THE CRANIAL NERVES 975 



The pia of the surfaces of the hemispheres, where it covers the gray substance 

 of the convolutions, is very vascular, and gives off from its inner surface a multi- 

 tude of minute vessels, which extend perpendicularly for some distance into the 

 cerebral substance. At the base of the brain, in the situation of the anterior and 

 posterior perforated substance, a number of long, straight vessels are given off, 

 which pass through the white substance to reach the gray substance in the interior. 

 On the cerebellum the membrane is more delicate, and the vessels from its inner 

 surface are shorter. The pia of the spinal cord is thicker, firmer, and less vascular 

 than that of the brain, and as it is traced upward over the medulla oblongata 

 it is seen to preserve these characters. At the upper border of the medulla ob- 

 longata it is prolonged over the lower half of the fourth ventricle, forming, before 

 it is reflected onto the under surface of the cerebellum, a covering for the fourth 

 ventricle called the tela chorioidea inferior (metatela; tela chorioidea ventriculi quartz); 

 this carries the choroid plexus of the fourth ventricle (plexus chorioideus ventriculi 

 quarti). 



The arteries of the pia (see pp. 617, 619, and 623) (Figs. 724 and 725) are the anterior, middle, 

 and posterior cerebral, anterior choroid and choroid, superior, anterior inferior, and posterior 

 inferior cerebellar. (The vessels of the cerebral ganglionic system and of the cortical arterial 

 system are considered on p. 620). 



The, veins of the pia (see pp. 721, 722, and 723) are the basilar vein, the velar veins (Fig. 

 723), the veins constituting the choroid plexuses of the third ventricle, the lateral ventricles, 

 and the fourth ventricle; the cerebral veins (Fig. 721) and the cerebellar veins (Fig. 721). 



The nerves of the pia accompany the branches of the arteries and are derived chiefly from 

 the sympathetic. A few fibres are derived from certain cranial nerves, all of which are prob- 

 ably of the afferent variety. 



THE CRANIAL NERVES (NERVI CEREBRALES). 



The irregularities of origin and distribution of the cranial nerves, as compared 

 with the relatively simple spinal nerves, are so great and their functions were for- 

 merly so little known that the older anatomists contented themselves with number- 

 ing them in order, beginning at the frontal end of the brain, and named them 

 with reference to their anatomic connections. The enumeration of the cranial 

 nerves was as variously given, almost, as there were writers upon the subject; 

 the systems of Willis and of Sommering were most in vogue for a time, but the 

 latter prevails today. In Sommering's arrangement twelve pairs of cranial nerves 

 are recognized, but, on morphological and functional grounds, the nerves of 

 the seventh and eighth pairs should each be considered as being composed of two 

 nerves, the eleventh pair should be included with the nerves of the tenth, and the 

 nerve" is rather a diverticulum of the brain itself than a nerve in the strict 



sense. Furthermore, while some of the nerves are sensor or motor in a sense 

 strictly comparable with the spinal nerves, others are mixed in function, and 

 yet others constitute nerves of special sense and lack general sensibility. The 

 numerical names, based upon the order in which they pass through the foramina 

 in the base of the cranium, are being abandoned gradually for the more appro- 

 priate functional names, but not yet entirely so. Thus glossopharyngeal has 

 not yet given way to "gustatory;" "vagus" is shorter than " pneumoqastric" 

 a term which is misleading for a nerve which is distributed not only to lungs 

 and stomach, but also to the meninges, the pharynx and oesophagus, larynx and 

 trachea, heart and pericardium, liver and spleen. 



The cranial nerves, as usually enumerated, together with their superficial 

 "origin" or attachment to the brain and their foramina of exit from the skull, are 

 tabulated on page 852. The central olfactory pathway is described on page 959, 

 and the central connections of the optic tracts are given on page 912. The central 

 connections of the remaining cranial nerves are described on pages 879 to 903. 



