THE MENINGES : 761 
impulses in a peripheral or central direction.1 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 falx cerebri-s'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. 
