908 THE NERVOUS SYSTEM 



The efferent veins of the cerebellum do not accompany the arteries; they spring from a 

 plexus in the pia mater which receives tributaries from the interior, and they form three groups 

 on each cerebellar surface, the vermian veins and the lateral veins. The superior vermian vein 

 runs forward on the superior surface of the vermis and terminates in the vein of Galen. The 

 inferior vermian vein runs posteriorly and ends in one of the transverse sinuses. The superior 

 lateral veins open into the superior petrosal or transverse sinuses, and the inferior lateral veins 

 into the inferior petrosal and transverse sinuses. The vein from the dentate nucleus usually 

 joins the inferior lateral veins. 



The blood-vessels of the pons. — The arteries to the pons are branches of the basilar artery, 

 and of its anterior inferior and superior cerebellar branches. The plexus in the pia mater 

 is comparatively unimportant, and the branches which enter the substance of the pons form 

 two main groups, the central and the peripheral. The central arteries spring directly from the 

 basilar. They pass backward along the raphe, giving branches to the adjacent parts, and they 

 terminate in the nuclei of the pons and those in the floor of the fourth ventricle. The peripheral 

 arteries are radicular and intermediate. The radicular branches spring from the peripheral 

 plexus and from the anterior inferior cerebellar arteries; they accompany the roots of the 

 trigeminus, abducens, facial, vestibular, and cochlear nerves, supply their fibres and the adjacent 

 parts, and they end in the grey nuclei with which the nerve-fibres are connected. The inter- 

 mediate arteries enter the surfaces of the pons irregularly and break up into capillaries in its 

 substance. The veins form a plexus on the surface. The dorsal and lateral part of this plexu- 

 is drained into the basilar vein on each side, and the inferior part is connected by efferent 

 channels with the inferior petrosal sinus and the cerebellar veins. 



The blood-vessels of the medulla oblongata. — The arteries of the medulla are derived 

 directly from the vertebral arteries, from their anterior and posterior spinal and posterior inferior 

 cerebellar branches, and from the basilar artery. The branches of these vessels form a plexus in 

 the pia mater from which, and from the arteries themselves, three main groups of vessels pass 

 into the medulla — the chorioidal, the central, and the peripheral. The chorioidal arteries are 

 derived claiefly from the posterior inferior cerebellar arteries. They supply the chorioid plexus 

 of the fourth ventricle. The anterior central arteries rise from the anterior spinal artery, from 

 the basilar artery, and from the peripheral plexus; they pass caudalward along the raphe, 

 supplying the adjacent parts of the ventral funiculi and the olivary bodies, and they break up 

 into fine terminals in the grey substance of the floor of the fourth ventricle around the nuclei 

 of the cranial nerves. The posterior central arteries spring from the posterior spinal arteries; 

 they pass down the median septum of the inferior part of the medulla and supply the adjacent 

 nervous substance. The peripheral arteries, like those of the spinal cord, are separable into 

 radicular and intermediate groups. The radicular arteries pass from the anterior and posterior 

 spinal branches and from the trunks of the vertebral arteries and accompany the fibres of the 

 last six cranial nerves into the substance of the medulla. They supply the nerve-roots and 

 adjacent white substance and they terminate in capillaries in the grey substance of the lateral 

 part of the floor of the ventricle. The intermediate peripheral arteries spring from the arteries 

 previously named and from the peripheral plexus, and they pass directly into the funiculi of 

 the medulla, where they terminate in a capillary plexus which supplies the white substance and 

 the grey nuclei; some of these arteries, more especially those derived from the posterior inferior 

 cerebellar and the posterior spinal arteries, extend inward to the lateral part of the floor of the 

 fourth ventricle. 



The veins which issue from the medulla form a peripheral plexus in the pia mater in which 

 there are two main longitudinal channels, an anterior median and a posterior median vein. 

 The former communicates posteriorly with the anterior median vein of the cord, and anteriorly 

 with the veins of the pons and with the veins which accompany the hypoglossal nerves. The 

 latter veins empty into the internal jugular veins. The posterior median vein is continuous 

 caudally with the corresponding vein of the cord, and anteriorly, in the region of the calamus 

 scriptorius, it divides into branches which join the radicular veins. The blood is carried 

 away from the peripheral plexus mainly by the radicular veins, which pass along the roots of 

 the last six cranial nerves. Those which accompany the hypoglossal nerves have aheady 

 been referred to. The others end in the terminal parts of the transverse sinuses, the inferior 

 petrosal sinuses, or the inferior part of the occipital sinuses. 



The nerve supply of the blood-vessels of the brain consists of a perivascular plexus of sympa- 

 thetic nerve-fibres upon the walls of the vessels and medullated fibres which accompany the 

 vessels and apparently terminate, for the most part, in the connective tissue about them. The 

 former are thought to be vaso-motor in function; the latter probably sensory fibres of the 

 cranio-spinal type. Nerves have been described only for the larger vessels. 



IV. THE MENINGES 



Three membranes, collectively called the meninges, envelope the entire cen- 

 tral nervous system, separate it from the walls of the bony cavities in which it lies, 

 and aid in its protection and support. They consist of feltworks in which white 

 fibrous connective tissue predominates, and through them pass the blood-vessels 

 which supply the central nerve-axis and the nerves by which the axis is connected 

 with the periphery. Though there are definite spaces or cavities between them, 

 the membranes are not wholly separated from each other, and they are both 

 continuous with and contribute to the walls of the blood-vessels and the sheaths 

 (epineurium) of the nerves passing through them. Beginning with the outermost, 

 they arc — (1) the dura mater, the thickest, most dense and resistant of the mem- 



