outwards and occupy an almost horizontal 

 plane, the roof plate meanwhile expanding. The basal and alar laminae now 

 form the floor of the bulbar part of the fourth ventricle, the basal lamina being 

 mesial to the dorsal lamina. 



3. The lumen of the neural tube, which is a mere dorso-ventral cleft in the 

 spinal portion of the tube, becomes wide in the region of the fourth ventricle. 



Roof-Plate. — This forms the roof-epithelium or ependyma of the bulbar 

 part of the fourth ventricle, and it becomes thickened in certain situations 

 (mesial and lateral), thus giving rise to (i) the obex (mesial), and (2) the 

 ligula, right and left (lateral). 



The roof-plate is connected, on either side, with the dorsal margin of the 

 corresponding alar plate, which margin becomes everted and forms the 

 rhombic lip. Subsequently this lip fuses with the contiguous part of the 

 outer surface of the alar lamina. 



The widened roof-plate is covered by vascular mesenchyme, representing 

 pia mater. At the level of the pontine flexure the ependymal or epithelial 

 roof is invaginated into the fourth ventricle in the form of a transverse fold — 

 plica choroidea, containing pia mater, which extends between the lateral 

 recesses of the ventricle. From this transverse fold two vertical folds, pliccs 

 choroidecB, hkewise containing pia mater, extend vertically downwards into 

 the ventricle, close to the median line. These ependymal folds, containing 

 pia mater, form the two choroid plexuses of the lateral ventricle, thus — [J. 



Lateral Walls. — Each lateral wall, as stated, consists of three zones — 

 ependymal, mantle, and marginal, from within outwards. On either side it 

 presents two laminae — dorsal or alar, and ventral or basal, separated by the 

 sulcus limitans. Moreover, as stated, these two laminae lie vertically, the 

 dorsal margin of the alar lamina forming the rhombic hp. The axons of the 

 neuroblasts of the mantle-layer of the basal lamina emerge as centrifugal or 



