914 



THE NERVOUS SYSTEM 



smooth and glistening, being lined by a layer of endothelium and containing a 

 small amount of the cerebro-spinal lymph. 



The subdural cavity of the base of the brain is prolonged a short distance outward along 

 the roots of the various cranial nerves before it is obliterated by the blending of the dura 

 mater with the sheaths of the nerves. This outward extension of the space is most marked 

 about the optic and auditory nerves. In the optic especially, the dura mater remains separate 

 from the nerve throughout its length, only fusing with its sheath upon the posterior surface of 

 the ocular bulb (fig. 718). 



One of the most striking differences between the cranial dura mater and that of 

 the spinal cord is that the inner layer of the former undergoes striking septa-like 

 duplications or folds, forming exceedingly strong partitions which project between 

 the larger subdivisions of the encephalon. These are four in number, two large 

 and two small — the falx cerebri and the tentorium cerebelli; the falx cerebelli 

 and the diaphragma sellse. The larger enclose within their folds the great venous 

 sinuses, into which most of the spent blood of the encephalon collects to pass out- 

 ward b}^ way of the internal jugular veins (figs. 720, 721). 



Fig. 720. — The Crajstium with Encephalon Removed to show the Falx Cerebri, the 

 Tentorium Cerebelli, and the Places where the Cranial Nerves pierce the Dura 

 Mater. (Sappey.) 



Trochlear nerve Oculomotor nerve 



Falx cerebri 



Optic nerve 



Superior sagit- 

 tal sinus 



Inferior sagit- 

 tal sinus 



Vein of Galen 



Straight sinus 



Internal 

 carotid artery 



Superior 

 petrosal sinus 



Falx cerebelli 



Facial and 

 auditory nerves 

 Glassopharyngeal, vagus 

 and accessory nerves 



Hypoglossal nerve 



Vertebral 

 Artery 



First Inferior Abducens Trigeminus 

 Second cervical nerve cervical petrosal nerve nerve 



Ligamentum denticulatum nerve sinus 



The falx cerebri is the most striking of these partitions. It is a sickle-shaped 

 fold which projects vertically from the vault into the longitudinal fissure between 

 the cerebral hemispheres. It begins attached to the crista galli in front, and 

 arches to terminate by blending with the superior surface of the hirozontally 

 placed tentorium cerebelli. Its convex, superior border joins the outer layer of the 

 dura along the medial plane of the vault, and encloses the superior sagittal sinus. 

 Its concave border is free and contains in its posterior two-thirds the smaller 

 inferior sagittal sinus. The anterior and narrower end is often perforated and 

 occasionally so much so as to appear as a coarse, fibrous reticulum. The pos- 

 terior part of the concave border touches the upper surface of the corpus callosum, 

 but the anterior part, which does not descend so low, is separated from the corpus 

 callosum by a part of the subarachnoid space. The base of the fold which slopes 

 downward and blends with the upper surface of the tentorium cerebelli, contains 

 the straight winus running along the line of junction. 



The tentorium cerebelli is a large transverse, semilunar fold, concave forward. 

 It descends from its central part which is elevated, and consequently it forms a 



