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Böhm AA. and M. Von Davidoff. (translated Huber GC.) A textbook of histology, including microscopic technic. (1910) Second Edn. W. B. Saunders Company, Philadelphia and London.

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Special Histology

VII. The Central Nervous System

IN a study of the minute anatomy of the central nervous system consideration should be given to the arrangement of the nerve-cells and nerve-fibers in the various regions, and to the mutual relations which the elements of the nervous system bear to one another. In a text-book of this scope, however, we shall be unable to enter into the consideration of these subjects in detail, but must content ourselves with a very general discussion of the structure of certain regions of the central nervous system and an account of a few typical examples illustrating the mutual relationship of the nerve-elements to one another. We shall, therefore, give a general description of the structure of the spinal cord, cerebellum, cerebrum, olfactory lobes, and ganglia. In this description we have drawn freely from the results of the researches of Golgi (94), Ramon y Cajal (93 l )> von Lenhossek (95), Kolliker (93), and van Gehuchten (96). '

A. The Spinal Cord

The spinal cord extends from the upper border of the atlas to about the lower border of the first lumbar vertebra. It has the form of a cylindric column, which at its lower end becomes quite abruptly smaller, to form the conns medullaris, and terminates in an attenuated portion the filum terminate. It presents two fusiform enlargements, known as the cervical and lumbar enlargements respectively. The spinal cord is partly divided into two symmetric halves by an anterior median fissure and by a septum of connective tissue, extending into the substance of the cord from the pia mater (one of the fibrous tissue membranes surrounding the cord), and known as the posterior median septitm. Structurally considered, the spinal cord consists of white matter (mainly medullated nerve-fibers) and gray matter (mainly nerve-cells and medullated nerve-fibers). The white and the gray matter present essentially the same general features at all levels of the spinal cord, although the relative proportion of the two substances varies somewhat at different levels. The different portions of the cord present also certain structural peculiarities.

The distribution of the gray and the white substances of the spinal cord is best seen in transverse sections.

The varying shape of the spinal cord in the several regions and the changing relations of the gray to the white substance are shown in the illustrations of cross-sections of the adult human spinal cord (see p. 407).

The gray substance is arranged in the form of two crescents, one in each half of the cord, united by a median portion extending from one half of the cord to the other, the whole presenting somewhat the form of an H. The horizontal part contains the commissures and the central canal of the spinal cord, while the vertical limbs or crescents extend to the ventral and dorsal nerve-roots, forming the anterior and posterior horns. The former are, as a rule, the larger, and at their sides (laterally) the so-called lateral horns may be seen, varying in size in different regions. In each anterior horn are three main groups of ganglion cells : the ventrolateral, made up of root or motor nerve-cells ; the ventromesial, composed of commissural cells ; and the lateral (in the lateral horn), containing column cells. At the median side of the base of each posterior horn we find a group of cells and fibers known as the column of Clark, most clearly defined in the dorsal region, while in the posterior horn itself is the gelatinous substance of Rolando. Aside from these, numerous cells and fibers are scattered throughout the entire gray substance.

Fig. 327. Four cross-sections of the human spinal cord ; X 7 : ^ Cervical region in the plane of the sixth spinal nerve-root ; J3, lumbar region ; C, thoracic region ; Z>, sacral region (compare with Fig. 328). (From preparations of H. Schmaus.)

The motor nerve-cells lie in the ventrolateral portion of the anterior horn, their neuraxes extending into the anterior nerve-root. Their dendrites are distributed in a lateral, dorsal, and mesial direction, the two former groups ending in the anterior and lateral columns, the mesial in the region of the anterior commissure. Some of the mesial dendrites extend beyond the median line and form a sort of commissure with the corresponding processes of the other side. The commissural cells lie principally in the mesial group of the anterior horn, but occur here and there in other portions of the gray substance. Their neuraxes form the anterior gray commissure with the corresponding processes from the other side. After entering the white substance of the other side, these neuraxes undergo a T-shaped division, one branch passing upward and the other downward. The column cells are small multipolar elements, represented by the cells of the lateral horns, although they are also found throughout the entire gray mass. Their neuraxes pass directly into the anterior, lateral ^ and posterior horns.

The cells of the column of Clark, or micleus dorsalis, are of two kinds those in which the neuraxes pass to the anterior commissure (commissural cells) and those in which the neuraxes pass into the direct cerebellar tract of the same side. The plurifunicular cells are cells the neuraxes of which divide two or three times in the gray substance, the branches then passing to different columns of the white matter on the same or opposite side of the cord. In the latter case the branches must necessarily extend through the commissure. The cells of the substantia gclatinosa (Rolando) are cells with short, freely branching neuraxes, which end after a short course in the gray mass (Golgi's cells). The posterior horn contains marginal cells, spindle-shaped cells, and stellate cells. The first are situated superficially near the extremity of the posterior horn, their neuraxes extending for some distance through the gelatinous substance of Rolando and then into the lateral column. The spindle-shaped cells are the smallest in the spinal cord and possess a rich arborization of dendrites extending to the nerve-root of the posterior horn. Their neuraxes, which originate either from the cellbody or from a dendrite, pass over into the posterior column. The stellate cells are supplied with dendrites, which either branch in the substance of Rolando or extend into the column of Burdach.

The gray matter contains, further, numerous medullated nervefibers, in part the neuraxes of the nerve-cells previously mentioned, and in part collateral and terminal branches of the nerve-fibers of the white matter with their telodendria ; also supporting cells, known as neurogliar cells (to be discussed later), and blood-vessels.

The white matter of the spinal cord consists of medullated fibers, which are devoid of a neurilemma, of neurogliar tissue, and of fibrous connective tissue.

Fig. 328. Schematic diagram of the spinal cord in cross-section after von Lenhossek, showing in the left half the cells of the gray matter, in the right half the collateral branches ending in the gray matter.

In each half of the cord the white substance, which surrounds the gray, is separated by the gray matter and its nerve-roots into three main divisions or columns: The first division, lying between the anterior median fissure and the anterior horn, is the anterior column ; the second, lying between the anterior and posterior horns, is the lateral column (since the anterior and lateral columns belong genetically to each other, the term anterolateral column is often used) ; and the third, lying between the posterior nerve-root and the posterior median septum, is the posterior column.

By means of certain methods it has been possible to separate the white substance into still smaller divisions, the most important of which may here be described.

In each anterior column is found a narrow median zone extending along the entire length of the anterior median fissure and containing nerve-fibers which come from the pyramids of the medulla. The majority of the pyramidal fibers cross from one side of the cord to the other in the lower portion of the medulla, at the crossing of the pyramids, and form a large bundle of nerve-fibers found in each lateral column, which will receive attention later. Some of the pyramidal fibers descend into the cord on the same side, to cross to the opposite side at different levels in the cord. These latter fibers constitute the narrow median zone, on each side of the anterior median fissure previously mentioned, forming the anterior or direct pyramidal tract, or the column of Tiirck. Between the direct pyramidal tract and the anterior horn lies the anterior ground bundle.

In the lateral columns are found a number of secondary columns, which may now be mentioned. In front of and by the side of the posterior horn in each lateral column lies a large group of nerve-fibers, forming a bundle which varies somewhat in size and shape in the several regions of the spinal cord, but which has in general an irregularly oval outline. These nerve-fibers are the pyramidal fibers, previously mentioned, which in the lower part of the medulla cross from one side to the other, and for this reason are known as the crossed pyramidal fibers, forming the crossed pyramidal columns. External to these columns and to the posterior horns, and extending from the posterior horns half-way around the periphery of the lateral columns, lie the direct cerebellar columns, consisting of the neuraxes of the cells of the columns of Clark, which have an ascending course. Lying just external to and between the anterior and posterior horns is a somewhat irregular zone, the mixed lateral column, containing several short bundles of fibers, the anterior of which are probably motor ; the posterior, sensory. In the ventrolateral portions of the lateral columns, between the mixed lateral and the direct cerebellar columns and extending as far backward as the crossed pyramidal columns, lie two not well-defined columns, known as the ascending anterolateral or Gowers's columns and the descending anterolateral columns ; the former are nearer the outer portion of the cord.

