922 



THE NERVOUS SYSTEM 



The cranial pia mater is closely applied to the external surface of the brain, 

 clipping into all the fissures, furrows, and sulci. It is connected with the arach- 

 noid by numerous filaments of the spongy subarachnoid tissue and by the blood- 

 vessels 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 Older surface bounds the subarachnoid cavity. It is with difficulty separable into two 

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

 nective-tissue cells enmeshed between them. Its inner surface sends a large number of fibrous 

 processes into the nervous substance, which blend with the neuroglia and aid in the support 

 of the nervous elements. The larger of these processes accompany 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 surfaces, 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 transverse 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 chorioid telce of these ventricles respectively. 



Fig. 727. — Diagram showing Chorioid Tela of Fourth Ventricle after Removal of 



Cerebellum. 

 (The trochlear nerve should be shown emerging from the frenulum veli.) 



Inferior quadrigeminate body 

 Trochlear nerve 



Superior medullary velum. 

 Brachium conjunctivum 



Brachium of pons 



Restiform body 



Ligula (taenia) 

 Chorioid tela of 

 fourth ventricle 

 Cuneate tubercle 

 Clava 

 Tubercle of Rolando 



Frenulum veil 

 Lateral lemniscus 



Lingula of cerebellum 



Fourth ventricle 



Vessels to chorioid plexus 



Inferior medullary velum 

 Chorioid plexus 



Foramen of Magendie 

 Obex 



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

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

 dorsal surface of the medulla (fourth ventricle). The two layers of this fold of 

 the pia remain separate and a portion of the cisterna posterior of the subarachnoid 

 cavity lies between them. The inferior of the layers is the tela chorioidea 

 (fig. 727.) It is triangular in shape, with its })ase cephalad at the nodule of the 

 vermis and its apex below at the level of the tuber vermis. The superior layer of 

 the fold is the pia mater of the vermis. The tela chorioidea is strengthened by the 

 epithelial roof (ependyma) of the fourth ventricle and is continuous with the pia 

 mater of the medulla ol)longata and spinal cord. In roofing over the fourth 

 ventricle the tela chorioidea of the fourth ventricle constitutes the ligula and 

 the obex. A little above the calamus scriptorius it is pierced by the foramen of 

 Magendie and the two lateral apertures into the fourth ventricle. 



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

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

 loVjulated 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 



