Book - A textbook of histology, including microscopic technic (1910) Special Histology 1
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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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- 1 I. Blood and Blood-Forming Organs, Heart, Blood-Vessels, and Lymph-Vessels
- 2 A. Blood And Lymph
- 3 B. Lymphoid Tissue, Lymph-Nodules, And Lymph Glands
- 4 C. The Spleen
- 5 D. The Bone-Marrow
- 6 E. The Thymus Gland
- 7 Technic (Circulatory System)
I. Blood and Blood-Forming Organs, Heart, Blood-Vessels, and Lymph-Vessels
A. Blood And Lymph
1. Formation Of Blood
EARLY in the development of the embryo there appear in a portion of the extra-embryonic area of the blastoderm, known as the area vasculosa, definite masses of cells, derived from the mesenchyme, and spoken of as blood islands, which are intimately connected with the formation of the blood. If these blood islands be examined at a certain stage, free cells are seen lying in their center, apparently derived from the central cells of the islands ; the cells surrounding them represent the elements which later go to form the primitive vascular walls. The free elements are the first blood-cells of the embryo. The blood-cells thus developed enter the circulation by means of blood channels formed by the confluence of the blood islands. These grow toward the embryo and later join the large central vessels. The origin of these blood islands is still an open question. Some authors contend that they arise from the mesoblast (P. Mayer, 87, 93 ; K. Ziegler ; van der Stricht, 92), others that they are of entodermic origin (Kupffer, 78 ; Gensch ; Riickert, 88 ; C. K. Hoffmann, 93, I ; 93, II ; Mehnert, 96). At a certain period the embryonic blood consists principally of nucleated red cells, which proliferate in the circulation by indirect division. The colorless blood-cells, the development of which is not yet fully understood, appear later. It is possible that they also are elements of the blood islands, which do not contain any hemoglobin. In a later period of embryonic life the liver becomes a blood-forming organ. Recent investigations have, however, shown that it does not take a direct part in the formation of the blood, but only serves as an area in which the blood-corpuscles proliferate during their slow passage through its vessels. The blind sac-like endings of the venous capillaries seem to be particularly adapted for this purpose, as in them the blood current stagnates, and it is here that the greater number of blood-cells reveal mitotic figures. The newly formed elements are finally swept away by the blood stream and enter the general circulation (van der Stricht, 92 ; v. Kostanecki, 92, III). Many investigators believe that the red blood-cells have an entirely different origin in the liver namely, from the large polynuclear, giant cells, which are thought to arise either from the cells of the capillaries or from the liver-cells (Kuborn, M. Schmidt).
Late in fetal life and in the adult, the red bone-marrow and the spleen are the organs which form the red blood-cells. The lymphatic glands and the spleen produce the white blood-cells. In addition to the nucleated red corpuscles which are present up to a certain stage of development, nonnucleated red blood-cells also appear. The number of the latter increases, until finally they are found almost exclusively in the blood of the new-born infant.
The blood of the adult consists of a fluid, coagulable substance, the blood plasma, and of formed elements suspended in this intercellular substance. The fluid medium of the blood is of a clear yellowish color and of alkaline reaction, having a specific gravity of about 1030. It is made up of water, of which it contains about 90 % , and various organic and inorganic substances. The formed elements are : (a) Red blood-corpuscles (erythrocytes) ; () white blood-corpuscles (leucocytes); and (r) the blood platelets of Bizzozero (82), hematoblasts of Hayem, or the thrombocytes of Dekhuysen. Besides these, there are present particles of fat, and, as H. F. Miiller (96) has recently shown, also hemokonia.
2. Red Blood-Corpuscles
In man and nearly all mammalia the great majority of the red blood-corpuscles are nonnucleated, biconcave circular discs with rounded edges. They have smooth surfaces, are transparent, pale yellowish-red in color, and very elastic. No method has as yet been devised to demonstrate a nucleus in these cells, and there is no doubt that the red blood-discs of the human adult and of mammalia are devoid, in the histologic sense, of a nucleus capable of differentiation (compare Lavdowsky; Arnold, 96). They are therefore peculiarly modified cells. They possess a somewhat more resistant external zone of exoplasm, which has been interpreted as a cell membrane by certain observers (Lavdowsky), but which does not present the characteristics of a true cell membrane.
If fresh blood be left for some time undisturbed, the blood-discs adhere to each other by their flattened surfaces, grouping themselves in rouleaux,
By certain reagents the clear and transparent contents of the blood-corpuscles can be separated into two substances a staining and a nonstaining. The first consists of the blood pigment, or hemoglobin, which can be dissolved ; the second of a colorless substance, the strorna, which presents itself in various forms (protoplasm of the cell). The stroma probably contains the hemoglobin in solution.
Hemoglobin is a very complex proteid which may be decomposed into a globulin and a pigment hematin. The hemoglobin of the majority of animals crystallizes in the form of rhombic prisms ; in the squirrel, however, in hexagonal plates, and in the guinea-pig in tetrahedra. Hematin combines with hydrochloric acid to form hemin, or Teichinanris crystals, of brownish color, rhombic shape, and microscopic size. They are of much value in lego-medical work, since they may be obtained from blood, no matter how old, and are characteristic of hemoglobin. They may be obtained from very small quantities of blood pigment.
If a small drop of blood pressed from a small puncture is placed on a slide and covered with a cover-glass, the red bloodcells soon become changed. This is due to the evaporation of water in the blood plasma, causing an increased concentration of the sodium chloride contained, which in turn draws water from the blood-cells The shrinkage which follows produces a characteristic change in the form of the cells, which assume a crenated or stellate shape. The red blood-cells of blood mounted in normal salt become crenated in a short time for the same reason. Red bloodcells are variously affected by different fluids. In water they become spheric and lose their hemoglobin by solution. Their remains then appear as clear, spheric, indistinct blood shadows, which may, however, be again rendered distinct by staining with iodin. Dilute acetic acid has a similar but more rapid action, with this peculiarity, that before becoming paler the blood-cells momentarily assume a darker hue. Bile, even when taken from the animal furnishing the blood, exerts a peculiar influence upon the red blood-cells ; they first become distended, and then suddenly appear to explode into small fragments. Dilute solutions of tannic acid cause the hemoglobin to leave the blood-cells, and coagulate in the form of a small globule at the edge of the blood-cell. In alkalies of moderate strength the red blood-cells break down in a few moments.
Fig. 151. Hu- Fig. 152. So-called Fig. 153. Hemin, or man red blood-cells; "rouleau" formation of Teichmann's crystals, from X 1500: a, As seen human erythrocytes ; X blood stains on a cloth, from the surface ; 6, 1500. as seen from the edge.