In the posterior column we distinguish a median and a lateral column. The former lies along the posterior median septum, and may even be distinguished externally by an indentation ; its upper portion tapers into the fasciculus gracilis. This is the column of Goll, and it contains ascending or centripetal fibers. The lateral tract lies between the column of Goll and the posterior horn, and is known as the column of Burdach, posterior ground-bundle, or posterolateral column. It contains principally the shorter tracts, or bundles of longitudinal fibers connecting the adjacent parts of the spinal cord with one another.

Many of the nerve-fibers of the posterior column are the neuraxes of spinal ganglion cells which enter the spinal cord through the posterior roots. The cell-bodies of the spinal ganglion or sensory neurones are situated in the spinal ganglia found on the posterior roots of the spinal nerves. In the embryo they are distinctly bipolar, but during further development their two processes approach each other, and then fuse for a certain distance, forming finally single processes which branch like the letter T. In reality, then, there are two processes which are fused for a certain distance from the cell-body of each neurone. The peripherally directed process is regarded as the dendrite of the cell, and the proximal as the neuraxis passing to the spinal cord. The neuraxes enter the spinal cord through the posterior roots and pass to the posterior columns, where they divide, Y-shaped, into ascending and much shorter descending branches, from each of which numerous collateral branches are given off.

From the preceding account of the white matter of the spinal cord, it may be seen that it consists of longitudinally directed neuraxes arranged in so-called short and long tracts or columns. The neuraxes constituting the former, after a short course through the gray matter, emerge from it, and after giving off various collaterals, again penetrate into the gray matter, where their telodendria enter into contact with the ganglion cells. The long columns consist of the neuraxes of neurones the cell -bodies of which are situated in the cerebrum or cerebellum, and of neurones the cell-bodies of which are in the spinal cord or spinal ganglia and the neuraxes of which terminate in the medulla or cerebellum. The nerve-fibers of the various columns give off numerous collaterals which enter the gray matter to end in telodendria. The collaterals of the posterior columns end : (i) between the cells of the gelatinous substance of the posterior horns ; (2) in the columns of Clark ; (3) in the anterior horns, these constituting the principal portion of the so-called reflex bundles ; (4) in the posterior horn of the opposite side. The collaterals of the lateral columns pass horizontally toward the central canal, some ending in the anterior horn, others closely arranged near the columns of Clark, and some arching around the central canal, forming with the corresponding fibers of the other side the anterior bundles of the posterior commissure. The collaterals of the anterior columns form well-marked plexuses in the anterior horns of the same and opposite sides.

We have still to describe the two commissures. The anterior consists of: first, neuraxes from the commissural cells ; second, dendrites from the lateral group of the anterior horn cells ; and, third, the collaterals of the anterolateral column, which end in the gray substance of the other side of the cord. The posterior commissure is probably composed of the collaterals from all the remaining columns. The posterior bundle of this commissure comes from the posterior column ; the middle, from the posterior portion of the lateral column ; and the anterior, or least developed, from the anterior portion of the lateral column, possibly also from the anterior column.

In the gray commissure, nearer its anterior border, is situated the central canal of the spinal cord, continuous above with the ventricular cavity of the medulla and terminating caudally in the filum terminale. This canal is not patent in the majority of adults, being occluded from place to place. The canal is lined by a layer of columnar cells, developed from columnar cells, known as spongioblasts, lining the relatively larger canal of the embryonic spinal cord. In young individuals these cells are ciliated and their basal portions terminate in long, slender processes in which are embedded neuroglia fibers.

B. The Cerebellar Cortex

In the cerebellar cortex we distinguish three general layers the outer molecular, the middle granular (rust-colored layer), and the inner medullary tract.

Fig. $30. Section through the human cerebellar cortex vertical to the surface of the convolution. Treatment with M tiller's fluid ; X XI 5 The molecular layer contains three varieties of nerve-cells, those of Purkinje, which border upon the granular layer, the stel late cells, and the small cortical cells. The cells of Purkinje possess a large flask-shaped body (about 60 p. in diameter), from which one or more well -developed dendrites pass toward the periphery. The latter branch freely and the main arborization has in each case the general shape of a pair of deer's antlers. These dendrites extend nearly to the periphery of the cerebellar cortex. In a section horizontal to the surface of the organ the dendrites of the Purkinje's cells are seen to lie in a plane very nearly vertical to the surface of the convolutions, so that a longitudinal section through the latter would show a profile view of the cells. In other words, they have an appearance much like that of a vine trained upon a trellis. The neuraxes of the cells of Purkinje arise from their basal (inner) ends and extend through the granular layer into the medullary substance. During their course they give off a few collaterals, which pass backward to the molecular layer and end in telodendria near the bodies of the cells of Purkinje. The stellate cells lie in various planes of the molecular layer. Their peculiar interest lies in the character of their neuraxes. The latter are situated in the same plane as the dendrites of the cells of Purkinje, run parallel to the surface of the convolution, and possess two types of collaterals. Those of the first are short and branched ; those of the second branch at a level with the cells of Purkinje, and form, together with their telodendria, basket-like nets around the bodies of these cells. The small cortical cells of the molecular layer are found in all parts of this layer, but are more numerous in its peripheral portion. They are multipolar cells with neuraxes which are not readily stained and concerning the fate of which little is known.

Fig. 332. Cell of Purkinje from the human cerebellar cortex. Chrome-silver method ; X I2

Fig. 333. Granular cell from the granular layer of the human cerebellar cortex. Chromesilver method ; X IO

The granular layer contains two varieties of ganglion elements, the so-called granular cells (small ganglion cells) and the large stellate cells. The dendrites of the granular cells are short, few in number (from three to six), branch but slightly, and end in short, claw-like telodendria. Their neuraxes ascend vertically to the surface and reach the molecular layer. At various points some of them are seen to undergo a T-shaped division, the two branches then running parallel to the surface of the cerebellum in a plane vertical to that of the dendrites of the cells of Purkinje. Large numbers of these T-shaped neuraxes produce the striation of the molecular layer of the cerebellum. It is very probable that during their course these parallel fibers come in contact with the dendrites of the cells of Purkinje. The large stellate cells are fewer in number and lie close to the molecular layer, some of them even within this layer. Their dendrites branch in all directions, but extend principally into the molecular layer. Their short neuraxes give off numerous collaterals which end in telodendria among the granular cells.

The medullary substance is composed of the centrifugal neuraxes of the cells of Purkinje and of two types of centripetal neuraxes, the mossy and the climbing fibers. The position of their corresponding nerve-cells is not definitely known. The mossy fibers branch in the granular layer into numerous twigs, and are not uniform in diameter, but are provided at different points with typical nodular swellings. These fibers do not extend beyond the granular layer. The climbing fibers pass horizontally through the granular layer, giving off in their course numbers of collaterals, which extend to the cells of Purkinje, up the dendrites of which they seem to climb.

In the medullary portion of the cerebellum are found a number of groups of ganglion cells known as central gray nuclei. The nerve-cells of these nuclei are multipolar, with numerous, oftbranching dendrites and a single neuraxis.

C. The Cerebral Cortex

The cell-bodies of the neurones of the cerebrum are grouped in a thin layer of gray matter, varying in thickness from 2 to 4 mm., which, as a continuous sheet, completely covers the white matter of the hemispheres, and in larger and smaller masses of gray matter, known as basal nuclei. In our account of the histologic structure of the cerebral hemispheres we shall confine ourselves in the main to a consideration of the cerebral cortex, the thin layer of gray matter investing the white matter.

From without inward the following layers may be differentiated in the cerebral cortex : (i) a molecular layer ; (2) a layer of small pyramidal cells ; (3) a layer of large pyramidal cells ; (4) a layer of polymorphous cells ; and (5) medullary substance or underlying nerve-fibers.

