THE MENINGES THE DURA MATER 645 



respectively. A Imndlc of fibers which passes from one nerve-trunk to another is 

 called an anastomotic branch (Ramus anastomoticus). In some situations the 

 exchange of branches between adjacent nerves is so free as to constitute a nerve- 

 plexus (Plexus nervorum). The term ramus communicans is properly restricted 

 to branches which connect the ventral divisions of the s])inal nerves with adjacent 

 ganglia or nerves of the sympathetic system. The terminal twigs of the nerves are 

 designated according to their tlistribution as muscular branches (Kami musculares), 

 cutaneous nerves (Nervi cutanei), and articular nerves (Nervi articulares). The 

 muscular branches are motor in function, the cutaneous and articular sensory, but 

 all contain vasomotor fibers which control the caliber of the blood-vessels. 



THE MENINGES 



The central organs of the nervous system are inclosed in three meninges or 

 membranes. From without inward these are: (1) the dura mater, (2) the aracli- 

 noidea, and (3) the pia mater. 



THE DURA MATER 



The dura mater is a dense resistant membrane of white fibrous tissue; in the 

 fresh state it is bluish-white in color. On account of the difTerence in its arrange- 

 ment within the cranium from that in the spinal canal it is customary to describe 

 it as consisting of two parts, cerebral and spinal; these portions are continuous with 

 each other at the foramen magnum. 



The cerebral dura mater (Dura mater encephali) is adherent to the interior 

 of the cranium, and may l^e regarded as forming an internal periosteum for the 

 bones here as well as being an envelope of the brain. Its outer surface is connected 

 with the bony wall of the cranial cavity by numerous fine fibrous strands and by 

 blood-vessels; hence it appears rough in many places when separated from the wall. 

 The degree of adhesion varies greatly at different points. It is most firmly attached 

 at the various projections, e. g., the internal sagittal crest, the tentorium osseum, 

 the petrosal crest; also at the base and the foramen magnum. Before the sutures 

 are closed the dura is connected with the sutural ligaments and through them is 

 continuous with the external periosteum. The cranial nerves receive sheaths from 

 the dura, which is thus continuous without the cranium with the epineurium and 

 periosteum. Along the roof and sides (except as noted above) the adhesion is 

 relatively slight, and lymph-spaces are said to exist between the dura and the bone. 

 The internal surface of the dura is smooth and glistening, since it is lined by 

 endothelium and is moistened by a fluid reseml^ling lymph; it forms the outer 

 boundary of the subdural space. In accordance with its doul)le function the dura 

 is composed of two layers, which are, however, intimately adherent to each other 

 in most places in the adult. The venous sinuses are channels between the two 

 layers and are lined by endothelium. They have been descril^ed with the other 

 vessels. 



Several folds or septa are given off from the inner surface of the dura; these 

 project into the cavity betAveen the gross subdivisions of the brain. These septa 

 are: (1) the falx cerebri; (2) the tentorium cerebelli; and (3) the diaphragma sellae. 



The falx cerebri is a sickle-shaped median partition which projects downward 

 in the great longitudinal fissure between the cerebral hemispheres. It is attached 

 above to the internal sagittal crest, in front to the crista galli, and behind to the 

 tentorium osseum. Its superior border is convex and separates into two layers 

 which inclose the superior longitudinal sinus. Its lower border is concave and 

 lies over the corpus callosum. The falx is thick above, but much thinner below, 

 and in some places cribriform. 