Fig. 154. " Crenated" human red blood- Fig. 155. Red blood-corpuscles sub cells; X 1500. jected to the action of water; X 1 5: a, Spheric blood -cell ; b, "blood shadow."
Besides the disc-shaped red blood-cells, every well-made preparation shows a few small, spheric, nonnucleated cells containing hemoglobin. These, however, have received as yet but little attention.
M. Bethe makes the statement that human blood and the blood of mammalia contain corpuscles of different sizes, bearing a definite numerical relationship to each other. " If they be classified according to their size, and the percentage of each class be calculated, the result will show a nearly constant proportional graphic curve varying but slightly, according to the animal species." According to M. Bethe, dry preparations of human and animal blood may be distinguished from each other, with the exception of the blood of the guinea-pig which presents a curve identical with that of human blood.
Fig. 156. Red blood-corpuscles from various vertebrate animals; X IOO (Walker's model) : a, From proteus (Olm) ; b, from frog ; c, from lizard ; d, from sparrow ; e, from camel ; /and^-, from man ; h, from myoxus glis ; i, from goat ; k, from musk-deer.
The red blood-cells of mammalia, excepting those of the llama and camel species, are in shape and structure similar to those of man. The red blood-cells of the llama and camel have the shape of an ellipsoid, flattened at its short axis, but also nonnucleated.
We have already made mention of the fact that the embryonal erythrocytes are nucleated ; the question now arises as to how, in the course of their development, they lose their nuclei. Three possibilities confront us : First, either the embryonal blood-cells are destroyed and gradually replaced by previously existing nonnucleated elements ; or, second, the nonnucleated red cells are formed from the nucleated by an absorption of the nucleus (or what appears to be such' to the eye of the observer, Arnold, 96) ; or, finally, the nucleus is extruded from the original nucleated cell. According to recent investigations (Howell) the third possibility represents the change as it actually takes place.
In all vertebrate animals except mammalia, the red bloodcorpuscles are nucleated. They are elliptic discs with a biconvex center corresponding to the position of the nucleus. The bloodcells of the amphibia (frog) are well adapted for study on account of their size. They are long and, as a rule, contain an elongated nucleus with a coarse, dense chromatin framework, which gives them an almost homogeneous appearance. The cell-body may be divided, as in mammalia, into stroma and hemoglobin. When subjected to certain reagents, the contour of the cells appears double and sharply defined. This condition is, however, no proof of the existence of a membrane. The blood-cells of birds, reptiles and fishes are similarly constructed.
The diameter of the erythrocytes varies greatly in different vertebrate animals, but is constant in each species. The red blood-cells of man measure on the average 7.5 // (7.2 fj. to 7.8 //), in their long diameter, and I.6// to 1.9^ in their short diameter. We append a table of their number in a cubic millimeter and size in man and certain animals as compiled by Rollett (71, II) and M. Bethe.
|Species||Size (in cubic millimeter_||No.|
|Monkey (Cercopith. ruber)||70||6,355,000|
|Hare (Lepus cuniculus)||7.16||6,410,000|
|Guinea-pig (Cavia cob.)||7-48||5,859,500|
|Dog (Cam's fam.)||7-2||6,650,000|
|Cat (Felis dom.)||6.2||9,900,000|
|Horse (Equus cab.)||5-58||7,403,50|
|Musk-deer (Moschus jav.)||2.5|
|Spanish goat (Capra /its.)||4.25||19,000,000|
|Domestic chaffinch (Fringilla dom.)||Length, 11.9 Breadth, 6.8|
|Chicken ( Callus dom. )||12. 1||7.2|
|Duck (Anas bosch.)||L. 12.9 B. 8.0|
|Tortoise L.||21.2 B. 12.45||629,000|
|Lizard (Lacerta agil.)||L. 15-75 B. 9-1||1,292,000|
|Snake (Coluber natr.)||L. 22. B. 13.0||829,400|
|Frog (Rana temp.)||L. 22.3 B. 15-7||393,200|
|Toad (Bufo vulg.)||L. 21.8 B. 15-9||389,000|
|Triton (Triton crist.)||L. 29-3 B. 19-5||103,000|
|Salamander (Salamandra mac.}||Length, 37.8 Breadth, 23.8||80,000|
|Carp (Cyprinus Gobio)||L. 58 B. 35||35,000|
|(Proteus angu.)||L. 17.7 B. 10. i|
|Sturgeon (Acipenser St.)||L. 13.4 B. 10.4|
3. White Blood-Corpuscles
The white blood-cells contain no hemoglobin and are nucleated elements which, under certain conditions, possess ameboid movement. Their size varies from 5 fj. to 12 //, and they are less numerous than the red blood-corpuscles, one white blood-cell to from three hundred to five hundred red cells being a normal proportion.
Fig. 157. From the normal blood of man; X I2O (from dry preparation of H. F. Miiller) : a, Red blood-cell ; b, lymphocyte ; c and d, mononuclear leucocytes ; e t transitional leucocyte ; f and g, leucocytes with polymorphous nuclei.
Flemming ascribes a fibrillar structure to the protoplasm of white blood-cells, and was the first to observe a centrosome situated near the nucleus. M. Heidenhain made the observation that the white blood-cells possessed several centrosomes grouped to constitute a microcenter (microcentrum) about which the fibrillar structure of the protoplasm was arranged radially." The meshes of the fibrillar network are filled with a more fluid interfibrillar substance, in which are found the specific granules to be mentioned later. In the normal blood the white blood-cells vary in size and structure, and the following varieties are distinguished : (i) Small and large lymphocytes ; (2) mononuclear leucocytes ; (3) transitional leucocytes ; (4) leucocytes, either polymorphonuclear or polynuclear.
The lymphocytes form about 20% of the white blood-cells.
They vary in size from 5 // to 7. 5 fj. and possess a relatively large nucleus, the chromatin of which is in the form of relatively large granules, which stain rather deeply. The nucleus is surrounded by a narrow zone of protoplasm, often seen clearly only to one side of the cell in the form of a crescent. It does not stain readily in acid dyes.