Aside from neurogliar tissue, we find in the molecular layer a large number of nerve-fibers, which cross one another in all directions, but, as a whole, have a direction parallel with the surface of the brain. Within this layer there are found : (i) the tuft-like telodendria of the chief dendritic processes of the pyramidal cells ; (2) the terminations of the ascending neuraxes, arising mostly from the polymorphous cells ; and (3) autochthonous fibers /. e., those which arise from the cells of the molecular layer and terminate in this layer. The cells of the molecular layer may be classed in three general types polygonal cells, spindle-shaped cells, and triangular or stellate cells. The polygonal cells have from four to six dendrites, which branch out into the molecular layer and may even penetrate into the underlying layer of small pyramidal cells. Their neuraxes originate either from the bodies of the cells or from one of their dendrites, and take a horizontal or an oblique direction, giving off in their course a large number of branching collaterals, which terminate in knob-like thickenings. The spindle=shaped cells give off from their long pointed ends dendrites which extend for some distance parallel with the surface of the brain. These branch, their offshoots leaving them at nearly right angles, the majority passing upward, assuming as they go the characteristics of neuraxes having collaterals. The arborization is entirely within the molecular layer. The triangular or stellate cells are similar to those just described, but possess not two, but three, dendrites. The triangular and spindle-shaped cells, with their numerous dendritic processes resembling neuraxes, are characteristic of the cerebral cortex.

The elements which are peculiar to the second and third layers of the cerebral cortex are the small (about 10 // in diameter) and large pyramidal cells (from 20 //to 30 , in diameter). They are composed of a triangular body, the base of the triangle being downward and parallel to the surface of the brain, of a chief, principal, or primordial dendrite ascending toward the brain-surface, of several basilar dendrites arising from the basal surface of the cell-body, and of a neuraxis which passes toward the medullary substance and which has its origin either from the base of the cell or from one of the basilar dendrites. The ascending or chief dendrite gives off a number of lateral offshoots which branch freely and end in terminal filaments. The main stem of the dendrite extends upward to the molecular layer, in which its final branches spread out in the form of a tuft. The neuraxis, during its course to the white substance, gives off in the gray substance from six to twelve collaterals, which divide two or three times before terminating.

Aside from the fact that the layer of polymorphous cellt contains a few large pyramidal cells, it consists principally of (i) multipolar cells with short neuraxes (Golgi's cells) and (2) of cells with only slightly branched dendrites and with neuraxes passing toward the surface of the brain (Martinotti's cells). Both these types of cells are, however, not found exclusively in the layer of polymorphous cells, but may be met with here and there in the layers of the small and large pyramidal cells. The dendrites of the cells of Golgi are projected in all directions, those in the neighborhood of the medullary substance even penetrating into this layer. The neuraxes break up into numerous collaterals, the telodendria of which lie adjacent to the neighboring ganglion cells. The cells of Martinotti, which, as we have seen, occur also in the second and third layers, are either triangular or spindle-shaped. The neuraxis of each cell originates either from the cell-body or from one of its dendrites, and ascends (giving off collaterals) to the molecular layer, in which it finally divides into two or three main branches ending in telodendria. Occasionally it divides in a similar manner in the layer of small pyramidal cells.

Fig. 334. Portions of vertical section of human cerebral cortex, treated by the Golgi method; X 7- The figure shows the arrangement of the different cells of the cerebral cortex : gP, Layer of large pyramidal cells ; kF, layer of small pyramidal cells ; pZ, layer of polymorphous cells (Sobotta, "Atlas and Epitome of Histology").

Fig. 335. Large pyramidal cell from the human cerebral cortex. Chrome-silver method ; X I 5

In the medullary substance the following four classes of fibers are recognized : (i) The projection fibers (centrifugal) i. e., those which indirectly connect the elements of the cerebral cortex with the* periphery of the body ; their course may or may not be interrupted during their passage through the basal nuclei ; (2) the commissural fibers, which, according to the original definition, pass througli the corpus callosum and anterior commissure, thus joining corresponding parts of the two hemispheres ; (3) the association fibers, which connect different parts of the gray substance of the same hemispheres ; and (4) the centripetal or terminal fibers i.e., the terminal arborizations of those neuraxes, the cells of which lie in some other region of the same or opposite hemisphere, or even in some more distant portion of the nervous system. The projection fibers originate from the pyramidal cells, some of them perhaps from the polymorphous cells. The commissural fibers are also derived from the pyramidal cells, and lie somewhat deeper in the white substance than the association fibers. With the exception of those which join the cunei and those which lie in the anterior commissure, all the commissural fibers are situated in the corpus callosum. They give off during their passage through the hemispheres large numbers of collaterals, which penetrate at various points into the gray substance and end there in terminal filaments. In this respect their arborization is contrary to the old definition of these fibers, and the latter must be completed by the statement that, besides joining symmetric points of the two hemispheres, they also, by means of their collaterals, may connect other areas of the gray substance with the peripheral regions supplied by their end-tufts (Ramon y Cajal, 93). The association fibers have their origin also in the pyramidal cells. In the medullary substance their neuraxes divide T-shaped, and after a longer or shorter course penetrate into the gray substance of the same hemisphere, where they end as terminal fibers. A few collaterals are, however, previously given off which also terminate in the same manner in the gray substance. The association fibers form the bulk of the medullary rays.

Fig. 336. Schematic diagram of the cerebral cortex : a, Molecular layer with superficial (tangential) fibers ; b, striation of Bechtereff-Kaes ; c, layer of small pyramidal cells; d, stripe of Baillarger; e, radial bundles of the medullary substance ; y, layer of polymorphous cells.

On examining a vertical section through one of the cerebral convolutions a number of successive striations may be seen. These are more or less distinct, according to the region, and consist of strands of medullated nerve-fibers between the layers of cells, and parallel with the surface of the convolution. The most superficial form a layer of tangential fibers. Between the molecular layer and the layer of small pyramidal cells is the striation of Bechtereff and Kaes, and in the region of the large pyramidal cells the striation of Baillarger (Gennari) corresponding to the striation of Vicq d'Azyr in the cuneus. In figure 336 the medullary substance is seen below, \vith rays, composed of parallel bundles of fibers, passing upward into the gray substance ; in reality these fibers penetrate much higher than is shown in the illustration.

The olfactory bulb is composed of five layers, which are especially well marked on its ventral side : first, the layer of peripheral nerve-fibers ; second, the layer of olfactory glomeruli ; third, the stratum gelatinosum, or molecular layer ; fourth, the layer of pyramidal cells (mitral cells) ; and, fifjth, the granular layer with the deeper nerve-fibers.

The layer of peripheral fibers is composed of the nervebundles of the olfactory nerve which cross one another in various directions and form a nerve-plexus. The glomerular layer contains peculiar, regularly arranged, round or oval, and sharply defined structures, which were first accurately studied by Golgi. They are known as glomeruli (from 100 [J. to 300 /JL in diameter), and are in reality complexes of intertwining telodendria. As we shall see, the epithelial cells of the olfactory region of the nose must be regarded as peripheral ganglion cells and their centripetal (basal) processes as neuraxes. The telodendria of these neuraxes, together with those of the dendrites from the mitral or other cells, come in contact with each other within the olfactory glomeruli. The molecular layer consists of small, spindle-shaped ganglion cells. Their neuraxes enter the fifth layer and their short dendrites end in terminal ramifications in the glomeruli. The mitral cells give off neuraxes from their dorsal surfaces which also enter the granular layer, but the majority of their dendrites break up into terminal ramifications in the olfactory glomeruli, as just described. The granular layer (absent in the illustration) is made up of nerve-cells and nerve-fibers ; but, aside from these, we find also large numbers of peculiar cells with a long peripherally and several short centrally directed dendrites. No neuraxes can be demonstrated in these cells (granular cells). This layer also contains the stellate ganglion cells. The latter are not numerous, but lie scattered, and each possesses several short dendrites and a peripherally directed neuraxis which ends in the molecular layer in a rich arborization. The deep nerve-fibers are grouped into bundles which inclose between them the granular and stellate cells just mentioned. These nerve-fibers are derived partly from the neuraxes of the pyramidal or mitral cells and partly from the cells of the molecular layer, while some of them are centripetal fibers from the periphery, which end between the granules of the fifth layer.

Fig. 337. The olfactory bulb, after Golgi and Ram6n y Cajal. The granular layer is not shown.

