two or three strata of nuclei. The basilar processes of the cells are very long, may be branched, and, as a rule, describe a tortuous

course.

The shape and structure of the neurogliar cells (spider-cells) vary somewhat in different parts of the central nervous system. From the bodies of these cells numerous delicate processes are sent out, which in the one variety of cell-that occurring principally in the white matter-do not branch. Similar cells, with shorter but occasionally dividing processes, are situated for the most part in the gray matter. Other neurogliar cells may be distinguished from the varieties just described by the smaller number of their processes and by their correspondingly larger bodies.

A large proportion of the fine fibers found in the gray and the white matter are processes of the neurogliar cells. Whether the spinal cord contains other similar cellular and fibrillar elements of mesodermic origin is an unsettled question. There seems to be no doubt, however, that connective tissue (other than that composing the pial processes) always accompanies the numerous blood-vessels penetrating into the spinal cord.

The majority of investigators have described the various fibers and fibrils brought to view by certain methods as processes of the neurogliar cells. Weigert (95) and Mallory have demonstrated, by means of special methods, the existence of neuroglia-fibers in the adult human brain which are nowhere connected with cellular elements, although they frequently group themselves around a cell as an axis, and thus simulate with the latter the "spider-cells" of some authors. The neurogliar elements of the embryo and fetus have as yet never been demonstrated by Weigert's method, but have, as a rule, been studied by means of Golgi's method. Reinke (97) has very recently found in the white matter of the adult human spinal cord neurogliar cells with processes and neuroglia-fibers having no connection with cells.

I. THE MEMBRANES OF THE CENTRAL
NERVOUS SYSTEM.

The membranes of the central nervous system (meninges) are three in number: the outer, or dura mater; the middle, or arachnoid; and the inner, or pia mater.

Around the brain the dura mater is very intimately connected. with the periosteum and presents a smooth inner surface. It consists of an inner and an outer layer, the two being separated from each other only in certain regions. At such points either the inner layer is pushed inward to form a duplicature, as in the falx cerebri and falx cerebelli, tentorium, and diaphragma sellæ, or the outer layer is pushed outward to form small, blindly ending sacs. The venous and lymphatic sinuses lie between the two layers. The outer

layer of the dura is continued some distance along the cerebrospinal

nerves.

The dura mater of the spinal cord does not form the periosteum for the bones forming the vertebral canal; these possess their own periosteum. The spinal dura mater is covered on its outer surface by a layer of endothelial cells and is separated from the wall of the vertebral canal by the epidural space, containing a venous plexus imbedded in loose areolar connective tissue and adipose tissue.

The dura consists chiefly of connective-tissue bundles having a longitudinal direction along the spinal cord. Within the cranium, however, the bundles of the inner and outer layers cross each other; those of the outer having a lateral direction anteriorly and a mesial posteriorly; those of the inner, a mesial direction anteriorly and a lateral posteriorly. In the falx cerebri, tentorium, etc., the fibers are arranged radially, extending from their origin toward their borders. The shape and size of the connective-tissue cells vary greatly, and their processes form a network around the bundles of connective tissue. Few elastic fibers are present, and, according to K. Schultz, these are entirely absent in the new-born; they are somewhat more numerous in the dura of the spinal cord. The dura is very rich in blood-capillaries, and the presence of lymphatic channels in communication with the subdural space may be demonstrated by means of puncture-injections. The inner surface of the dura mater is covered by a layer of endothelial cells.

The dura mater is quite richly supplied with nerves, especially in certain regions. These are of two varieties : (1) Vasomotor fibers, which form plexuses in the adventitial coat of the arteries, and would seem to terminate in the muscular coat of the arteries; (2) medullated nerve-fibers, which either accompany the blood-vessels in the form of larger or smaller bundles or have a course independent of the vessels. After repeated division these medullated nerve-fibers lose their medullary sheaths and terminate between the connective-tissue bundles in fine varicose fibrils, which may often. be traced for long distances (Huber, 99).

The arachnoid is separated from the dura by a space which is regarded as belonging to the lymphatic system-the subdural space. The outer boundary of the arachnoid consists, as does the inner lining of the dura, of a layer of flattened endothelial cells. The arachnoid is made up of a feltwork of loosely arranged connective-tissue trabecula, which also penetrate into the lymph-space between it and the pia-the subarachnoid space. For a short distance from their points of origin the cerebrospinal nerves are accompanied by arachnoid tissue. In the brain the arachnoid covers the convolutions and penetrates with its processes into the sulci. These processes are especially well developed in the so-called cisterns; in the cisterna cerebellomedullaris, fossæ Sylvii, etc. In the spinal cord the subarachnoid space is separated by the ligamenta denticulata into two large communicating spaces—a dorsal and a ven

tral. The dorsal space is further divided by the septum posticum, best developed in the cervical region.

At certain points, usually along the superior longitudinal sinus, the outer surface of the arachnoid is raised into villi, which are covered by the inner layer of the dura, and form with the latter the Pacchionian bodies or granulations. These villi are connected with the arachnoid by pedicles so delicate that they often seem to be suspended free in the venous current of the sinus.

The subarachnoid space contains numerous blood-vessels, some of which are free and others attached to the arachnoid. Their adventitia is covered by endothelium; hence the subarachnoid space would seem to assume

here the character of a perivascular space.