The leucocytes vary in size from 7 /j. to 10 fi. The mononuclear leucocytes, constituting about 2^ to 4^ of the white blood-cells, have a nearly round or oval nucleus, which usually does not stain very deeply, and which is relatively smaller than that of the lymphocytes. The transitional leucocytes, forming also about 2 ^ to 4^ of the white blood-cells, are developed from the mononuclear variety and represent transitional stages in the development of mononuclear leucocytes to those with polymorphous nuclei. The nucleus in the transitional form is similar in size and structure to that of the mononuclear variety, but Of a more or less pronounced horseshoe-shape. The leucocytes with polymorphous miclei, developed from the transitional forms, are very numerous in the blood, forming about 70^ of the entire number of white blood-cells. They are also the cells which show the most active ameboid movement when examined on the warm stage. They possess variously lobulated nuclei, the several nuclear masses often being united by delicate threads of nuclear substance. A leucocyte with a polymorphous nucleus becomes a polynu clear cell in case the bridges of nuclear substance uniting the several lobules of the nucleus break through. In the protoplasm of the transitional leucocytes, the polymorphonuclear, and the polynuclear forms are found fine and coarse granules. Our knowledge of these granules has, however, been greatly extended since Ehrlich has shown that the granules of leucocytes show specific reactions toward certain anilin stains, or combinations of such stains. He divides the granules of the leucocytes into five classes which he terms respectively a-, (3-, 3-, ?-, and egranules. In human blood are found the a-granules, which show an affinity for acid-anilin stains, are therefore known as acidophile granules, and, since they are most readily stained in eosin, are generally spoken of as eosinophile granules. In normal blood from I ^ to 4^ of the polymorphonuclear leucocytes and now and then a transitional cell have eosinophile granules. The granules are coarse and stain bright red in eosin. Nearly all the leucocytes with granules (from 65 ^ to 68 % of all white blood-cells) have e-granules or, since they are stained in color mixtures formed by a combination of acid and basic anilin stains, neutropliile granules. The neutrophile granules are much finer than the eosinophile and are not stained in acid stains. The y- and ^-granules are stained in basic anilin stains, and are known as basophile granules. They are coarse and irregular, and the leucocytes containing them form from o. 5 % to I <fo of the white blood-cells.
Fig. 158. Ehrlich's leucocytic granules; X '800 (from preparations of H. F. Miiller) : a, Acidophile or eosinophile granules, relatively large and regularly distributed ; e, neutrophile granules ; /3, amphophile granules, few in number and irregularly distributed ; y, mast cells with granules of various sizes ; t!, basophile granules, (a, 6, and e, From the normal blood ; y, from human leukemic blood ; /3, from the blood of guinea-pig.)
It cannot at this time be definitely stated whether the different varieties of granules are to be looked upon as specific products of the protoplasm of the leucocytes, possibly of the nature of granules which may be likened to the secretory granules of glandular cells, or whether they are to be regarded as cell inclusions. It has also not been clearly shown whether one variety of granules may develop into another variety, neutrophile into eosinophile, although this has been suggested. According to Weidenreich, eosinophile granules are thought to represent fragments of erythrocytes, enclosed within the protoplasm of leucocytes.
The polymorphism of the leucocyte-nucleus has induced many investigators to advance the theory that a direct division takes place (fragmentation Arnold, Lowit). Flemming (91, III), however, succeeded in demonstrating that true rnitotic processes actually take place, so that in this respect there really exists no difference between leucocytes and other cells (compare also H. F. Miiller, 89, 91). It is only in the formation of polynuclear leucocytes that the polymorphous nucleus sometimes undergoes a fragmentation process which results in several parts. But even in this case pluripolar mitoses have been observed. A division of the cell-body subsequent to that of the nucleus is lacking in the processes just described. As a result a single cell with several nuclei is formed (polykaryocyte). The fate of such cells is still in doubt.
The extraordinary motility which most leucocytes possess, is in great part responsible for their wide distribution, even outside of the vascular system. They have the power of creeping through the walls of the capillaries (diapedesis, Cohnheim 67, I), and of penetrating into the smallest connective-tissue clefts, between the cells of epithelia, etc., whence they either pass on (migratory cells) or remain stationary for a time. An important function falls to the lot of the leucocytes in the absorption of superfluous tissue particles or in the removal of foreign bodies from certain regions of the body. In the first case they take part in a process of tissue-disintegration ; in the second, they take up the particles by means of their pseudopodia for the purpose either of assimilation or of removal (phagocytes). It may be readily understood that the latter function of the leucocytes is of the greatest importance in certain pathologic processes.
It is somewhat venturesome at the present state of our knowledge to make definite statements as to the origin in postembryonic life of the various forms of white blood-cells above described. The following statement, however, seems warranted from the evidence at hand.
The lymphocytes would seem to be developed in the meshes of adenoid tissue, especially in the so-called germ centers of Flemming, m the adenoid tissue of lymph-glands and lymph-follicles (see under these). Here the cells undergo active karyokinetic division, but where the cells which pass through the process originate is a matter concerning which there is a difference of opinion. Some investigators believe that they penetrate the germ centers with the lymph, and find there a suitable place for division. Again, others see in Flemming's germ centers permanent organs whose elements remain stationary and supply the blood with a continuous quota of lymphocytes. Be this as it may, the fact remains that the germ centers are the most important regions for the formation of lymphocytes. From these they pass out with the lymph current into the blood circulation, or directly into the blood-vessels, there to enter upon the functions which they are called upon to perform. The leucocytes with neutrophile granules are probably developed in the blood and lymph from mononuclear leucocytes which have their origin in the spleen pulp, possibly also in trie bone-marrow. The leucocytes of circulating blood with eosinophile granules in all probability come from mononuclear cells with such granules found in bone-marrow. Under certain conditions it would seem that they also develop in connective tissue. The leucocytes with the basophile granules probably enter the circulation from the connective tissue of certain regions. The lymphocytes and leucocytes found in the blood are also found in the lymph-vessels and lymph-spaces.
4. Blood Platelets Thrombocytes
The third element of the blood is the blood platelets (Bizzozero) (blood-placques, Laker ; hematoblasts, Hayem ; thrombocytes, Deckhuysen). They are extremely delicate and transitory structures, whose existence in the living blood was denied for a long time by many investigators, but whose presence in the wing vessels of the living bat was conclusively demonstrated by Laker (84). They are free from hemoglobin, are of round or oval shape, and in mammals measure about 3 p. in diameter. Owing to the fact that they readily clump together when blood leaves the vessels, and undergo change, it is somewhat difficult to give an estimate of their number. They are said to be present in human blood to the extent of 200,000 to 300,000 in every cubic millimeter. By the exercise of great care and the employment of special methods on the part of a number of recent observers (Detjen, Deckhuysen, Kopsch and Argutinsky), they have been able to show that these structures present a more complicated structure than was formerly thought. When examined in an isotonic salt solution (for mammals 0.9 to 0.95 sodium chlorid solution), they present an oval or short spindle-shaped form, and in them there can be made out a relatively large structure, which stains in certain basic aniline stains and is interpreted as a nucleus (Deckhuysen). When examined after a method suggested by Detjen (with a i per cent, agar solution there is mixed O.6 per cent, sodium chlorid, 0.3 per cent, of sodium metaphosphate and dipotassiuin phosphate; a thin layer of this agar mixture is spread on the slide and a drop of blood mounted between it and the cover), the blood platelets or thrombocytes may be observed on the warm stage for several hours, and it may be seen that they present ameboid movement, in that short, thread-like processes pass out from the cell, which may alter their shape and position and which may be again withdrawn.