E. Epiphysis and Hypophysis

In mammalia the epiphysis, or pineal gland, consists of a fibrous capsule derived from the pia mater, from which numerous fibrous tissue septa and processes pass into the gland, uniting to form quite regular round or oval compartments in which closed follicles or alveoli, whose walls consist of epithelial cells, are found. In the lower portion of the epiphysis there is found a relatively large amount of neuroglia tissue, consisting of coarse fibers, as has been shown by Weigert. The epithelial cells forming the walls of the follicles are of cubic or short columnar shape, and may be arranged in a single layer or may be pseudostratified or stratified. Follicles completely filled with cellular elements are found. Other follicles contain peculiar concretions, known as brain-sand or acervulus, of irregular round or oval or mulberry shape. Medullated nerve-fibers have been traced into the epiphysis, but their mode of termination is not known.

The hypophysis, or pituitary body, consists of two lobes. The posterior or infundibular lobe is developed from the floor of the first primary brain-vesicle, and remains attached to the floor of the third ventricle by a stalk, known as the infundibulura ; the anterior or glandular lobe develops from a hollow protrusion derived from the primary oral ectoderm. The distal end of this protrusion or pouch comes in contact with the anterior surface of the lower portion of the infundibulum, and becomes loosely attached to it. As the bones at the base of the skull develop, the attenuated oral end of this pouch atrophies, the distal end becoming finally completely severed from the buccal cavity.

In the infundibular lobe of the hypophysis of the dog, Berkley (94) described three portions presenting different microscopic structure. His account will here be followed :. (i) An outer stratum consisting of three or four layers of cells resembling ependymal cells, which are separated into groups by thin strands of fibrous tissue entering from the fibrous covering of this lobe. (2) A zone consisting of glandular epithelial cells which in certain places are arranged in the form of alveoli, often containing a colloid substance. This zone merges into the central portion, (3), containing variously shaped cells and connective-tissue partitions with blood-vessels. In this portion neurogliar cells (see these) and nerve-cells were stained by the chrome-silver method.

The glandular or anterior lobe resembles slightly in structure the parathyroid. This lobe is surrounded by a fibrous tissue capsule and within it are found variously shaped alveoli or follicles, or, again, columns or trabeculae of cells separated by a very vascular connective tissue. In the alveoli or columns of cells are found two varieties of glandular cells, which may be differentiated more by their staining reaction than by their size and structure, although they present slight structural differences. One variety of cells possesses a protoplasm which shows affinity for acid stains ; these are known as chromophilic cells. They are of nearly round or oval shape, with nuclei centrally placed, and have a protoplasm presenting coarse granules. The other variety of cells, known as chief cells, are more numerous than the chromophilic. They are of cubic or short columnar shape, with nuclei placed in the basal portions of the cells and with protoplasm showing a fine granulation and with an affinity for basic stains. Now and then alveoli containing a colloid substance, similar to that found in the alveoli of the thyroid gland, may be observed. The blood-vessels of the glandular portion are relatively large, the majority of them having only an endothelial lining which comes in contact with the glandular cells.

The circulation of the hypophysis must be regarded as sinusoidal. In the glandular portion of the hypophysis of the dog, Berkley (94) found small varicose nerve-fibers belonging to the sympathetic system. From the larger bundles, which follow the blood-vessels, are given off single fibers, or small bundles of such, which end on the glandular elements in numerous small nodules.

F. Ganglia

In the course of peripheral nerves are found numerous larger and smaller groups of nerve-cells, known as ganglia. The neurones of these ganglia are in intimate relation with the neurones of the central nervous system, and may, therefore, be discussed with the latter. According to the structure and function of their neurones, the ganglia are divided into two groups (i) spinal or sensory ganglia and (2) sympathetic ganglia.

Fig. 338. Longitudinal section of spinal ganglion of cat.

The spinal ganglia are situated on the posterior roots of the spinal nerves. Certain cranial ganglia namely, the Gasserian, geniculate, and auditory ganglia, the jugular and petrosal ganglia of the glossopharyngeal nerves, and the root and trunk ganglia of the vagi are classed with the spinal ganglia, since they present the same structure. The spinal and sensory cranial ganglia are surrounded by firm connective- tissue capsules, continuous with the perineural sheaths of the incoming and outgoing nerve-roots. From these capsules connective-tissue septa and trabeculae pass into the interior of the ganglia, giving support to the nerve-elements. The cell-bodies (ganglion cells) of the neurones constituting these ganglia are arranged in layers under the capsule and in rows and groups or clusters between the nerve-fibers in the interior of the ganglia. More recent investigations have shown that several types of neurones are to be found in the spinal and cranial sensory ganglia ; of these, we may mention the following : (i) Large and small unipolar cells with T- or Y-shaped division of the process. These neurones, which constitute the greater number of all the neurones of the ganglia under discussion, consist of a round or oval cell-body, from which arises by means of an implantation cone a single process, which, soon after it leaves the cell, becomes invested with a medullary sheath and usually makes a variable number of spiral turns near the cell-body. According to Dogiel, this process divides into two branches, usually at the second or third node of Ranvier, sometimes not until the seventh node is reached. Of these two branches, the peripheral is the larger, and enters a peripheral nerve-trunk as a medullated sensory nerve-fiber, terminating in one of the peripheral sensory nerve-endings previously .described. The central process, the smaller of the two, becomes a medullated nerve-fiber, which enters the spinal cord or medulla in a manner described in a former section. The cell-body of each of these neurones is surrounded by a nucleated capsule, continuous with the neurilemma of the single process. (2) Type II spinal ganglion cell of Dogiel. Dogiel has recently described a second type of spinal ganglion cell which differs materially from the type just described. The cell-bodies of these neurones resemble closely those of the typical spinal ganglion neurones. Their single medullated processes divide, however, soon after leaving the cells into branches which divide further and which do not pass beyond the bounds of the ganglia but terminate, after losing their medullary sheaths, in complicated pericapsular and pericellular end-plexuses surrounding the capsules and cell-bodies of the typical spinal ganglion cells. (3) Multipolar ganglion cells ; in nearly all spinal and cranial ganglia there are found a few multipolar nerve-cells, which in shape and structure resemble the nerve-cells of the sympathetic system.

Fig. 339. Ganglion cell from the Gasserian ganglion of a rabbit ; stained in methylene blue (intra vitani).

Fig. 340. Diagram showing the relations of the neurones of a spinal ganglion ; p. r., posterior root; a. r., anterior root; /. s., posterior branch and a. s., anterior branch of spinal nerve ; w. r., white ramus communicans ; a, large, and 6, small spinal ganglion cells with T-shaped division of process ; c, type II spinal ganglion cells (Dogiel); s, multipolar cell ; d, nerve-fiber from sympathetic ganglion terminating in pericellular plexuses (slightly modified from diagram given by Dogiel).

Entering the spinal ganglia from the periphery are found a relatively small number of small, medullated or nonmedullated nervefibers, probably derived from sympathetic ganglia. These nervefibers, medullated and nonmedullated, the former losing their medullary sheaths within the ganglia, approach a spinal ganglion cell, and after making a few spiral turns about its process, terminate in pericapsular and pericellular end-plexuses. Dogiel believes that the cell-bodies and capsules thus surrounded by the terminal branches of the sympathetic fibers terminating in the spinal ganglia belong to the spinal ganglion cells of the second type first described by him. In figure 340 is represented by way of diagram the structure of a spinal ganglion.

In the medium-sized cells (from 30 fJ. to 45 fj. in diameter) of the spinal ganglia of the frog, von Lenhossek (95) found centrosomes surrounded by a clear substance (centrospheres). The entire structure lay in a depression of the nucleus and contained more than twelve extremely minute granules (centrosomes), which showed a staining reaction different from that of the numerous concentrically laminated granules present in the protoplasm. This observation is interesting in that it proves that centrosome and sphere occur also in the protoplasm of cells which have not for a long time undergone division and in which there is no prospect of future division.

