tory segments, the cells are arranged in two layers. The membrana propria is very delicate, and in both regions of the gland apparently structureless. External to the basement membrane is a fine connective-tissue sheath. A marked peculiarity of the secretory portion of the gland consists in a longitudinal layer of smooth musclefibers between the membrana propria and the glandular epithelium. The presence of this structure can be accounted for only by assuming that it is an epithelial derivative.

The changes in the gland cells during secretion have not been sufficiently studied, but this much is certain, that the secretory phenomena are not similar to those in the sebaceous glands (see below). To the glandular secretion must be added also the serum-like fluid oozing from the canalicular lymph-spaces in the stratum Malpighii into the epidermal portion of the excretory duct (Unna).

The development of the sweat-glands begins in the fifth month of fetal life. At first solid cords grow from the stratum germinativum of the epidermis into the corium. Later, in the seventh month, these become hollow.

Capillary networks surround the secreting portions of the sweat-glands.

The nerves of the sweat-glands have been studied with the aid of the methylene-blue method by Ostroumow, working under Arnstein's direction. These glands receive their innervation through the neuraxes of sympathetic neurones, the terminal branches of which form an intricate network just outside of the basement membrane, known as the epilamellar plexus. From this plexus fine, varicose nerve-fibers pass through the basement membrane, and, after coursing a shorter or longer distance with or without further division, end on the gland-cells, often in clusters of small terminal granules united by delicate threads.

2. The Sebaceous Glands.-The distribution of the sebaceous glands in the skin is closely connected with that of the hair follicles into which they pour their contents. Exceptions to this rule occur in only a few regions of the body, as, for instance, in the glans penis and foreskin (Tyson's glands), in the labia minora, angle of the mouth, glandulæ tarsales, and the Meibomian glands of the eyelids, etc. As a rule the sebaceous gland empties by a wide excretory duct into the upper third of the hair follicle. The walls of the duct also produce secretion, and can therefore hardly be differentiated from the rest of the gland. At its base the duct widens and is provided with a number of simple or branched alveoli. The sebaceous glands are therefore of the type of compound alveolar glands, varying in length from 0.2 mm. to 5 mm. They are surrounded by connective-tissue sheaths, which at the same time cover the hair follicles. Inside of the sheath is the membrana propria, which is a continuation of the glassy membrane of the follicle. The two or three basal strata of glandular cells must be regarded as a direct continuation of the elements of the external root-sheath. In the

more centrally placed strata the cells are distinctly changed in character; their contents consist of fat globules, varying in size and distributed throughout the protoplasm, giving this a reticular appearance, while the nuclei suffer compression from the accumulation of the fat globules and gradually become smaller and more angular. Finally, the cells change directly into secretion, which is then poured into the hair follicle as sebum. It is thus seen that in the secretion of sebum the cells are consumed and must be replaced. This renewal takes place by the constant proliferation of the basilar cells, which push the remains of the secreting cells upward and finally take their places. The final disintegration of the cells occurs either within the gland itself or between the hair follicle and the hair. The secretion contains fatty globules of varying size, which occur either free or attached to cellular detritus.

3. The Mammary Glands.-The mammary glands are also included among the cutaneous glandular structures. They are developed early, but not until the fifth month is it possible to distinguish a solid central portion, with radially arranged tubules terminating in dilatations. The structures are all derived from the basal layers of the epidermis. From birth to the age of puberty the organs are in a state of constant growth, and are early surrounded by a connective-tissue sheath. The alveoli, which have been developed in the mean time, are still solid and relatively small. Up to the twelfth year the glands remain identical in structure in boys and girls. In the female the mammary glands continue to develop from the age of puberty; in the male, on the other hand, they undergo a retrograde metamorphosis, ending, finally, in the atrophy of all except the excretory ducts. The mammary glands

do not attain their full stage of development in women until the last months of pregnancy, and are functionally active at parturition.

The human mammary gland when fully developed has the following structure: It consists of about twenty lobes, separated from each other by connective-tissue septa. These lobes are again divided into a larger number of lobules, and these in turn are composed of numerous alveoli, which, as in the case of the lung, present lateral sacculations. The alveoli are provided with small excretory passages, which unite and finally form the larger ducts. Shortly before terminating at the surface of the mammilla, each

"Die

mammary duct widens into a vesicle, the sinus lactiferus. The number of excretory ducts corresponds to that of the larger lobes. The ducts are lined by simple cubical epithelium, except near the termination in the nipple, where they are lined by stratified pavement epithelium, and surrounded by a fibrous tissue sheath.