When the blood leaves the blood vessels, the blood platelets or thrombocytes break down very quickly, unless the above-mentioned methods are made use of, so that in ordinary
fresh preparations or generally in dried films they are not to be observed in an unaltered state. The nuclei disappear and the protoplasm becomes granular or vacuolated. The breaking down of the blood platelets or thrombocytes is accompanied by the formation of fibrin (coagulation of the blood), the fibrin threads beginning at the borders or processes of the platelets, and radiating in all directions (Kopsch).
Hemokonia. H. F. Muller (96) found in the blood of healthy and diseased individuals highly refractive, colorless, and round (seldom rodlike) bodies, which he terms " hemokonia. " Their numbers vary, although they are normal constituents of the blood. Their nature and origin are obscure. They do not dissolve in acetic acid, nor are they blackened by osmic acid. The latter would seem to indicate that they do not consist of ordinary fat substance, although they are probably composed of a substance closely allied to fat. They are usually i p. in diameter.
Fig. 159- Fibrin from laryngeal vessel of child ; X about 30x3.
5. Behavior of Blood-Cells in the Blood Current
In the circulating blood the behavior of the formed elements varies. The more rapid axial current contains very nearly all the erythrocytes, and as a consequence very few are found adjacent to the walls of the vessels. In the peripheral current, on the other hand, are found most of the leucocytes, and in a retarded circulation they are seen to glide along the walls of the vessels. At the bifurcations of the vessels, especially of the capillaries, the erythrocytes are sometimes caught and elongated by the division of the current, the one-half of the cell extending into the one and the other half into the other branch of the vessel, while the corpuscle oscillates back and forth. When again free the cell immediately resumes its original shape. From this it is seen that erythrocytes are very elastic structures. In the smaller vessels and capillaries, especially when the latter are altered by pathologic conditions, the leucocytes may be seen passing out of the vessels, and it would seem that they are able to penetrate through the walls and even through the endothelial cells lining the blood-vessels (compare also Kolossow, 93). First, they send out a fine process, which is probably endowed with a solvent action. This penetrates the wall of the vessel, after which the remainder of the cell pushes its way through slowly.
B. Lymphoid Tissue, Lymph-Nodules, And Lymph Glands
As to the origin of lymphoid tissue, the lymph-glands, and the spleen, there is still considerable difference of opinion. Most authors believe that these structures are developed from the middle germinal layer (Stohr, 89 ; Paneth ; J. Schaffer, 91 ; Tomarkin). Others believe in an entodermic origin (Kupffer, 92 ; Retterer ; Klaatsch ; C. K. Hoffmann, 93, II).
The framework of lymphoid tissue is a reticular connective tissue (adenoid connective tissue His, 61). This consists of a network of fine fibrils of reticular and white fibrous connective tissue aftd of cells (endoplasm and nuclei) which are situated on the reticulum, often at nodal points. Within its meshes the lymph-cells lie in such numbers and so densely arranged that on microscopic examination the network is almost entirely covered unless very thin sections are used. The cells may be removed from the meshes of the reticulum by stippling and brushing section with a fine brush or by placing sections in a test-tube partly filled with water and subjecting them to vigorous shaking, or, still better, by subjecting sections or pieces of lymphoid tissue to digestion with pancreatin.
Lymph tissue may be diffuse, as in the mucous membrane of the air-passages and as in that of the intestinal tract, uterus, etc. (vid. Sauer, 96). Lymphoid tissue may be also sharply defined in the form of round nodules, simple follicles or nodules. These are either single, solitary lymph-follicles, or gathered into groups, agminated lymph-nodules. They are found scattered in the mucous membrane of the mouth, pharynx, and intestine. In lymph-nodules also we find the characteristic lymph-cells and the adenoid reticulum. As a rule, the former are arranged concentrically at the periphery ; and in the center of the nodule the reticular tissue usually has wider meshes, and the lymph-cells are less densely placed. (Fig. 160.) In the center of the nodule the cells often show numerous mitoses, and it is here that an active proliferation of the cells takes place. The cells may either remain in the lymph-follicle or the newly formed cells are pushed to the periphery of the nodule, and are then swept into the circulation by the slow lymph current which circulates between the wide meshes of the reticular connective tissue. Flemming (85, II) has called that central part of the nodule containing the proliferating cells the germ center or secondary nodule (compare p. 194). The germ centers are transitory structures, and are consequently found in different stages of development. They may even be absent for a time.
Fig. 160. A solitary lymph-nodule from the human colon. At a is seen the pronounced concentric arrangement of the lymph-cells.
The lymph-glands are organs of a more complicated structure, but also consist of lymphoid tissue. They are situated here and there in the course of the lymph-vessel and are widely distributed. Their size varies greatly. In shape they are much like a bean or kidney, and the indentation on one side is known as the hilum. The afferent lymph-vessels, the vasa affercntia, enter at the convex surface of the organ, while the efferent vessels, the vasa efferentia, pass out at the hilum. The whole gland is surrounded by a capsule consisting of two layers : the outer is made up of a loose, and the inner of a more compact, connective tissue in which elastic fibers and a few smooth muscle-fibers are imbedded. Portions of the inner layer pass into the substance of the gland to form septa, or trabeculce, by means of which the organ is divided into a number of imperfectly separated compartments. These trabeculae may be very well developed, as in the lymph-glands of the domestic cattle, or only poorly developed, as in the human lymph-glands, where they are often almost wanting. The lymphoid tissue of the gland is so distributed that at its periphery a large number of more or less clearly defined lymph-nodules are found, which are in part separated from each other by the trabeculae just described, the cortical nodules. The nodules are structural units and have a typical blood supply, and are in structure like the lymph-nodules of simple and agminated follicles above mentioned. They form a peripheral layer which is, however, not clearly defined in the neighborhood of the hilum. This layer is known as the cortex of the lymph-gland. (Fig. 161.) The lymphoid tissue of the interior of the gland, the medullary substance, is in the shape of cords medullary cords which are continuous with the lymphoid nodules of the cortical portion. These connect with each other and form a network of lymphoid tissue, in the open spaces of which lie the trabeculae. At their periphery the nodules and medullary cords are bordered by a wide-meshed lymphatic tissue, the lymph-sinus of the gland, parts of which lie (1) between the capsule and the cortical substance, (2) between the nodules of the latter and the trabeculae, (3) between the medullary cords and the trabeculae, and (4) between the medullary substance and the capsule at the hilum. At the hilum the loose lymphoid tissue represents a terminal sinus (Toldt). These sinuses are lined throughout by endothelial cells, which are continuous with those of the afferent and efferent lymph-vessels. The lymph flows into the gland through the afferent vessels, and passes along into the interior through the spaces offering the least resistance (sinuses). The latter represent those peripheral portions of the nodules and of the medullary cords in which the lymphoid tissue is present in Joose arrangement. The lymph consequently envelops not only the lymph-nodules of the cortical substance, but also the medullary cords, and finally streams into the terminal sinus and then into the efferent channels. As a result the lymph takes with it the newly formed cells of the lymph-nodules and the medullary cords, and passes out richer in cellular elements than on its entrance.