Sympathetic Ganglia. The ganglia, of the sympathetic nervous system comprise those of the two great ganglionated cords, found on each side of the vertebral column and extending from its cephalic to its caudal end, with which may be grouped certain cranial ganglia having the same structure, namely, the sphenopalatine, otic, ciliary, sublingual, and submaxillary ganglia ; also three unpaired aggregations of ganglia, found in front of the spinal column, of which the cardiac is in the thorax, the semilunar in the abdomen, and the hypogastric in the pelvis ; and further, large numbers of smaller ganglia, the greater number of which are of microscopic size and are found in the walls of the intestinal canal and bladder, in the respiratory passages, in the heart, and in or near the majority of the glands of the body.

Fig. 341. Neurone from inferior cervical sympathetic ganglion of a rabbit; methylene blue stain.

The sympathetic ganglia are inclosed in fibrous tissue capsules continuous with the perineural sheaths of their nerve-roots. The thickness of the capsule bears relation to the size of the ganglion, being thicker in the larger and thinner in the smaller ones. From these capsules thin connective-tissue septa or processes pass into the interior of the ganglia, supporting the nerve elements.

The sympathetic neurones, the cell-bodies and dendritic processes of which are grouped to form the sympathetic ganglia, are variously shaped unipolar, bipolar, and multipolar cells, the cell-bodies of which are surrounded by nucleated capsules, continuous with the neurilemma of their neuraxes. In the sympathetic ganglia of mammalia and birds the great majority of sympathetic neurones are multipolar, although in nearly all ganglia a small number of bipolar and unipolar cells are to be found, usually near the poles of the ganglia.

The dendrites of the sympathetic neurones in any one ganglion branch repeatedly. Of these branches, some extend to the periphery of the ganglion, where they interlace to form a peripheral subcapsular plexus, while others interlace to form plexuses between the cell-bodies of the neurones in the interior of the ganglion pericellular plexuses. These pericellular plexuses are external to the capsules surrounding the cell-bodies of the sympathetic neurones.

Fig. 342. From section of semilunar ganglion of cat ; stained in methylene-blue, intra vitam (Huber, Journal of Morphology, 1899).

The neuraxes of the sympathetic neurones, the majority of which are nonmedullated, the remainder surrounded by delicate medullary sheaths, arise from the cell-bodies either from implantation cones or from dendrites at variable distances from the cellbodies, leave the ganglion by way of one of its nerve-roots, and terminate in heart muscle tissue, nonstriated muscle, and glandular tissue, and to some extent in other ganglia, both sympathetic and spinal. Terminating in all sympathetic ganglia are found certain small medullated nerve-fibers, varying in size from about 1.5 // to 3 fjt. The researches of Gaskell, Langley, and Sherrington have shown that these small medullated nerve-fibers leave the spinal cord through the anterior roots of the spinal nerves from the first dorsal to the third or fourth lumbar and reach the sympathetic ganglia through the white rand comnmnicantes. Similar small medullated nerve-fibers are found in certain cranial nerves. These small medullated nerve-fibers, which may be spoken of as white rami fibers, after a longer or shorter course, in which they may pass through one or several ganglia without making special connection with the neurones contained therein, terminate in some sympathetic ganglion in a very characteristic manner. After entering the sympathetic ganglion in which they terminate, they branch repeatedly while yet medullated. The resulting branches then lose their medullary sheaths and divide into numerous small, varicose nerve -fibers, which interlace to form intracapsular plexuses, which surround the cell-bodies of the sympathetic neurones. In the sympathetic ganglia of mammalia such intracapsular pericellular plexuses may be very simple, consisting of only a few varicose nerve-fibers, or very complicated, consisting of many such fibers. In the sympathetic ganglia of reptilia, in which are found very large sympathetic neurones, the white rami fibers are wound spirally about the cell-bodies of such neurones before terminating in complicated pericellular plexuses. In the frog and other amphibia the sympathetic neurones are unipolar nerve-cells. The white rami fibers terminating in the sympathetic ganglia of amphibia are wound spirally about the single processes of these unipolar cells while yet medullated fibers, but they lose their medullary sheaths before terminating in the intracapsular pericellular plexuses. From what has been said concerning the white rami fibers and their relation to the sympathetic neurones, it is evident that the sympathetic neurones, the cell-bodies and dendrites of which are grouped to form the sympathetic ganglia, form terminal links in nerve or neurone chains ; the second link of these chains is formed by neurones the cell-bodies of which are situated in the spinal cord or medulla, the neuraxes leaving the cerebrospinal axis through the white rami as small medullated nerve-fibers, which terminate in pericellular plexuses inclosing the cell-bodies of the sympathetic neurones.

Fig. 343. From section of stellate ganglion of dog, stained in methylene-blue and alum carmin : a, white ramus fiber ( Huber, Journal of Morphology, 1899).

Fig. 344. From section of sympathetic ganglion of turtle, showing white rami fibers wound spirally about a large process of a unipolar cell, and ending in pericellular plexus (Huber, Journal of Morphology, 1899).

Large medullated nerve -fibers, the dendrites of spinal ganglion neurones, reach the sympathetic ganglia through the white rami.

Fig. 345. From section of sympathetic ganglion of frog, showing spiral fiber (white ramus fiber) and pericellular plexus (Huber, Journal of Morphology, 1899).

They make, however, no connection with the sympathetic neurones, but pass through the ganglia to reach the viscera, where they terminate in special sensory nerve-endings or in free sensory nerveendings.

G. General Survey of the Relations of the Neurones to One Another the Central Nervous System

The following figures illustrate the modern theories with regard to the relationship of the neurones in a sensorimotor reflex cycle. The pathway along which the impulse from the stimulated area of the body is transmitted to the motor nerve end-organ traverses two neurones (primary neurones) which are in contact by means of their telodendria situated within the gray matter of the spinal cord. The cell-body of the sensory neurone lies within the spinal ganglion ; that of the motor neurone, in the anterior horn of the spinal cord. The dendrite of the sensory neurone commences as a telodendrion in the skin or perhaps also in more deeply seated structures, and transmits a cellulipetal impulse, while its cellulifugal neuraxis and telodendrion (the latter in the gray matter of the cord) transfer the impulse to the cellulipetal telodendrion of the motor neurone. The cellulifugal neuraxis of the latter finally ends as a telodendrion in the muscle. (Figs. 346 and 347.)

Fig- 346. Schematic diagram of a sensorimotor reflex arc according to the modern neurone theory ; transverse section of spinal cord : mN, Motor neurone ; sN, sensory neurone ; C 1 , nerve-cell of the motor neurone ; C 2 , nerve-cell of the sensory neurone ; d, dendrite ; n, neuraxis of both neurones ; f, telodendria ; M, muscle-fiber ; A, skin with peripheral telodendrion of sensory neurone.

In the case of longer tracts the conditions are somewhat more complicated, as, for instance, in tracing the impulse along the sensory fibers to the cortex of the brain, and from there along the motor fibers to the responding muscle. In such cases secondary neurones are called into play by means of their telodendria, which are necessarily in contact with the primary neurones just described.

When we take into consideration the simplest possible case, that of the motor segment of such a neurone-chain, we find, for instance (Fig. 348), that the neuraxis of a pyramidal cell in the brain cortex (psychic cell) enters the white substance and traverses it as a nervefiber through the peduncle and the pyramid into the crossed pyramidal tract of the opposite side. Here its telodendria come in contact with those of the motor neurone of the anterior horn.

In the foregoing instance the motor nerve tract is composed of two neurones of a motor neurone of the first order, extending from the cortex of the brain to the anterior cornua of the spinal cord, and of a motor neurone of the second order, the elements of which extend from the anterior cornua to the telodendria in the muscle.

Fig. 347. Schematic diagram of a sensorimotor reflex cycle ; sagittal section of the spinal cord: C 1 , Motor cells of the anterior cornua; , , neuraxes ; sN, sensory neurone ; C 2 , spinal ganglion cell ; C, collaterals of the sensory neuraxes ; </, dendrite of sensory neurone ; the broken lines at the cells on the left indicate their dendrites.