The epithelium of the alveoli differs according to the state of functional activity. In a state of rest it consists of a single layer of glandular cells of nearly cubical shape which stain deeply, the internal surfaces projecting into the lumen. At the beginning

of secretion the cells increase in length and fat globules make their appearance in their distal ends. At the same time a corresponding increase in size occurs throughout the entire alveolus. Finally, the free ends of the cells, which contain the most fat globules, are constricted off, after which the fat globules are freed in the lumen. The secretory portion of the alveolus is then composed of low epithelial cells, in which the process begins anew. The process of milk secretion therefore consists in throwing off the inner halves of the cells containing the fat globules, and in regeneration of the cells from the nucleated remains of the glandular epithelium. Whether a karyokinetic division of the nuclei occurs in this process is not known, and how often the process of regeneration may be repeated in a single cell is not capable of demonstration. It is certain, however, that entire cells are destroyed, to be replaced later by new elements. The membrana propria of the alveoli appears homogeneous. Between it and the glandular cells are so-called basket cells, similar to those in the salivary glands. Benda regards the basket cells as nonstriated muscle elements, having a longitudinal direction, making the structure of the alveoli of the mammary gland similar in this respect to that of the secreting portion of the sweat-glands.

The skin of the mammilla is pigmented, and the papillæ of its corium are very narrow and long. In the corium are also found large numbers of smooth muscle-fibers, which form circular bundles around the excretory ducts. In the areola of the mammæ are the so-called glands of Montgomery, which very probably represent accessory mammary glands. These are especially noticeable during lactation.

The mammary glands possess many lymphatics. These are especially numerous in the connective-tissue stroma between the lobules and alveoli. The vessels form capillary networks surrounding the alveoli. The lymph-vessels collect to form two or three larger vessels, which empty into the axillary glands. The mammary gland receives its nerve supply from the sympathetic and cerebrospinal nervous systems through the fourth, fifth, and sixth intercostal nerves. The terminations of the nerves in the mammary gland have been studied by means of the methylene-blue method by Dmitrewsky, working in the Arnstein laboratory, who finds that the terminal branches form epilamellar plexuses outside of the basement membrane of the alveoli, from which fine nerve branches pass through the basement membrane and end on the gland cells in clusters of terminal granules united by fine filaments. The nipple has a rich sensory nerve supply. In the connective-tissue papillæ are found tactile corpuscles of Meissner.

The milk consists of fat globules of varying size, which, however, do not coalesce-an attribute due to the presence of albuminous haptogenic membranes surrounding the globules. Shortly before, and for some days after, parturition the milk contains true

nucleated cells in which are fat globules; these are known as the colostrum corpuscles. They probably represent cast-off glandular cells in a state of fatty degeneration. Some authors regard them as leucocytes which have migrated into the lumen of the gland and there undergone fatty degeneration. This milk is known as colostrum.

TECHNIC.

287. Good general views of the skin can be obtained only from sections. Any fixation method may be employed, although alcohol is preferable on account of the better subsequent staining. For detail work Flemming's solution, corrosive sublimate, or osmic acid is the best. Sectioning of the skin is attended with many difficulties, and large pieces can be cut only in celloidin. Small and medium-sized pieces may be cut in paraffin; but even in this case the skin must be rapidly imbedded in the paraffin—¿. e., it must not remain too long in either alcohol or toluol-and the paraffin must have only the consistency necessary to cut well (about 50° C. meltingpoint). In order to obtain good paraffin sections of the skin the following procedure is recommended: Pieces fixed in Flemming's solution or osmic acid are kept in 96% alcohol, then placed for not more than twentyfour hours in absolute alcohol and imbedded in paraffin by means of the chloroform method. In the chloroform, chloroform-paraffin, and pure paraffin they remain for one hour each. The paraffin used should consist of two parts paraffin of 42° C., and one part paraffin of 50° C. melting-point. The thermostat must be kept at 50° C. (R. Barlow). The sections should not be mounted by the water-albumen method.

288. In sections of epidermis which have been freshly fixed with osmic acid, the stratum corneum may be clearly differentiated into three layers (probably because of the defective penetration of the reagent)— into a blackened superficial, a middle transparent, and a still lower black layer (vid. Fig. 304).

289. In tissue fixed in alcohol or corrosive sublimate the stratum lucidum stains yellow with picrocarmin, but is very weakly colored by basic anilin stains. In unstained preparations the stratum lucidum is glass-like and transparent. Eleidin is diffusely scattered throughout both the stratum lucidum and stratum corneum. Like keratohyalin, it stains with osmic acid and also with picrocarmin, but not with hematoxylin. Nigrosin stains eleidin, but not keratohyalin.

290. Keratohyalin is insoluble in boiling water and is not attacked by weak organic acids. It dissolves, however, in boiling acetic acid, but is not changed by the action of pepsin or trypsin. The keratohyalin granules of the stratum granulosum swell in from 1% to 5% potassiumhydrate solution; under the influence of heat these granules together with the cells containing them are finally dissolved. They are not attacked by ammonia, and remain unaffected for a long time in strong acetic acid. As ammonia and acetic acid render the remaining portions of the tissue transparent, these reagents may be employed for the rapid identification of keratohyalin. The larger flakes of keratohyalin swell in sodium carbonate solution (1%), but not the smaller granules, and it would seem that the larger granules have less power of resistance than the smaller. Keratohyalin remains unchanged in alcohol, chloroform, and ether, but