Fig. 161. Transverse section of human cervical lymph-gland, showing the general structure of a lymph-gland ; X J 8. Af> Blood-vessels ; ff, fibrous capsule ; h, hilum ; kz, germ-center ; nl, lymph-nodule ; sc, cortical substance ; gm, medullary substance ; tr, trabeculse ; via, afferent lymph-vessels ; vie, efferent lymph-vessels ( " Atlas and Epitome of Human Histology," Sobotta).
Fig. 162. From a human lymph-gland ; X 2 4- At a are seen the concentrically arranged cells of the lymph-nodules. (Fixation with Flemming's fluid.)
The lymph-glands receive their blood supply mainly through the hilum ; relatively small arterial branches may penetrate the capsule. Generally, a number of arterial branches enter at the hilum, from whence they may pass directly into the medullary substance, or pass for a distance in trabeculae. In their course branches are given off which pass to the medullary cords, in which they break up into capillary vessels situated in the periphery of the cords. These unite to form small veins which anastomose freely, and unite to form larger veins. The cortical nodules receive their blood supply from arterial branches which enter their proximal sides (side toward the hilum) and course through the center of the nodules, giving off capillary vessels which pass, without much anastomosis, to the periphery of the nodules, where they unite to form plexuses ; the capillaries of these plexuses join to form the veins of the nodules, which are thus situated at their periphery. These veins unite to form larger veins, which leave the glands at the hilum (Calvert).
Medullated and nonmedullated nerves penetrate the lymphglands accompanying the blood-vessels on which they terminate.
Hemolymph Glands. A typical lymph-gland possesses afferent and efferent lymph-vessels and a closed blood-vascular system completely separated from the lymph -vascular system, as may have been seen from the foregoing description. Attention has, however, been called in recent years to certain lymph -glands in which the complete separation of the vascular and lymphatic systems does not obtain, glands in which the formed elements of blood and lymph are intermingled in the meshes of the adenoid reticulum, and which contain blood-sinuses in place of the lymph-sinuses observed in the typical lymph-glands. These have been designated as hemolymph glands (Blutiymphdrusen, hemal glands, hemal lymphatic glands). In the typical hemolymph glands there are no afferent and efferent lymphatic vessels; the. glands are intercalated in the vascular system. Certain less clearly defined hemolymph glands possess afferent and efferent lymphatics and bloodsinuses, the two systems being not completely separated. These may be considered transitional forms.
Lymph-glands with blood-sinuses were first described by Gibbes, who found such glands in the region of the renal artery. They were further considered and more fully described by Robertson, to whom the term hemolymph glands is to be credited, and by Clarkson, Vincent and Harrison, Drummond, Warthin, Weidenreich and Lewis. It appears from their description that they are widely distributed among vertebrates, although not equally well developed in the different types studied. Warthin has discussed more fully than other observers the hemolymph glands of man, and his account will here be followed in the main. It may be parenthetically stated that the hemolymph glands are numerous and well developed in the sheep (Warthin, Weidenreich) ; not so well differentiated in the dog and cat ; on the other hand, well developed in the rat (Lewis).
We learn from the account of Warthin that the hemolymph glands are numerous in man, in the prevertebral retroperitoneal region, in the cervical region, and less numerous in the thorax. They vary in size from that of several millimeters to that of several centimeters. They present a variety of structure, depending mainly upon the arrangement of the lymphoid tissue and blood-sinuses. The great majority of these glands show a resemblance in structure to splenic tissue (splenolymph glands) ; others resemble more closely marrow-tissue (marrow lymph-glands). Between the two varieties of lymph-glands there are found transition forms, as also between these and lymph-glands (Warthin).
The hemolymph glands (splenolymph glands) are surrounded by a capsule varying in thickness and composed of white fibrous and elastic tissue and nonstriated muscle-cells. From it trabeculae of the same structure pass into the gland, which after division are lost in the substance of the gland. Beneath the capsule there is found a continuous or discontinuous blood-sinus, bridged over by reticular fibers, from which anastomosing sinuses pass to the interior of the gland. These blood-sinuses are, in part at least, lined by endothelial cells. The sinuses in the gland substance are also bridged by trabeculse and reticular fibers. The sinuses divide the lymphoid tissue into anastomosing masses and cords. This tissue consists of an adenoid reticulum, in the meshes of which are found white and red blood-cells. The small lymphocytes are numerous; next in frequency are found the rnononuclear leucocytes ; transitional and polymorphonuclear cells. Basophile and eosinophile cells are also found. According to Weidenreich, the eosinophile cells are numerous ; he is also of the opinion that the eosinophile granules are derived from disintegrating red blood-cells. In the reticulum and in the blood-sinuses are found mononuclear phagocytes, the origin of which has not been fully determined. Certain observers (Schumacher, Weidenreich) trace their origin to the cells of the reticulum ; Thoma regards them as developed from endothelial cells, while Drummond and others regard them as altered leucocytes. They contain disintegrating red blood-cells and pigment (according to Weidenreich, eosinophile cells). The majority of the hemolymph glands present a hilum through which the blood-vessels enter. The arteries, soon after entering the gland, divide into smaller branches, certain of which communicate directly through blood-capillaries with the blood-sinuses (Lewis) ; others pass to the adenoid tissue. The larger veins are in the trabeculae (at the hilum). On leaving the trabeculae their walls are formed of endothelium and adenoid reticulum, which separates them from the blood-sinuses. They end (or begin) in lacunae with thin walls which are perforated and communicate with the blood-sinuses (Weidenreich). Nerves have been traced to the hemolymph glands by Lewis (dog, rat). They probably end in the involuntary muscle of the capsule and trabeculae. Typical hemolymph glands have no lymph-vessels. In certain glands both blood- and lymph-sinuses are found. In such glands there is apparently an intermingling of blood and lymph, so that red blood-cells may pass into the lymphatics.