The sensory tract may likewise be composed of neurones of the first and second orders. The cellulifugal neuraxis arising from a cell of the spinal ganglion passes to the posterior column of the cord, gives off collaterals to the latter, and then passes upward by means of its ascending branch through the posterior column to the medulla. Although here the relationship is not so clearly defined as in the motor tract, it may nevertheless be assumed that the cellulifugal (but centripetally conducting) neuraxis at some point or other terminates in telodendria (sensory neurone of the first order), which enter into contact with the corresponding structures of a cell of the spinal cord or medulla oblongata. These cells would then constitute the sensory neurones of the second order. Exactly how their cellulifugal neuraxes end has not as yet been fully determined, but it is very probable that in this case the telodendria are represented by the coarse end-fibers which penetrate into the brain cortex, and here seem to come in contact with the dendrites of the pyramidal cells.

Fig. 348 Schematic diagram of the reflex tracts between a peripheral organ and the brain cortex: H, Cerebral cortex; mJV 1 , motor neurone of the first, sJV 2 , sensory neurone of the second, degree ; C 1 , motor cell of the spinal cord ; C 2 , sensory cell of a spinal ganglion ; C 3 , pyramidal cell of the brain cortex (pyschic cell) ; C 4 , nerve-cell of a sensory neurone of the second degree ; , n, n, n, neuraxes ; </, d, dendrites ; f, c, c, c, collaterals; /, t, telodendria; sJV 1 , sensory neurone first degree; mJV*, motor neurone second degree.

H. The Neuroglia

The neuroglia tissue is an especially differentiated supporting tissue found in the central nervous system, the optic chiasm, optic nerve and retina and for some distance, at least, in the olfactory nerve. Its relation to other tissues has long been a matter of controversy, but modern observers have shown quite conclusively that neuroglia tissue is of ectodermal origin. It should not be understood, however, that the neuroglia tissue forms the only supporting tissue of the central nervous system. In all parts of the central nervous system, more especially, however, in the spinal cord, there is found true connective tissue of mesoblastic origin, more especially in connection with the blood-vessels.

At an early stage of embryonic development there are seen in the spinal cord, and also in the brain, elements radially disposed around the neural canal, which upon closer observation appear to be processes emanating from the epithelial cells lining the neural canal. These processes may undergo repeated dichotomous division, ending finally in a swelling near the periphery of the cord. These cells are known as ependymal cells, and are differentiated from ectodermal cells, called spongioblasts. In later stages the radial arrangement is still preserved, but the cell-bodies no longer all border upon the central canal, many being found at varying distances from the latter. At this stage in the development of the spinal cord, the elements retaining their original characteristics are situated only in the region of the ventral and dorsal fissures of the spinal cord, and during further development increase in number.

These observations would seem to indicate that at least a portion of the neurogliar cells, which develop from the ependymal cells previously mentioned, originate from the epithelium of the central canal, and that from here they are gradually pushed toward the periphery of the cord. This assumption is still further strengthened by the fact that later the epithelial cells of the central canal still continue to divide. Later observations (Schaper, 97) show, however, that neurogliar cells develop also from certain undifferentiated germinal cells of the neural canal, of ectodermal origin,, which wander from their position near the neural canal toward the periphery of the medullary tube, where they develop into neuroglia cells.

Fig. 349. Neurogliar cells : <?, From spinal cord of embryo cat ; 6, from brain of adult cat ; stained in chrome-silver.

Owing to the fact that of the several methods now at hand for studying neuroglia tissue no two give identical results, the views concerning this tissue are still at variance. The Golgi or chromesilver method was for many years the only method by means of which the elements of neuroglia tissue were brought to light with any degree of clearness. In preparations of the central nervous system treated with this method all the neuroglia elements appear as cells with processes. The cell bodies of these cells as also the processes being stained black or nearly black (as seen with transmitted light) so that the relations of the processes to the cellular constituents can not be ascertained, investigators who have made use of this method in their study of neuroglia distinguish two essentially different cellular elements of the neuroglia: ependymal cells, previously mentioned, and neuroglia cells, so-called spider cells or astrocytes. The astrocytes are grouped under two main heads : short-rayed astrocytes, possessing a few shart processes, found in the gray matter, and long-rayed astrocytes with many fine and long processes, which do not appear to branch, found both in the gray and white matter. The two types of astrocytes are not clearly defined, as intermediate types are also found. In figure 349 are shown two astrocytes (long-rayed) as seen in chrome-silver preparations.

A number of investigators have in recent years perfected methods by means of which neuroglia tissue could be stained differentially Weigert, Mallory, Benda. In tissues treated after any one of these rather complicated differential staining methods the processes of the neuroglia cells as seen in chrome-silver preparations appear in the form of well-contoured fibrils, which are not interrupted by the cellbodies of the neuroglia cells, from which they are either entirely separated or are seen to pass through the protoplasm of the cells without losing their identity. In preparations of the central nervous system stained after Benda's differential neuroglia tissue staining method, numerous neuroglia cells may be observed both in the gray and white matter. Certain of these cells possess very little protoplasm, others and these are in the majority present it to an appreciable extent. The shape of such cells varies. When situated in the main mass of the white matter of the spinal cord, and seen in cross-sections of the cord, they present an irregular triangular and quadrangular form, with protoplasmic branches which arise from the angles and which extend for a variable distance between the nervefibers. In such preparations it may be seen that the neuroglia fibers pass in close proximity to the neuroglia cells, apparently embedded in the outermost part of their protoplasm, and often following the protoplasmic processes. This view of the structure of neurogliar tissue is more in accord with recent investigations on this subject (Weigert, Mallory, Benda, Krause, Hardesty, Huber). In figure 350 are shown two neuroglia cells from a cross-section of a human spinal cord, in which the relation of neuroglia fibers to neuroglia cells is shown.

Fig. 350. Typical neuroglia cells, from cross-section of the white matter of the human spinal cord, stained after Benda' s selective neuroglia tissue staining method; X 1 200 (Huber, "Studies on Neuroglia Tissue," Vaughan Festschrift, 1903).

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 sellae, 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 : (i) 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 trabeculse, 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, fossae Sylvii, etc. In the spinal cord the subarachnoid space is separated by the ligamenta denticulata into two large communicating spaces a dorsal and a ventral. 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 adventitta is covered by endothelium ; hence the subarachnoid space would seem to assume here the character of a perivascular space.

The trabeculse 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 arachnoid possesses neither bloodvessels nor nerves.

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 consists 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. Just beneath the pia there is found in the spinal cord of man a well-developed layer of neuroglia fibers. 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.

Fig. 351. Section through the cerebral cortex of a rabbit. The blood-vessels are injected ; X 4

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 : (i) 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, 9.9)

J. Blood-Vessels of the Central Nervous System

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

The spinal cord receives its arterial blood mainly through vessels which accompany the spinal nerve roots and through numerous anastomoses from a plexus in the pia mater in which there may be recognized a median ventral unpaired line of anastomosis and along each half of the spinal cord four other lines of anastomosis. From the median unpaired line of anastomosis some 200 to 2 50 branches pass into the anterior fissure, each of which generally divides into a right and left branch just in front of the commissure, each branch being distributed to the gray matter in its immediate vicinity. The white matter receives its blood-supply from vessels of the plexus in the pia mater, from which numerous fine branches are given off which terminate in capillary networks and extend as far as the gray matter. The veins return the blood to the veins of the pia mater, following in the main the course of the arteries. The central and peripheral arteries do not anastomose except through capillaries and now and then precapillaries (Adamkiewicz and Kadyi).

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.

Lymphatic vessels with definitive walls have thus far not been discovered in the central nervous system. The blood-vessels through the central nervous system are, however, surrounded by perivascular spaces, which may be regarded as performing the function of lymphatic vessels.

Technic

The organs of the central nervous system are best fixed in Miiller's fluid, washed with water, cut in celloidin, and stained with carmin. Such preparations are suitable for general topographic work.

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

The medullary sheath may be demonstrated as follows (Weigert): Pieces of tissue (spinal cord, for instance), fixed as usual in Miiller's or Erlicki's fluid, 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 : i 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 this stain the sections are placed (at room -temperature) for a day, and then in a thermostat (40 C.) for a few hours. The sections, now quite dark, are washed in distilled water and then placed in the so-called differentiating fluid. The latter consists of borax 2 gm., ferrocyanid of potassium 2.5 gm., and distilled water 100 gm. In this fluid the color of the sections is differentiated by virtue of the circumstance that the medullary sheath retains the dark stain, while the remaining structures, such as the ganglion cells, etc. , are bleached to a pale yellow. The time required for this differentiation varies, but it is usually complete at the end of a few minutes. The sections are then washed in distilled water, dehydrated in alcohol, cleared in carbol-xylol (carbolic acid i part, xylol 3 parts) and mounted in balsam.

