THE BRAIN. 



of the next division, the pons Varolii. Beyond the upper margin of the pons the 

 brain-stem is represented by a third division that ventrally is separated by a (hep 

 recess into two diverging limbs, the cerebral peduncles, or critra cercbri, to corre- 

 spond with the halves or hemispheres of the cerebrum, each of which receives one 

 of the crura and in this manner is connected with the lower levels of the cerebro- 

 spinal axis. The greater part of the medulla and pons is covered dorsully by the 

 cerebellum, whose large lateral expansions, or hemispheres, project on either side 

 as conspicuous masses, distinguished by the closely set plications and intervening 

 fissures that mark their surface. Of the five component parts of the brain medulla, 

 pons, cerebral peduncles, cerebrum, and cerebellum the last two are coated with 

 the cortical gray matter, in which, broadly speaking, are situated the neurones 

 that constitute the end-stations for the sensory impulses conveyed by the various 

 corticipetal paths and the centres controlling the lower-lying nuclei of the motor 

 nerves. .The brain-stem, on the other hand, whilst containing numerous stations 

 for the reception and distribution of sensory impulses, is primarily the great pathway 

 by which the cerebrum and the cerebellum are connected with each other and with 

 the spinal cord. 



Viewed in a mesial sagittal section (Fig. 910), each of these divisions is seen to 

 be related to some part of the system of communicating spaces that, as the lateral 

 and third ventricles, the aqueduct of Syhmis and the fourth ventricle, extend from 

 the cerebral hemispheres above, through the brain-stem and beneath the cerebellum, 

 to the central canal of the spinal cord below. Since the lateral ventricles are two in 

 number, in correspondence with the cerebral hemispheres in which they lie, their 

 position is lateral to the mid-plane and hence only one of the openings, fat foramina 

 of Monro, by which they communicate with the unpaired and mesially placed third 

 ventricle, is seen in sagittal sections. 



Both the roof and the floor of the irregular third ventricle are thin, whilst its 

 lateral walls are formed by two robust masses, the optic thalami, the mesial surface 



Corpus callosu 

 Septum lucidum 



FIG. 910. 



Frontal lobe, mesial surfa 



Anterior commissure 



Foramen of Monro 



Lamina cinerea 



Optic commissure 



Floor of third ventricle 



Mammillary body 



Aqueduct of Sylvius 



Pons 



Fo 



Optic thalamus, dorsal surface 



Lateral wall of third 

 ventricle (optic thalamus) 



Cerebral peduncle 



Roof of Sylvian aqueduct 



Occipital lobe 

 -Superior medullary velum 



White core of cerebellum 

 Inferior medullary velum 



Spinal cord 



Simplified drawing of brain as seen in mesial section, showing relation of brain-stem, cerebrum and cerebellum, 



and ventricular spaces. 



of one of which forms the background of the space when viewed in sagittal section. 

 The roof vi the ventricle is very thin and consists of the delicate layer of cpcndvwa, 

 as the immediate lining of the ventricular spaces is designated, supported by the 

 closely adherent fold of pia mater which in this situation pushes before it the neural 

 wall and contains within its lateral border a thickened fringe of blood-vessels, the 



6? 