The marrow lymph-glands are not so numerous. They have a thin capsule consisting of fibrous tissue but containing little elastic and muscular tissue. The blood-sinuses are not so well developed. In the lymphoid tissue the basophile and eosinophile cells are more numerous than in the splenolymph glands, and large cells similar to the large bone-marrow cells are now and then met with.
As appears from the accounts of the majority of observers who have studied hemolymph glands, they have a hemolytic function, in that the red blood-cells are destroyed in them. Robertson and Clarkson ascribe to them a blood-forming function. This has also been observed by Warthin in the case of marrow lymph-glands, under certain conditions. The hemolymph glands are seats of origin for the white blood-cells which appear also to be destroyed here (eosinophile cells, Weidenreich).
C. The Spleen
The spleen is a blood-forming organ, in which white blood-cells and, in embryonic life and under certain conditions in adult life also, red blood-cells are formed the former in the adenoid tissue (Malpighian corpuscles) and spleen pulp, the latter only in the spleen pulp.
The spleen is covered by peritoneum, and possesses a capsule consisting of connective tissue, elastic fibers, and nonstriated musclecells. This capsule sends numerous processes or trabeculae into the interior of the organ, which branch and form a framework in which the vessels, especially the veins, are imbedded. This connective-tissue framework breaks up to form the reticular tissue which constitutes the ground substance of the spleen.
On examining a section of the spleen with the low-power magnifying glass, sections of the trabeculae, and round or oval masses of cells, having a diameter of about 0.5 mm., and in structure and appearance similar to the lymph-nodules (Malpighian corpuscles), are clearly defined ; between and around these structures is a tissue rich in cells, blood-vessels and blood-corpuscles, known as the spleen pulp.
The organ has a very typical blood supply. Its arteries enter at the hilum, or indented surface, and its veins pass out at the same place. On the penetration of the vessels through the capsule, the latter forms sheaths around them (trabeculse), but as soon as the arteries and veins separate, the trabeculae envelop the veins alone. The arteries break up into smaller branches, which in turn divide into a large number of tuft-like groups of arterioles. Soon after their separation from the veins, the adventitia (outer fibrous tissue coat) of the arteries begins to assume a lymphoid character. This lymphoid tissue increases here and there to form true lymphoid nodules, possessing all the characteristics already mentioned reticular tissue, germ centers, etc. These are the Malpighian bodies, or corpuscles ; they are not very plentifully represented in man. The Malpighian bodies with their germ centers are formative centers for the lymphocytes. The newly formed cells pass into the pulp and mix with its elements, which are then bathed by the blood emptying from the arterial capillaries into the channels of the pulp. The lymphoid sheaths and nodules derive their blood supply from arteries which arise from the lateral branches of the splenic vessels, and which divide into capillaries inside of the lymph sheaths or nodules, and only assume a venous character outside of the lymphoid substance. These vessels constitute the nutritive vascular system of the spleen.
Fig. 163. Portion of section of human spleen ; X 15- The figure gives a general view of the structure of the spleen : a, Arteries with lymphoid sheaths ; cf, fibrous capsule ; Mk, Malpighian corpuscle ; //, spleen pulp ; tr, trabeculae ; v, vein in trabecula ("Atlas and Epitome of Human Histology," Sobotta).
The small arterial branches above mentioned break up into very fine arterioles which gradually lose their lymphoid sheath, so that branches with a diameter of 0.02 mm. no longer possess a lymphoid sheath, but again assume an adventitia of the usual type. The smallest arterioles now pass over into capillaries which are for a time accompanied by the adventitia (capillary sheath), while the terminal branches have the usual structure of the capillary wall and are gradually lost in the meshes of the pulp. (See below.) On the other hand, the beginnings of the venous capillaries may be distinctly seen in the pulp spaces. Groups of these capillaries combine to form larger vessels, which, however, still retain a capillary structure, and these again form small veins which unite to form the larger veins.
F. P. Mall, whose recent contributions on the structure of the spleen have greatly extended our knowledge of the microscopic anatomy of this organ, states that the trabecular and vascular systems together outline masses of spleen pulp about i mm. in diameter, which he has named spleen lobules. Each lobule is bounded by three main interlobular trabeculae, each of which sends three intralobular trabeculae into the lobule which communicate with each other in such a manner as to divide the lobule into about ten smaller compartments. An artery enters at one end of the lobule and, passing up through its center, gives off a branch to the spleen pulp found in each of the ten compartments formed by the intralobular trabeculae. . The spleen pulp in these compartments is arranged in the form of anastomosing columns, or cords, to which Mall has given the name of pulp cords. The branches of the main intralobular artery, going to each compartment, divide repeatedly ; the terminal branches course in the spleen-pulp cords, and in their path give off numerous small side branches which end in small expansions known as the ampulla of Thoma. An ampulla of Thoma may be divided into three parts. The first part, which is the ampulla proper, is lined by spindle-shaped cells, directly continuous with the endothelial cells of the artery. The second third, which often communicates with neighboring ampullae, contains large side -openings. The remaining third, which is the intermediary segment of Thoma (Thoma 's Zivisckenstuck], is difficult to demonstrate. It is bridged over by fibrils of reticulum, and its communication with the vein is not wide. The circulation through the spleen is therefore not a closed one, through a system of capillaries completely closed, but rather through spaces in the spleen -pulp, certain of which are more direct, leading from the terminal arteries to the veins. According to this view, then, "the blood passes from the ampullae into the pulp spaces, then through the pores into the walls of the veins to form columns of blood discs which are pushed from the smaller to the larger veins of the spleen." The pulp spaces usually contain very few bloodcorpuscles, in preparations fixed and prepared in the usual way, since on removal from the animal the muscular tissue of the capsule and trabeculae contracts and presses the blood from pulp spaces into the veins. If, however, the muscular tissue of the spleen is paralyzed before the tissue is fixed, numerous blood-corpuscles are found in the pulp spaces. In the above account of the ultimate distribution of the splenic vessels we have followed very closely the recent observations of F. P. Mall. The accompanying diagram (Fig. 164), slightly, though immaterially, modified from one given by F. P. Mall, shows clearly the trabecular and vascular systems of a spleen lobule. In larger spleens there may be some two hundred thousand of these lobules. In a dog weighing 10 kg. there are on an average some eighty thousand (F. P. Mall).
