is digested in trypsin and pepsin (not, however, the keratin). Keratohyalin can be stained with hematoxylin and most of the basic anilin dyes.

291. The prickles of the cells composing the stratum Malpighii may be seen in very thin sections (not over 3 in thickness) of skin previously fixed in osmic acid. In this case it is best to employ not Canada balsam, but glycerin, which does not have so strong a clearing action. Isolation of the prickle cells is best accomplished as follows (Schiefferdecker): A fresh piece of epidermis is macerated for a few hours in a filtered, cold-saturated, aqueous solution of dry pancreatin; the whole may then be preserved for any length of time in equal parts of glycerin,

Fig. 304.-Transverse section through the human skin. Treated with osmic acid; X 30: a, Part of the tortuous duct of a sweat-gland in the epidermis; b, duct of same sweat-gland in the corium.

water, and alcohol. Small pieces taken from such specimens are readily teased and show both isolated and small groups of attached prickle cells.

292. The distribution of the pigment in the skin is best studied in unstained sections. With a nearly closed diaphragm and under medium. magnification the pigment granules appear darker on raising the tube and. lighter upon lowering it.

293. In sections of skin treated with Flemming's fluid, the structure of the cutis also may be studied. The medullary sheaths of the nerve

fibers and the fat appear black. In preparations stained with safranin the

elastic fibers are colored red and are very distinct (Stöhr and O. Schultze). For the orcein method according to Unna, see T. 138.

294. Hair may be examined in water without further manipulation. The cuticle is then seen to consist of polygonal areas, the border-lines of which correspond to the limits of the flattened cells. By slightly lowering the objective the cortical substance comes into view with its indistinct striation and occasional pigmentation. The medullary substance, if present, may also be seen with its vesicles containing air. Both the cortical and cuticular cells may be isolated, the process consisting in treating the hairs for several days with 33% potassium hydrate solution at room temperature, or in heating the whole for a few minutes. Concentrated or weak sulphuric acid produces the same result. On warming a hair in sulphuric acid until it begins to curl and then examining it in water, we find that the cortical and medullary layers as well as the cuticle are separated into their elements. Treatment of the skin with Müller's fluid, alcohol, or sublimate is recommended for the examination of hair and hair follicles. The orientation of the specimen should be very precise, in order to obtain exact longitudinal or cross-sections of the hair. There is hardly a structure of the body which is more suitable for staining with the numerous coal-tar colors than the hair and its follicle (Merkel).

295. The corpuscles of Meissner may be best obtained from the end of the finger. After boiling a piece of fresh skin from the finger-tip for about a quarter of an hour, the epidermis may be easily removed; the papillæ are now seen on the free surface of the cutis. A portion of the latter is cut away with a razor and examined in a 3% solution of acetic acid. The corpuscles are readily distinguished. Their relations to the nerves should be studied in specimens fixed with osmic acid or gold chlorid. The terminations of the nerves in these end-organs are best seen in preparations stained after the intra vitam methylene-blue method.

296. The corpuscles of Herbst and Grandry are found in the waxy skin covering the bill, and in the palate of the duck (especially numerous in the tongue of the woodpecker). For the study of the nervous elements the following method is useful: Pieces of the waxy skin are removed with a razor and placed for twenty minutes in 50% formic acid. After washing the specimens for a short time in distilled water they are transferred to a small quantity of 1% gold chlorid solution (twenty minutes), then again rinsed in distilled water, and placed for from twentyfour to thirty-six hours in the dark in a large quantity (1 liter) of Pichard's solution (amyl alcohol 1 part, formic acid 1 part, water 100 parts). After again washing in water the specimens are transferred to alcohols of gradually increasing strengths and finally imbedded in celloidin or celloidin-paraffin.

297. The Pacinian corpuscles occur in the mesentery of the cat and may be examined in physiologic saline solution.

298. The nerves of the epidermis are demonstrated by the goldchlorid method (see p. 166). But even here the chrome-silver method and the intra vitam methylene-blue method yield extremely good results, and may be used with great advantage in the study of the nerves in the cutis.

299. The so-called tactile menisci are very numerous in the snout of the pig and the mole. Bonnet recommends for these structures fixation in 0.33% chromic acid solution (vid. T. 26), overstaining with hematoxylin, and differentiation in an alcoholic solution of potassium ferricyanid.

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), Ramón y Cajal (93, 1), von Lenhossék (95), Kölliker (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 conus medullaris, and terminates in an attenuated portion-the filum terminale. 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 septum. 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. 366).

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 commis

Fig. 305.-Four cross-sections of the human spinal cord; X7: A, Cervical region in the plane of the sixth spinal nerve-root; B, lumbar region; C, thoracic region; D, sacral region (compare with Fig. 306). (From preparations of H. Schmaus.)

sures 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.

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. 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 nucleus 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 gelatinosa (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 pos