THE MENINGES 761 



impulses in a peripheral or central direction, i On the same basis the nerve-fibers 

 are designated efferent and afferent respectively. A bundle of fibers which passes 

 from one nerve-trunk to another is called an anastomotic branch (Ramus anasto- 

 moticus). 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 spinal nerves with adjacent ganglia or nerves of the sympathetic system. 

 The terminal twigs of the nerves are designated, according to their distribution, as 

 muscular branches (Rami 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 enclosed in three meninges or 

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

 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 difference 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 be 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 parietal 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 endo- 

 thelium and is moistened by a fluid resembling lymph; it forms the outer boundary 

 of the subdural space. In accordance with its double 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 described with the other vessels. Two folds or 

 septa given off from the inner surface of the dura project into the cranial cavity 

 between the gross subdivisions of the brain. These are: (1) the falx cerebri; (2) 

 the tentorium cerebelli. 



The fabc cerebri is a sickle-shaped median partition which is situated in the 

 longitudinal fissure between the cerebral hemispheres. It is attached dorsally 

 to the internal parietal crest, in front to the crista galli, and behind to the. 

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



1 These cell-groups are the real origin or termination of a nerve. The term "apparent 

 origin " is a convenient designation for the point at which a nerve is connected with the surface 

 of the brain. 