Fig. 164. Diagram of lobule of the spleen (Mall, "Johns Hopkins Hospital Bulletin," Sept., Oct., 1898).'
The splenic pulp consists of a reticulum, in the meshes of which are found (i) fully developed red blood-cells; (2) now and then nucleated red blood-cells; (3) in many animals giant cells ; (4) cells containing red blood-corpuscles and the remains of such, with or without pigment ; (5) the different varieties of white blood-cells, especially a relatively large proportion of mononuclear leucocytes. Pigment granules, either extra- or intracellular, also occur in the splenic pulp. The pigment probably originates from disintegrating erythrocytes. Besides these are found, especially in teased preparations, long, spindle-shaped and flat cells, which are probably derivatives of the connective-tissue cells of the pulp and of the endothelium and muscular fibers of the vessels.
Fig. 165. Cells containing pigment, blood-corpuscles, and hemic masses from the spleen of dog ; X 1800 (from cover-glass of H. F. Miiller).
Fig. 166. From the human spleen ; X 8 (chrome-silver method) : a, Larger fibers of a Malpighian body ; b, reticular fibrils (Gitterfasern).
In embryonic life and under certain conditions in postembryonic life (after severe hemorrhage and in certain diseases) red blood-cells are developed in the spleen pulp. The nucleated red blood-cells from which they develop may lose their nuclei in the spleen pulp or only after entering the circulation (compare Bone-marrow).
Lymphatic vessels have been observed in the capsule and trabeculae, but not in the spleen pulp nor Malpighian corpuscles.
The spleen receives medullated and nonmedullated nerve-fibers ; the latter are much more numerous. The medullated nervefibers are no doubt the dendrites of sensory neurones. Their mode of ending has, however, not been determined. It is probable that they will be found to terminate in the fibrous-tissue coat of the vessels, and in the trabeculae and capsule. The nonmedullated nerve-fibers, no doubt the neuraxes of sympathetic neurones, are very numerous ; they enter the spleen with the artery and mainly follow its branches. By means of the chrome-silver method, Retzius (92) has shown that in the rabbit and mouse these nervefibers follow the vessels, forming plexuses which surround them, the terminal branches of these plexuses terminating in free endings in the muscular coat of the arteries. Here and there a nerve-fiber could be traced into the spleen pulp. The mode of ending of such fibers could, however, not be determined. The nonstriated musclecells of the trabeculae and capsule no doubt also receive their innervation from the nonmedullated nerves (neuraxes of sympathetic neurones).
D. The Bone-Marrow
The ingrowing periosteal bud which ushers in the process of endochondral ossification constitutes the first trace of an embryonal bone-marrow (compare p. 117). It consists mainly of elements from the periosteum which penetrate with the vascular bud and later form the entire adult bone-marrow. The red bone-marrow is formed first. This is present in embryos and young animals, and is developed from the above elements during the process of ossification. As Neumann (82) has shown, the red bone-marrow of the human embryo is first formed in the bones of the extremities and gradually replaced in a proximal direction, so that in the adult it is found only in the proximal epiphyses, in the flat bones and in the bodies of the vertebras. In the remaining bones and parts of bones the red bone-marrow is replaced by the yellow bone-marrow (fatmarrow).
As a result of hunger and certain pathologic conditions the yellow bone-marrow changes into a gelatinous substance, which, however, may again assume its original character.
The red bone-marrow, surrounded by a delicate fibrous-tissue membrane, the endosteum, is a tissue consisting of various cellular elements imbedded in a matrix of reticular tissue, which has been demonstrated by Enderlen with the chrome-silver method, and which is similar to the adenoid reticulum. Aside from these cellular elements, the marrow contains numerous vessels (see below), fixed connective-tissue cells, etc.
The typical cellular elements of red bone-marrow are:
- The Marrow-cells, or Myelocytes. These are cells, slightly larger than the leucocytes, possessing a relatively large nucleus of round or oval shape, rarely lobular, containing a relatively small amount of chromatin. In the protoplasm of these cells are found (in man) neutrophile granules and now and again small vacuoles. They are said to contain various pigment granules. These cells are not found in normal blood, but are found in circulating blood in certain forms of leukemia, where they may be distinguished from the mononuclear leucocytes partly by their structure, more particularly by the presence of neutrophile granules not found in the mononuclear leucocytes.
- Nucleated Red Blood-cells containing Hemoglobin. Two varieties of these cells are recognized structurally, with intermediary stages, as one variety is developed from the other. The erythroblasts, being genetically the earlier cells, possess relatively large nuclei with distinct chromatin network, surrounded by a protoplasm tinged with hemoglobin, and are often found in a stage of mitosis. The other variety of nucleated red blood-cells, the normoblasts, are developed from the erythroblasts. They contain globular nuclei, staining deeply, in which no chromatin network is recognizable, and surrounded by a layer of protoplasm containing hemoglobin. The normoblasts are changed into the nonnucleated red blood-discs by the extrusion of the nucleus. This process occurs normally in the red bone-marrow, or in the venous spaces of the bone-marrow (see below). In certain pathologic conditions, nucleated red blood-cells are found in the circulation.
- Cells with Eosinophile Grannies. In the red bone-marrow are found numerous eosinophile (acidophile) cells, some with round or oval nuclei (mononuclear eosinophile cells), others with horseshoe-shaped nuclei (transitional eosinophile cells), and still others with polymorphous nuclei. The latter, which are the most numerous, are no doubt the mature cells, and are identical with those elements of the blood having the same structure.
- Cells with basophilic granules. In the bone-marrow are found mononuclear cells in which basophile granules may be differentiated with special reagents.
- The various forms of leucocytes and the lymphocytes found in blood and lymph.
- The giant cells (myeloplaxes), which are situated in the center of the marrow, and contain simple or polymorphous nuclei, or lie adjacent to the bone in the form of osteoclasts, which are, as a rule, polynuclear (compare p. 120). The physiologic significance of the giant cells is still obscure. They probably originate from single leucocytes by an increase in size of the latter, and not, as many assume, from a fusing of several leucocytes. The giant cells are endowed with ameboid movement, and often act as phagocytes (the latter quality is denied them by M. Heidenhain, 94).
Fig. 167. Cover-glass preparation from the bone-marrow of dog ; X 1200 (from preparation of H. F. Miiller) : a, Mast-cell ; b, lymphocyte ; c, eosinophile cell ; d, red blood-cell ; e, erythroblast in process of division ; f, f, normoblast ; g, erythroblast. Myelocyte not shown in this figure.
