670 



THE XERVOVS SYSTEM 



vesicles, an anterior, a middle, and a posterior. The ventricles of the brain are 

 afterwards develo})ed from these three cerebral vesicles, and the remainder of the 

 neural tube forms the central canal of the spinal cord. The substance of the brain 

 and cord is formed by thickenings of the wall of this neural tube. The terminal 

 or anterior cerebral vesicle liecomes divided into an anterior and a posterior division; 

 the latter ]:)ersistsas tlie third ventricle, which, with the parts around it. constitutes 

 the thalamencephalon. The anterior division is afterwards differentiated into the 

 cerebral hemispheres or prosencephalon. It becomes at first indented in the 

 mitldle line, and then eonjpletely Inlobed. In this way two symmetrical cavities, 

 the right and left lateral ventricles, are formed. 



The anterior part of the wall of the cerebral vesicle 1)etween the tAvo lienii- 

 spheres, thus mapped out, is called the lamina terminalis. The lamina terminalis 

 is carried backwards so as to l)ound the third ventricle in front, and the aperture of 

 communication (foramen commune anterius), connecting the third Avith the 

 lateral ventricles, Vu^comes narrowed. The cavity of the middle cerebral vesicle 

 becomes the aqueduct of Sylvius, and the parts developed around it constitute 

 the mid-brain or mesencephalon. The cavity of the posterior cerebral vesicle 

 l)ecomes the fourth ventricle, and its walls are differentiated into the cerebellum 

 and pons Varolii (epencephalon), and into the medulla oblongata (meten- 

 cephalon). 



The walls of the lateral ventricles become greatly thickened and form the 

 cerebral hemispheres, constituting the greater part of the mass of the brain. They 



Fig. 402.— Diagrammatic Horizontal Section of a Vertebrate Brain. (After Huxley.) 



EPESCEPHALOX OPTIC THALAMUS 



LATERAL 



VENTRICLE 

 RHINEN- 



CEPHALOS 

 LA MIS A 



TERMINALIS 



MEDULLA 

 OBLONG A TA 



CEREBELLUM 



CORPUS 

 STRIA TUM 



MESENCEPHA L ON PINE A L 

 BODY 



FORAMEN OF MONRO 



grow backwards from the position represented in the diagram (fig. 401), and cover 

 the mid- and hind-brain. In front of the lamina terminalis the walls of the lateral 

 ventricles remain thin, and approximate so as to enclose a narrow space, the fifth 

 ventricle. Thus the fifth ventricle differs entirely in its nature from the other 

 ventricles of the brain: it is a separated portion of the interhemispheric or great 

 longitudinal fissure, and has nothing to do with the true ventricular cavities; accord- 

 ing to Testut it is lined l)y a rudimentary layer of jna mater. Later on, a great 

 transverse commissure (corpus callosum), which i)asses from one hemisphere to the 

 other, is developed. It commences in front of the fifth ventricle in the secondarily 

 fused portions of the cerebral walls, and afterwards extends backwards as the 

 hemispheres grow over the mesencephalon. 



The oldest i>ortion of the cerebral hemisphere, l)oth in the scries of vertebrates 

 and in the development of the individual, is the island of Reil with the grey 

 masses which lie sul)jacent to it (nucleus caudatus and nucleus lenticularis of the 

 corpus striatum). These structures constitutes a central portion (Stammtheil of 

 Scliwall)e) which is almost completely invested l)y the remaining larger ])art of the 

 hemisphere. The latter forms th(^ less massive i)art of the wall of the lateral 

 ventricle, and may lie called the mantle-wall (Manteltheil of Schwalbe). 



The ventricular cavities of the brain and the central canal of the spinal cord 

 are lined by a layer of epithelial cells, immediately outside which is a stratum 

 of neuroglia, free from nerve-cells, which is called ependyma. The ependyma 

 around the central canal of the cord is often termed substantia gelatinosa centralis. 



