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sinus wall. On each side of the superior sagittal sinus there is a number of irregular spaces in the dura mater which communicate with the sinus either by a small aperture or a narrow channel. These spaces are called the lacunæ laterales, and certain of the meningeal veins and some of the diploic veins open into them. Granulations push themselves into the lateral lacunæ from below in such a manner that they receive a complete covering from the layer of dura mater which forms the sinus floor. Nor does the bone escape. As the granulations enlarge they cause absorption of the cranial wall, and small pits are hollowed out on its internal surface for their reception. It must be clearly understood, however, that in such cases the granulation is separated from the bone by the following:-(1) A continuation round the granulation of the subdural space; (2) the thinned floor of the lateral lacuna; (3) the lumen of the sinus; and (4) the greatly thinned upper wall of the sinus.
The granulations have a special function to perform. Through them fluid can pass from the subarachnoid space into the venous sinuses with which they stand in connexion. Whenever the pressure of blood in the sinuses is lower than that of the fluid in the subarachnoid space and the ventricles of the brain, the cerebro
Mouth of a vein
FIG. 598.-MEDIAN SECTION THROUGH THE CRANIAL VAULT IN THE FRONTAL REGION, ENLARGED. Displays a portion of the superior sagittal sinus and the arachnoideal granulations protruding into it.
spinal fluid filtrates through the granulations into the blood-sinuses. This is not the only way that subarachnoid fluid may obtain exit. The subarachnoid space is carried outwards for a short distance on the nerves in connexion with their arachnoideal sheaths, and communicates with the lymph channels of the nerves. This connexion is more complete in the case of the olfactory, the optic, and the acoustic nerves than in other nerves. A very free communication is said to exist between the subarachnoid space and the lymph-vessels of the nasal mucous membrane.
THE PIA MATER.
The pia mater forms the immediate investment of the brain and spinal medulla. It is a delicate and very vascular membrane.
Pia Mater Encephali.-The pia mater which covers the brain is finer and more delicate than that which clothes the spinal medulla. It follows closely all the inequalities on the surface of the brain, and in the case of the hemisphere it dips into each sulcus in the form of a fold which lines it completely. On the cerebellum the relation is not so intimate; it is only into the larger fissures that it penetrates in the form of folds.
The larger blood-vessels of the brain lie in the subarachnoid space. The finer twigs ramify in the pia mater before they proceed into the substance of the brain. As they enter they carry with them sheaths derived from the pia mater. When a portion of the membrane is raised from the surface of the encephalon, numerous fine processes are withdrawn from the cerebral surface. These are the blood-vessels with their sheaths, and they give the deep surface of the pia mater a rough and flocculent appearance.
As the pia mater is carried over the inferior part of the roof or posterior wall of the fourth ventricle of the brain it receives the name of the tela chorioidea
ventriculi quarti, and it is in connexion with this portion of the pia mater that the chorioid plexuses of that cavity are developed. The tela chorioidea ventriculi tertii (O.T. velum interpositum) is a fold of pia mater which is invaginated into the brain, so that it comes to lie over the third ventricle and to project, in the shape of chorioid plexuses, into the lateral ventricles. This invagination requires special notice.
The tela chorioidea ventriculi tertii (O.T. velum interpositum) is a double layer or fold of pia mater which intervenes between the body of the fornix, which lies above it, and the epithelial roof of the third ventricle and the two thalami, which lie below it. Between its two layers are blood-vessels, and some subarachnoid
trabecular tissue. In shape the chorioid tela of the third ventricle is triangular, and the narrow anterior end or apex reaches forwards as far as the interventricular foramina. The base lies under the splenium of the corpus callosum, and here the two layers of the tela separate and become continuous with the investing pia mater on the surface of the brain by passing out through a cleft called the transverse fissure.
Genu of corpus
Cavum septi pellucidi
- Septum pellucidum
Column of fornix
Chorioid tela of third
Along each margin the tela chorioidea of the third ventricle is bordered by the chorioid plexus of the central part of the lateral ventricle, which projects into the ventricular cavity from under cover of the free margin of the fornix. It should be borne in mind that the epithelial lining of the ventricle gives a complete covering to the chorioid plexus. Posteriorly the chorioid plexus is COontinuous with the similar structure in the inferior horn of the ventricle, whilst in front it narrows greatly, and becomes continuous across the median plane with the corresponding plexus of the opposite side, behind the epithelial layer which lines the interventricular foramen. From this median junction two much smaller chorioid plexuses run backwards on the under surface of the tela chorioidea, and project downwards into the third ventricle. These are the chorioid plexuses of the third ventricle.