Fig. 168. From a section through human red bone-marrow ; ^ 680. Technic No. 216 : a, f, Normoblasts ; b, reticulum ; c, mitosis in giant cell ; </, giant cell ; e, h, myelocytes ; g, mitosis ; ;', space containing fat-cells.
M. Heidenhain (94) has made a particular study of the giant cells. According to him the nuclei of these cells take the form of perforated hollow spheres whose thick walls contain " endoplasm." The latter is continuous with the remaining protoplasm of the cell, the " exoplasm " through the " perforating canals" of the nuclear wall. The exoplasm is arranged in three concentric layers, separated from each other by membranes, the external membrane of the outer zone being the membrane of the cell. The outer layer or marginal zone is of a transient nature, but is always renewed by the cell. Thus, the cell-membrane is replaced by the secondary membrane situated between the second and third zone. According to the same author the functions of the giant cells appear to consist in " the selection and elaboration of certain albuminoid substances of the lymph and blood currents, which are later returned to the circulation." The number of centrosomes occurring in the mononuclear giant cells of the bone-marrow is very large, and in some cases, as in a pluripolar mitosis, may even exceed one hundred in number.
The distribution of the blood-vessels in the bone-marrow is as follows : On entering the bone the nutrient arteries divide into a large number of small branches, which then break up into small arterial capillaries. The latter pass over into relatively large venous capillaries with relatively thin walls, which appear perforated in certain places, so that the venous blood pours into the spaces of the red bone-marrow where the current is very slow. The blood passes out by means of smaller veins formed by the confluence of the capillaries whigh collect the blood from the marrow. It is worth mentioning that the venous vessels, while inside of the bone-marrow, possess no valves ; but, on the other hand, they have an unusually large number of valves immediately after leaving the bone.
Yellow bone-marrow is derived from red bone-marrow by a change of the marrow-cells into fat-cells. The gelatinous marrow, on the contrary, is characterized by the small quantity of fat which it contains. Neither the yellow nor the gelatinous bone-marrow is a blood-forming organ (compare Neumann, 90; Bizzozero, 91 ; H. F. Miiller, 91 ; van der Stricht, 92).
E. The Thymus Gland
The thymus gland is usually considered as belonging to the lymphoid organs, although in its earliest development it resembles an epithelial, glandular structure. In the epithelial stage, this gland develops from the entoderm of the second and third gill clefts. Mesodermic cells grow into this epithelial structure, proliferate and then differentiate into a tissue resembling adenoid tissue. It retains this structure until about the end of the second year after birth, when it slowly begins to retrograde into a mass of fibrous tissue, adipose tissue, and cellular debris, which structure it presents in adult life.
By means of connective-tissue septa, the thymus is divided into larger lobes, and these again into smaller lobes, until finally a number of small, irregularly spheric structures are formed the lobules of the gland. These are, however, connected by cords of lymphoid tissue, the so-called medullary cords. The lobules of the thymus gland consist of a reticular connective tissue much more delicate at the periphery than at the center of the lobule. The reticulum supports branched connective -tissue cells, with relatively large nuclei. In the meshes of the reticular tissue are cellular elements, in structure similar to the lymphocytes, which are more numerous at the periphery of the lobule than at its center, so that we may here speak of the lobule as divided into a cortical and a medullary portion. Leucocytes with polymorphous nuclei, also leucocytes with eosinophile granules, are also found. The medullary portion is usually entirely surrounded by the cortical substance, but may penetrate to the periphery of the lobule, allowing the blood-vessels to enter and leave at this point. In the cortical substance occur changes which result in the formation of structures closely resembling the cortical nodules of lymph-glands.
Fig. 169. A small lobule from the thymus of child, with well-developed cortex, presenting a structure similar to that of the cortex of a lymph-gland ; X 6 : a, Hilus ; b, medullary substance ; <r, cortical substance ; d, trabecula.
Fig. 170. Hassal's corpuscle and a small portion of medullary substance, sliowing reticulum and cells, from thymus of a child ten days old.
Until recently, little was known of the significance of this organ. A careful study revealed a similarity between certain cellular elements of the thymus and the constituents of the blood-forming organs, a similarity still more striking from the presence of nucleated red blood-cells in the thymus. Logically, then, the embryonal thymus is to be regarded as one of the blood-forming organs (Schaffer, 93, I).
During embryonic life from the fourth month on and for some time after birth, there are found in the thymus peculiar epithelial bodies, known as the corpuscles of Hassal. They are spheric structures, about o. i mm. in diameter, whose periphery shows a concentric arrangement of the epithelial cells. In their central portions are found a few nuclear and cellular fragments. These bodies occur only in the thymus gland. They are remnants of the primary epithelial, glandular structure of the thymus, and are formed by an ingrowth of mesoderm which breaks down the epithelium into small irregular masses, mechanically compressed by the proliferating mesoderm.
The thymus gland has a relatively rich blood supply. Arterial branches enter the lobules usually near the medullary cords and form capillary networks at the boundaries of the medullary and cortical portions ; from this anastomosing capillaries radiate to the periphery of the lobules, joining to form a relatively dense capillary network under the connective-tissue covering. The veins arise from this capillary network and are situated mostly in the interlobular connective tissue. Certain of the veins are in the medullary portions of the lobules, where they accompany the arteries (Kolliker, v. Ebner).
The lymph-vessels are in the interlobular connective tissue in close apposition with the adenoid tissue.
Nerve-fibers accompanying the blood-vessels have been observed.
Technic (Circulatory System)
To obtain a topographical view of the layers composing the heart and vessels, sections are made of tissues that have been fixed in Miiller's fluid, chromic acid, etc. If the specimens are to be studied in detail, small pieces must be used, and are best fixed in chromic-osmic mixtures or corrosive sublimate. Celloidin imbedding is recommended for general topographic work. The further treatment is elective.
The endothelium of the intima may be brought to view by silver nitrate impregnation methods, by injecting silver solutions into the vascular system. The endothelial elements of the smallest vessels and capillaries are then clearly defined by lines of silver. Larger vessels must be cut open, the intima separated, and pieces of its lamellae examined.
Elastic elements, plates and networks are best observed in the tunica media of the vessels, very small pieces of which are treated for some hours with 33% potassium hydrate.
The appropriate stains for sectionwork are those which bring out the elastic elements and the smooth muscle-cells. For the former, orcein is used.
For demonstrating the distribution of the capillaries, the reader is referred to the injection methods. The lymph-capillaries are injected by puncture ; compare also the methods of Altmann.
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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.
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