io 5 8 HUMAN ANATOMY. 



choroid plexus. The two structures, the ependyma and the pia mater together, 

 constitute the membranous velum intcrpositum that forms the roof of the ventricle 

 and lies beneath the triangular/6>/v//.i , whose vaulted form is suggested by the arching 

 ridge that descends in front of the thalamus and marks the position of the anterior 

 pillar of the fornix. Behind, just over the upper end of the Sylvian aqueduct, lies 

 the cone-shaped pineal body that belongs to the third ventricle, from which it is an 

 outgrowth. The floor of the ventricle is also, for the most part, relatively thin and 

 irregular in contour. It corresponds to the median part of the lozenge-shaped area, 

 the interpeduncular space, which, seen on the inferior surface of the brain, is bounded 

 behind by the anteriorly diverging cerebral peduncles and in front by the optic chiasm 

 and the posteriorly diverging optic tracts. The posterior half of this area includes 

 the deep triangular recess at the bottom of which are seen the numerous minute open- 

 ings of the posterior perforated space through which small branches of the posterior 

 cerebral arteries pass to the optic thalamus and the crura. Passing forward, the 

 paired corpora mammillaria, the tuber cinereum, the stalk of the pituitary body 

 occupy successively the interpeduncular space. Anteriorly, between the trans- 

 versely cut optic chiasm below and the recurved portion of the great arching com- 

 missure, the corpus callosum, above, the third ventricle is closed by a thin sheet of 

 nervous substance known as the lamina cincrea. 



Through the foramina of Monro the lateral ventricles open into the third, and 

 the latter communicates with the fourth ventricle by way of the Sylvian aqueduct. 

 This narrow canal is surrounded below and laterally by the dorsal part or tegmentum 

 of the cerebral peduncles ; above it lies a plate of some thickness the dorsal surface 

 of which is modelled into two pairs of rounded elevations, the superior and inferior 

 corpora quadrigemina. 



In sagittal section, the fourth ventricle appears as a triangular space, the 

 anterior or basal wall being formed by the dorsal surface of the pons and medulla and 

 the posteriorly directed apex lying beneath the cerebellum. The upper half of the 

 thin tent-like roof of the ventricle is formed by the superior medullary velum, a thin 

 layer of white matter that stretches from beneath the inferior corpora quadrigemina 

 to the cerebellum. A similar lamina, the inferior medullary velum extends from the 

 cerebellum downward, but before reaching the dorsal surface of the medulla becomes 

 so attenuated that this part of the ventricular roof, known as the tela chorioidea, 

 consists practically of the pia mater, although the ependyma excludes the vascular 

 membrane from actual entrance into the ventricle. The pia, however, pushes in the 

 ependyrnal layer and in this manner produces the vascular fringes known as the 

 choroid plexus of the fourth ventricle. When viewed from behind, the ventricle 

 exhibits a rhomboidal outline, the lateral boundaries above being formed by two 

 arms, the superior cerebellar peduncles, that divergingly descend from the sides of the 

 corpora quadrigemina to the cerebellum. Similar bands, the inferior cercbcllar 

 peduncles, convergingly descend from the cerebellar hemispheres to the posterior 

 columns of the medulla and form the lower lateral boundaries of the fourth ventricle. 



Seen from directly above (Fig. 984), the cerebrum, divided into its hemi- 

 spheres by the deep sagittal fissure, is the only part of tin- brain visible, the other four 

 divisions being masked by the enormously developed overhanging cerebral mantle. 

 The effects of this expansion in displacing base-ward parts which, temporarily in man 

 and permanently in the lower vertebrates, occupy a superior position, are conspicuous 

 when the sagittal section of the developing (Fig. 913) and that of the fully formed 

 human brain (Fig. 910) are compared. It should be noted, that although in the 

 latter the brain-stem and the cerebellum are completely overhung by the cerebral 

 hemispheres, they still are in relation with the free surface of the brain, and by 

 passing beneath the posterior part of the cerebrum the dorsal surface of the cerebellum 

 and of the brain-stem may be reached without mutilation of the nervous tissue. 



THE GENERAL DEVELOPMENT OF THE BRAIN. 



Even before complete closure of the anterior end of the neural tube, which 

 takes place probably shortly after tin- end of the second week of foetal life, the 

 cephalic region of this tube, slightly flattened from side to side, exhibits the results 