Vena interna cerebri
Chorioid plexus of
FIG. 599.-DISSECTION TO SHOW THE CHORIOID TELA OF THE
TION TO IT.
The most conspicuous blood-vessels in the tela chorioidea are the two internal cerebral veins, which run backwards, one on each side of the median plane. In front, each is formed at the apex of the fold by the union of the vena terminalis and a large vein issuing from the chorioid plexus; behind, they unite to form the vena cerebri magna [Galeni], and this pours its blood into the anterior end of the straight sinus (Fig. 599, p. 674).
The continuous cleft in the brain through which the chorioid tela of the third ventricle and the chorioid plexuses of the inferior horns of the two lateral ventricles are introduced into the interior of the brain is sometimes called the transverse fissure. It consists of a superior intermediate part and two lateral parts. The former passes forwards between the corpus callosum and the fornix above and the roof of the third ventricle and the thalami below. It is limited on each side by the epithelial covering of the chorioid plexuses, which shuts out these structures from the cavity of the lateral ventricles. The
lateral part is the chorioidal fissure. This is continuous with the intermediate part, and has already been described in connexion with the inferior horn of the lateral ventricle (p. 636).
Pia Mater Spinalis.-The pia mater of the spinal medulla is thicker and denser than that of the brain. This is largely due to the addition of an outside fibrous layer, in which the fibres run chiefly in the longitudinal direction. The pia mater is very firmly adherent to the surface of the spinal medulla, and in front it sends a fold into the anterior-median fissure of the spinal medulla. The posterior median septum is likewise firmly attached to its deep surface. In front of the anterior-median fissure of the spinal medulla the pia mater is thickened in the form of a longitudinal glistening band, termed the linea splendens, which runs along the whole length of the spinal medulla, and blends with the filum terminale below. The blood-vessels of the spinal medulla lie between the two layers of the pia mater.
The nerves which leave both the brain and spinal medulla receive closely
Gyrus cinguli Indusium
Attachment of lamina chorioidea Thalamus (free surface)
Plexus chorioideus vent. tertii
applied sheaths from the pia mater. These blend with the connective-tissue sheaths of the nerves.
The ligamentum denticulatum is a strong fibrous band which stretches out like a wing from the pia mater on each side of the spinal medulla, so as to connect the pia mater with the dura mater. The pial or medial attachment of the ligament extends in a continuous line between the anterior and posterior nerve-roots, from the level of the foramen magnum above to the level of the first lumbar vertebra below. Its lateral margin is serrated or denticulated, and for the most part free. From twenty to twenty-two denticulations may be recognised. They occur in the intervals between the spinal nerves, and, pushing the arachnoid before them, they are attached by their pointed ends to the inner surface of the dura mater. The ligamenta denticulata partially subdivide the wide subarachnoid space in the vertebral canal into an anterior and a posterior compartment. The anterior nerve-roots traverse the anterior compartment, whilst the posterior nerveroots traverse the posterior compartment. Further, the posterior compartment is imperfectly subdivided into a right and a left half by the septum posterius.
By means of the ligamenta denticulata the spinal medulla is suspended in the middle of the tube of dura mater.
THE PERIPHERAL NERVES AND THE SYMPATHETIC
By A. MELVILLE PATERSON, M.D., F.R.C.S.
THE nervous mechanism comprised under this title is responsible for the transmission of peripheral impulses to the brain and spinal medulla, through afferent nerves, and for the distribution of central impulses to peripheral structures through efferent nerves. The peripheral nerves are at the outset divisible into two series:
FIG. 601.-SCHEME OF THE ARRANGEMENT OF THE MEMBRANES OF THE SPINAL MEDULLA AND
cerebral nerves, derived from or associated with the brain; and spinal nerves, in relation to the spinal medulla. Associated with the cerebro-spinal nerves is the sympathetic system. The animal body is naturally divided into two different areas or regions, the somatic area, forming the body wall and the associated limbs, innervated by the larger (somatic) parts of the spinal nerves; and the splanchnic