Weigert's new method is more complicated, but fruitful of correspondingly better results. The preliminary treatment remains the same. After the tissues have been imbedded in celloidin and this hardened in 80 % alcohol, they are transferred to a mixture composed of equal parts of a cold saturated aqueous solution of neutral copper acetate and 10% aqueous solution of sodium and potassium tartrate, and the whole is placed in the thermostat. Larger pieces as, for instance, the pons Varolii of man may remain in the solution longer than twenty-four hours, after which time, however, the mixture must be changed ; but in no case should the specimens be permitted to remain longer than forty -eight hours in this solution. The temperature in the thermostat should not be high, otherwise the specimens will become brittle. The objects are now placed in a simple aqueous solution of neutral copper acetate, either saturated or half diluted with water, and again put in the oven. They are then rinsed in distilled water and placed in 80 % alcohol ; after remaining in this for one hour, they are in a condition to cut, but may be preserved still longer if desired. Cut and stain in the customary manner. The staining solution is prepared as follows : (0) lithium carbonate 7 c.c. and distilled water 93 c.c. (saturated aqueous solution) ; (<) hematoxylin I gm. , absolute alcohol 10 c.c. ; both a and b keep for some time, and may be kept on hand as stock solutions. Shortly before using, 9 parts of a and i part of b are mixed. After remaining in this solution for from four to five hours at room -temperature the sections are well stained, but do not overstain even if allowed to remain in the solution for twenty-four hours. In the case of loose celloidin sections the use of the differentiating fluid is superfluous. Hence this method is particularly advantageous when the gray and the white matter can not be distinguished macroscopically. Finally, the sections are washed in water, placed in 95% alcohol, cleared with carbol-xylol or anilin-xylol (in the latter case carefully washed with xylol), and mounted in xylol -balsam. The medullated fibers appear dark blue to black, the background pale or light pink, and the celloidin occasionally bluish. In order to remove the latter color, it is only necessary to wash the sections in 0.5% acetic acid instead of ordinary water ; a process, however, not to be recommended in the case of very delicate preparations as, for instance, the cerebral cortex. In applying Weigert's methods a certain thickness of section (not exceeding 25 i^ is essential, since in thicker sections the medullary sheaths are not sharply differentiated from the surrounding tissue.

For thick sections the modified Weigert method, or Pal's method, is employed. After the specimens have been treated according to Weigert's method up to the point of staining with hematoxylin, they are placed for from twenty to thirty minutes in a 0.25% solution of potassium permanganate. As differentiating fluid a solution of oxalic acid i gm., potassium sulphite i gm., and water 200 c.c. is used, care being taken, as in the case of Weigert' s differentiating fluid, that the gray matter is thoroughly bleached (here entirely colorless) and the white matter dark. By this method the medullary sheaths are stained blue, while the rest of the structure remains colorless. The staining is very precise, but not so intense as by Weigert' s method. Hence its adaptability for thicker sections.

Benda's method is a modification of the Weigert-Pal methods. The tissues are hardened in Miiller's or Erlicki's fluid, imbedded in celloidin, and cut. The sections are then subjected to the action of the following mordant for from twelve to twenty-four hours : liquor ferri ter sulphatis i part, distilled water 2 parts. They are then thoroughly rinsed in two tap-waters and one distilled water and then stained in the following hematoxylin solution: hematoxylin i gm., absolute alcohol 10 c.c., distilled water 90 c.c.; in which they remain for twenty-four hours. They are next washed in tap -water for from ten to fifteen minutes and treated with a 0.25% aqueous solution of permanganate of potassium until the gray and the white matter are differentiated, after which they are rinsed in distilled water and bleached in the following solution until the gray matter has a light yellow color : hydric sulphite 5 to 10 parts, distilled water 100 parts. The sections are then washed in tap -water for from one to two hours, rinsed in distilled water, dehydrated, cleared in carbol-xylol, and mounted in balsam. Medullary sheaths will be stained a bluish-black ; other structures, a light yellow. Sections stained after the Weigert, Pal, or Benda method may be counterstained in Van Gieson's picric-acidfuchsin stain (i% aqueous solution of acid fuchsin, 15 parts; saturated aqueous solution of picric acid, 50 parts ; distilled water, 50 parts) . The fibrous connective tissue in the sections and degenerated areas stains a light red.

Apathy (97) demonstrates the fibrillar elements of the nervous system in invertebrates and vertebrates by means of his gold method. Fresh tissue may be used, or tissue already fixed. In the first case thin membranes are placed for at least two hours in a i <f solution of yellow chlorid of gold in the dark, then carried over without washing into a i % solution of formic acid (sp. gr. 1.223), an d finally exposed for from six to eight hours to the light (the formic acid may have to be changed). These specimens are best mounted directly in syrup of acacia or in concentrated glycerin. In his second method Apathy fixes vertebrate tissues for twenty-four hours in sublimate-osmic acid (i vol. saturated solution of corrosive sublimate in 0.5% sodium chlorid solution combined with i vol. 1% osmic acid solution), washes repeatedly in water, and places for twelve hours in an aqueous iodo-iodid of potassium solution (potassium iodid i Jc and iodin 0.5%). The further treatment is the same as after ordinary corrosive sublimate fixation. Finally,' the specimens are imbedded in paraffin with the aid of chloroform, cut, and mounted by the water method. The whole process, up to the point of imbedding in paraffin, is carried out in the dark. The sections are then passed through chloroform and alcohol into water, where they are allowed to remain for at least six hours ; or they may be washed in water, placed for one minute in i % formic acid, again washed in water, immersed for twenty-four hours in a i <f solution of gold chlorid, rinsed in water, and finally placed in a i / formic acid solution and exposed to the light. For the latter purpose glass tubes are employed in which the slides are placed obliquely, with the sections downward. A uniform illumination of the section with "as intense a light and low a temperature " as possible are conditions indispensable to the attainment of successful results. The sections are then again washed in water and mounted in glycerin or syrup of acacia, or in Canada balsam after being dehydrated. Thin membranes are stretched upon small frames of linden wood especially prepared for this purpose. Their further treatment is then the same as that of sections fixed to the slide. If successful, the nerve-fibrils appear dark violet to black. The large ganglia in the spinal cord of lophius, the calf, etc., are especially recommended as appropriate vertebrate material.

Bethe (1900) has recommended the following method for staining neurofibrils and Golgi-nets in the central nervous system of vertebrates:

The perfectly fresh tissue is cut in thin lamellae, varying in thickness from 4 to 10 mm. These are placed on pieces of filter-paper and then in 3 to 7-5% nitric acid, in which they remain twenty-four hours. From the hardening fluid the pieces of tissue are transferred into 96% alcohol, where they remain for from twelve to twenty-four hours. They are then placed in a solution of ammonium -alcohol, ammonium (sp. gr. 0.95 to 0.96), i part; distilled water, 3 parts ; 96% alcohol, 8 parts, in which they remain for from twelve to twenty-four hours. The temperature of this solution should not exceed 20 C. The tissues are then placed for from six to twelve hours in 96% alcohol, and next in a hydrochloric acid-alcohol solution, concentrated hydrochloric acid (sp. gr. 1.18 37%), i part; distilled water, 3 parts; and 96% alcohol, 8 to 12 parts, in which they remain for several hours. The temperature of this solution should not exceed 20 C. The tissues are then again placed in 96% alcohol for frofri ten to twenty-four hours, and next in distilled water for from two to six hours. The tissues are now placed for twenty-four hours in a 4% aqueous solution of ammonium molybdate. (This solution should be kept at a temperature varying from 10 to 15 C., if it is desired to stain the neurofibrils ; or at a temperature varying from 10 to 30 C., if it is desired to bring out the Golgi-nets.) After the ammonium molybdate treatment, the tissues are rinsed in distilled water, placed in 96% alcohol for from ten to twenty- four hours, then in absolute alcohol for a like period, cleared in xylol or toluol, and imbedded in paraffin. Sections having a thickness of 10 // are now cut and fixed to slides with Mayer's albumin-glycerin. Since the various solutions used in the fixation and further treatment of the tissues do not act with the same intensity on all parts of the piece of tissue to be studied, and since the differentiation and staining depend on a relative proportion (not yet fully determined) of the mordant (ammonium molybdate) and the stain in a given section, it is advised by Bethe to cut large numbers of sections and fix to each slide about three sections from different parts of the series. After fixation of the sections to the slide the paraffin is removed with xylol ; and they are then carried through absolute alcohol into distilled water, in which, however, the sections remain only long enough to remove the alcohol. The slides (with the sections fixed to them) are then taken from the water and rinsed with distilled water from a water-bottle. The slide is then wiped dry on its under surface and edges with a clean cloth, and about i c.c. to 1.5 c.c. of distilled water placed on the slide over the sections. The slides are now placed in a warm oven with a temperature of 55 C. to 60 C. for a period of time varying from two to ten minutes. No definite time can here be given ; sections from each block of tissue need to be tested until the right stay in the warm oven is ascertained. The slides are then taken from the warm oven and rinsed two or three times in distilled water and again dried as previously directed. They are then covered with the following staining solution and again placed in the warm oven for about ten minutes : toluidin-blue, i part ; distilled water, 3000 parts. The stain is washed off with distilled water and the sections are placed in 96% alcohol until no more stain is given off usually for from three-fourths to two minutes. They are then dehydrated in absolute alcohol, passed through xylol twice, and mounted in xylol balsam. For a fuller discussion of this method the reader is referred to Bethe's account in " Zeitsch. f. Wissensch. Mikrosk.," vol. xvn, 1900.

For staining neuroglia Weigert (95) has recommended a method, from which we give the following : A solution is made consisting of 5% neutral acetate of copper, 5% ordinary acetic acid, and 2.5% chrome -alum in water. The chrome-alum and water are first boiled together, the acetic acid then added, and finally the finely pulverized neutral copper acetate, after which the mixture is thoroughly stirred and allowed to cool. To this solution 10% formalin may be added. Objects not over 0.5 cm. in diameter are placed in this fluid for eight days, the mixture being changed at the end of a few days. By this means the pieces of tissue are at the same time fixed and prepared for subsequent staining by the action of the mordant. If separation of the two processes be desired, the specimens are fixed for about four days in a 10% formalin solution (which is changed in a few days), and then placed in the chrome-alum mixture without the addition of formafin. Specimens thus fixed may be preserved for years without disadvantage, and may then be subjected to further treatment by other methods, Golgi's for instance. Washing with water, dehydration in alcohol, and imbedding in celloidin are the next steps. The sections are then placed for about ten minutes in a 0.33% solution of potassium permanganate, washed by pouring water over them, and placed in the reducing fluid (5% chromogen and 5% formic acid of a specific gravity of 1.20; then filter carefully, and add 10 c.c. of a 10% solution of sodium sulphite to 90 c.c. of the fluid). The sections, rendered brown by the potassium permanganate, readily decolorize in a few minutes, but it is better to leave them for from two to four hours in the solution. If it be desirable to decolorize entirely the connective tissue, no further steps need be taken preliminary to staining ; if not, the reducing fluid is poured off and the sections are rinsed twice in water and then placed in an ordinary saturated solution of chromogen (5% chromogen in distilled water, carefully filtered). The sections are left in this solution overnight, and the longer they remain in it, the more marked will be the contrast, as far as stain is concerned, between the connective and nervous tissues ; then water is again twice poured upon the sections and they are ready for staining. This process consists in a modified fibrin stain (yid. Technic). The iodo-iodid of potassium solution is the same (saturated solution of iodin in a 5 % iodid of potassium solution). Instead of the customary gentian-violet solution, a hot saturated alcoholic (70% to 80% alcohol) solution of methyl-violet is made, and, after cooling, the clear portion decanted off; to every 100 c.c. of this fluid 5 c.c. of a 5% aqueous solution of oxalic acid is added. The staining takes place in a very short time. The sections are then rinsed and normal salt solution and the iodo-iodid of potassium solution poured over them (5% iodid of potassium solution saturated with iodin), and washed off with water and dried with filter-paper and decolorized in the anilin oil-xylol solution in the proportion of 1:1. The reactions are rapid, and the thickness of the section should not exceed 20 //. This method is best adapted to the central nervous system of the human adult ; it has as yet not been sufficiently tested for other vertebrates.

Mallory' s Selective Neuroglia Fiber-Staining Methods. Fix tissues in io c /c formalin four days ; place in saturated aqueous solution of picric acid four days ; place in 5 f/ c aqueous solution of ammonium bichromate four to six days in warm oven at 38 C.; dehydrate and imbed in celloidin ; sections may be stained in Weigert's fibrin stain and differentiated with equal parts of anilin oil and xylol, or they may be treated as follows: Place sections in 0.5% aqueous solution of permanganate of potassium twenty minutes ; wash in distilled water one to three minutes ; place in i f/ c aqueous solution of oxalic acid thirty minutes ; wash in distilled water ; stain in phosphotungstic-acid-hematoxylin solution (hematoxylin i g., distilled water 8oc.c.,io% aqueous solution of phosphotungstic acid [Merk], 20 c.c., peroxid of hydrogen [U.S. P.], 2 c.c.) for twelve to twenty-four hours ; rinse in distilled water and place for five to twenty minutes in an alcoholic solution of ferric chlorid (ferric chlorid 30 g-> 2>% alcohol 100 c.c.) ; rinse in distilled water and dehydrate quickly, clear in oil of bergamot, and mount in xylol-balsam.

Benda' s Selective Neuroglia Staining Method. Benda has for some years concerned himself with perfecting selective staining methods for differentiating certain constituents of the protoplasm of cells, and has recently published a number of staining methods, by all of which neuroglia fibers may be more or less successfully differentiated. According to him, certain hematoxylin solutions, used after proper fixation and mordanting of the tissues, maybe used for neuroglia stains; also hematoxylin staining, followed by staining with an acid-anilin water crystal violet solution. These will not be considered here. We wish, however, to call especial attention to the following method for staining neuroglia tissue, suggested by Benda, since it has certain advantages not possessed by other selective neuroglia stains. Fix small pieces of tissue in 10% formalin; place in Weigert's chrome-alum solution (formula given above), four days in warm oven at 38 C. ; wash in water twenty-four hours ; dehydrate in graded alcohols ; imbed in paraffin ; cut thin sections and fix these to slides with the albumin -glycerin fixative ; remove paraffin and place sections in mordant consisting of a 4% aqueous solution of ferric alum ; rinse thoroughly in two tap waters and one distilled water ; place in a sodium sulphalizarate solution (add to distilled water a sufficient quantity of a saturated solution of sodium sulphalizarate in 70% alcohol to give it a sulphur-yellow color) twenty-four hours ; rinse in distilled water ; stain for fifteen minutes in a o. i % aqueous solution toluidin blue, which should be heated after the sections are in the stain until the solution steams ; allow the stain to cool ; rinse in distilled water ; wash in a i C J C aqueous solution of glacial acetic acid for a few seconds or in acid alcohol (six drops of hydrochloric acid ; 70% alcohol looc.c. ) for a few seconds ; dry sections with filterpaper ; dip sections a few times in absolute alcohol ; differentiate in creosote, ten minutes to an hour control now and then under the microscope ; wash in several xylols and mount in xylol -balsam. Neuroglia fibers blue, chromatin of neuroglia cell nuclei a purplish blue, protoplasm of neuroglia cells brownish red to bluish red.

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Reference: Böhm AA. and M. Von Davidoff. (translated Huber GC.) A textbook of histology, including microscopic technic. (1910) Second Edn. W. B. Saunders Company, Philadelphia and London.

Cite this page: Hill, M.A. (2024, April 19) Embryology Book - A textbook of histology, including microscopic technic (1910) Special Histology 7. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_A_textbook_of_histology,_including_microscopic_technic_(1910)_Special_Histology_7

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