896 



THE NERVOUS SYSTEM 



traversing the subarachnoid cavity. It is also pierced by the cranial nerves, and 

 furnishes them their sheaths, which become continuous with the arachnoid and 

 dura mater. Its outer surface, bounding the subarachnoid cavity, is covered by a 

 layer of endothelium. The fibrous portion below is with difficulty separable into two 

 layers of mixed white fibrous and elastic connective tissue, with slightly pigmented 

 connective-tissue cells enmeshed between them. Its inner surface sends a large num- 

 ber of fibrous processes into the nervous substance, which blend with the neuroglia 

 and aid in the support ofthe nervous elements. The larger of these processes ac- 

 company the central arterial and venous branches of the rich superficial plexuses of 

 blood-vessels contained in the pia on the surface of the brain. Pieces of the pia when 

 pulled off and placed in water present a flocculent appearance as to their inner sur- 

 faces, due to these processes having been pulled out. 



The cranial pia mater sends strong, vascular duplications into two of the great 

 fissures of the encephalon; viz., the transverse cerebellar fissure, between the cere- 

 bellum and the medulla oblongata, and the transrtrse cerebral fissure, between the 

 cerebellum, mesencephalon, and thalamencephalon, and the overhanging cerebral 

 hemispheres. These duplications are spread over the cavities of the fourth and third 

 ventricles, and are known as the telce chorioidea? of these ventricles respectively. 



FIG. 667. DIAGRAM SHOWING TELA CHORIOIDEA OF FOURTH VENTRICLE AFTER REMOVAL OE 



CEREBELLUM. 



INFERIOR 

 Q UA DRIGEMINA TE BODY 



FOURTH NERVE 



SUPERIOR MED UL LARY 

 VELUM 



BRACIIIUM CONJUNCTIVUM 



BRA CHIUM PONTIS 



RESTIFORM BODY 



TELA CHORIOIDEA 



LIGULA 



TUBERCUL UM CUNEA TUM 



CLA VA 



LATERAL FUNICULVS 



i- 1: i:\ri.f.v VELI 



LATERAL L KM X ISC US 

 LING ULA CKREBELLI 



FOVRTII VENTRICLE 



INFERIOR MEDULLARY 

 VELUM 



CHORlfllll PI.KXUS 



FORA MEN OF MA GENDIE 



OBF.X 



The tela chorioidea of the fourth ventricle lies in the transverse cerebellar 

 fissure, between the under surface of the cerebellum (vermis chiefly) and the dorsal 

 surface of the medulla (fourth ventricle). The two layers of this fold of the pia re- 

 main separate and a portion of the cisterna posterior of the subarachnoid cavity lies 

 between them. It is a triangular fold, with its base above at the nodule of the ver- 

 mis and its apex below at the level of the tuber vermis. The upper layer adheres to 

 the vermis; the lower strengthens the epithelial roof of the fourth ventricle and is 

 continuous with the pia mater of the medulla oblongata and spinal cord. This 

 lower layer is the tela of the fourth ventricle proper. In roofing over the calamus 

 scriptorius it constitutes the ligula and the obex. A little above the calamus scrip- 

 torius the lower layer is pierced by the foramen of Magendie. 



In front of the foramen of Magendie the vessels of the tela chorioidea, which are 

 derived from the posterior inferior cerebellar arteries, form two longitudinal, lobulated , 

 strands which invaginate the epithelial roof of the ventricle, one on either side of the 

 mid-line, and project into its cavity. These form the chorioid plexus of the fourth 

 ventricle. At the base of the tela the two chorioid plexuses join each other and then 

 turn transversely outwards into the lateral recesses of the ventricle, where they pass 

 behind the restiform bodies and form the 'cornucopia.' 