The trabeculæ and membranes composing the arachnoid tissue show a great similarity to those of the mesentery, and especially to those of the omentum. The whole constitutes a typical areolar connective tissue, interrupted at numerous points and covered by a continuous layer of endothelial cells. Large numbers of spiral fibers are found here twining around single or groups of connective-tissue fibers.

The pia mater covers the entire surface of the brain and spinal cord, dipping down into every fissure and crevice. In the spinal cord it con

sists of an outer and an inner lamella, the former being composed of bundles of connective tissue containing elastic fibers. As a rule, the course of the fibers is longitudinal. Externally this layer is covered by a layer of endothelium. The bloodvessels lie between the outer and inner layers of the pia. The inner layer (pia intima) is made up of much finer elements, and is covered on both sides by endothelium. It is this layer which accompanies the blood-vessels penetrating into the spinal cord, surrounding their adventitia and forming with the latter the limits of their perivascular spaces. These are in communication with the

interpial spaces, and, by means of the adventitia of the blood-vessels, with the subarachnoid space. Aside from those just described, numerous fine, nonvascular, connective-tissue septa penetrate from the pia mater into the substance of the spinal cord. Wherever the pia mater penetrates the spinal cord, the latter is hollowed out, forming the so-called pial funnels.

The pia does not everywhere lie in direct contact with the surface of the spinal cord; for between the cord and the pia there is generally found a neurogliar covering, formed by the expanded ends of the radial processes from the neurogliar cells (glia covering or subpia). The posterior longitudinal septum of the spinal cord consists (in the thoracic region) exclusively of neurogliar elements, but in the cervical and lumbar regions the pia also enters into its peripheral formation.

In the brain, however, the conditions are somewhat different. Here the external layer of the pia disappears, leaving only a single layer analogous to the pia intima of the spinal cord.

The pia mater enters into the formation of the choroid plexus. This structure consists of numerous freely anastomosing bloodvessels, which form villus-like processes, the surfaces of which are covered by squamous or cubic epithelial cells. This epithelium is regarded as a continuation of the ventricular epithelium, and is ciliated, at least in embryonic life and in the lower classes of vertebrates. From an embryologic point of view the whole structure represents the brain-wall reduced to a single layer of epithelium (internal epithelial investment) pushed forward into the ventricle by the vessels and pia mater.

Since the dura and arachnoid accompany the cerebrospinal nerves for some distance, it is obvious that the lymph-vessels of the nasal mucous membrane (see these) may also be injected from the subarachnoid space (compare also Key and Retzius).

The pia mater, like the dura mater, receives two varieties of nerve-fibers (1) Vasomotor fibers, which form plexuses in the adventitial coat of the arteries and terminate in the muscular layer of the arteries. These may be traced to the small precapillary branches of the vessels. (2) Larger and smaller bundles of relatively large, medullated nerve-fibers, which accompany the larger pial vessels, forming loose plexuses in or on the adventitial coat of the vessels. After repeated divisions these medullated nerves lose their medullary sheaths and terminate, in the adventitia of the vessels, in long, varicose fibrils or in groups of such fibrils (Huber, 99).

J. BLOOD-VESSELS OF THE CENTRAL NERVOUS

SYSTEM.

The blood-vessels of the central nervous system present certain peculiarities, which deserve special consideration.

In the spinal cord the arteries, surrounded by pial tissue (connective-tissue septa), extend as far as the gray matter, but give off numerous lateral branches during their course through the white matter. The capillaries form a much closer meshwork in the gray matter than in the white.

The perivascular spaces throughout the central nervous system are separated from the substance of the brain and spinal cord by an endothelial membrane, the internal endothelial layer of the pia intima (Key and Retzius), and are easily injected from the pia.

In the cerebral cortex the capillaries are particularly numerous, and are closely meshed wherever groups of ganglion cells occur. In the medullary substance they are somewhat less closely arranged, their meshes being oblong.

In the cerebellum the arrangement is analogous. Of all the layers composing the cerebellum the granular is the most vascular; within it the capillaries are also densely arranged, and form a close network.

TECHNIC.

300. The organs of the central nervous system are best fixed in Müller's fluid (vid. T. 27), washed with water, cut in celloidin, and stained with carmin. Such preparations are suitable for general topographic work.

301. Special structures-as, for instance, the medullary sheaths of the nerve-fibers, the ganglion cells, the relations of the different neurones and dendrites to one another, etc.-require different treatment.

302. The medullary sheath may be demonstrated as follows (Weigert): Pieces of tissue (spinal cord, for instance), fixed as usual in Müller's or Erlicki's fluid (vid. T. 27 and 29), are transferred without washing to alcohol, imbedded in celloidin, and cut. Before staining the sections it is necessary to subject them to the mordant action of a neutral copper acetate solution (a saturated solution of the salt diluted with an equal volume of water). The sections may be subjected to the mordant action of this solution, but the following procedure is more convenient: The specimens, imbedded in celloidin and fastened to a cork or a block of wood, are placed for one or two days in the copper solution just described. At the expiration of this time the pieces of tissue will have become dark, and the surrounding celloidin light green. They are then placed in 80% alcohol, in which they may be preserved for any length of time. The sections are then stained in the following solution: 1 gm. of hematoxylin is dissolved in 10 c.c. absolute alcohol, and 90 c.c. of distilled water are then added (the fluid must remain exposed to the air for a few days); the addition of an alkali-as, for instance, a cold saturated solution of lithium bicarbonate (I c.c. to 100 c.c. of hematoxylin solution)-brings out the staining power of the solution at once.

In