848 ANATOMY [ANATOMY OF iG. 33. A, polyhedral gland cells from the liver; B, spheroidal gland cells from the saliva. are either spheroidal or cylindrical; but as the cavities lined by these cells are shut off from the air, the cells ought rather to be referred to the eudothelial than the epithelial series of structures. Cilia occasion currents in the fluid in which they move, and play an important part in the economy of many animals; in some of the invertebrata they serve as organs of locomotion, in others they propel currents over respiratory surfaces, and iu others aid in bringing food within the animal s reach. Spheroidal or glandular epithelium is situated on the free surface of the follicles or ultimate secreting apparatus of glands, and the commencement of gland ducts. The cells are often spheroidal in form, though not unfrequently they are polyhedral. Their contents are specially differentiated into the secretion of the particular gland in which they are situated. The epithelial cells of a Secreting Gland rest upon a sub- epithelial tissue. Not unfrequently this tissue has the appearance of a membrane; it represents, indeed, the base ment membrane of Bowman, and is called membrana pro- pria. Deeper than this apparent membrane is a delicate connective tissue in which the blood and lymph vessels and the nerves of the gland ramify. The anatomical structures necessary for secretion are cells, blood-vessels, and nerves- The blood-vessels convey the blood from Avhich the secretion has to be derived; the cells, as Goodsir showed by a variety of proofs, are the active agents in separat ing the secretion from the blood; the nerves regulate the size of the blood-vessels, and therefore the amount of blood which circulates through the gland, and perhaps also exercise some direct influence on the activity of the cells. The connective tissue and the membrana propria are merely supporting structures for the cells, vessels, and nerves. All secreting glands have the same general type of structure, though they differ from each other, as will be pointed out when the individual glands are described, in the degree of complexity in which their constituent parts are arranged. Transitional epithelium is the name applied to epithelial cells, situated on some free surfaces, which possess transi tional forms either between the columnar and tessellated epithelia, or the columnar and spheroidal. The epithelium of the mucous lining of the bladder is transitional between the columnar and scaly varieties; and in many glands the continuity of the epithelial layer from the spheroidal epithelium of the gland folh cles to the columnar epi thelium of the ducts is preserved by the interposition of intermediate transitional forms of cells. The epithelial surfaces of the upper part of the mucous lining of the nose and of the back of the tongue are specially modified in connection with the senses of smell and taste localised in those regions, as will afterwards be considered when their anatomy is described. ENDOTHELIUM. The free surfaces covered by an endo thelium are the serous membranes, the inner surface of the walls of the lymph and blood vessels and of the heart, the synovial membranes of the joints and of synovial bursse, the free surface of the osseous and membranous labyrinth of the internal ear, and the free surface of the ventricular cavities of the brain and central canal of the spinal cord. The tubes, canals, and cavities lined by an endothelium are shut off from all communication with the external atmosphere. The cells of the endothelium are arranged so as to give perfect smoothness to the sur face which they cover. In the blood and lymph vessels this smoothness of surface is in order to facilitate the flow of the blood and lymph in the course of the circulation. The serous and synovial membranes are found covering the surfaces 01 parts which move on eacn other, and the smoothness of their respective surfaces, by permitting free dom of movement, diminishes the friction. Each Serous Membrane consists of a portion which invests Serou the viscus or organ, named the visceral layer, and a portion mem ~ which lines the walls of the cavity in which the organ is situated, named the parietal layer. Between these two layers is the so-called serous cavity, the wall of which is formed by the smooth surfaces of both the parietal and the visceral layers. The serous membranes are as follows : The two pleurae situated in the cavity of the chest, one invest ing each lung, and lining the interior of that part of the thoracic cavity in which the lung is situated; the pericar dium, which invests the heart, and lines the bag in which the heart is contained; the peritoneum, which invests the abdominal viscera, and lines the, abdominal cavity; and the arachnoid membrane, which invests the brain and spinal cord, and is regarded by many as lining the dura mater, which encloses these important organs. The smooth free surfaces of the serous membranes are moistened by a limpid fluid, or serum, which facilitates their movement on each other, just as the free smooth surfaces of the synovial membranes are lubricated by the viscid synovia which they secrete. Endothelial cells form usually only a single layer, and are, as a rule, flattened scale-like cells, arranged after the manner of a tessellated epithelium. Endothelium, like epithelium, is non-vascular, and, so far as is known, non-nervous. The endothelial cells rest upon a sub-endothelial tissue, consisting of a delicate modification of the fibrous form of connective tissue. Here, as in the surfaces covered by epithelium, a basement membrane was at one time sup posed to intervene between the cells and the connective tissue; but it is now believed that the cells are in direct contact, by their deeper surface, with the connective tissue itself. In the serous membranes and in the coats of the larger blood-vessels elastic fibres are present in considerable numbers in the sub-endotlielial tissue, which serves as the framework of support for the blood and lymph vessels and the nerves of the part. In the serous membranes the lymph- vessels are very abundant in the sub-epithelial tissue, where they form a layer parallel to the free surface of the mem brane, from which short vessels pass vertically to open by minute orifices into the serous cavity. The serous mem branes are attached by the sub-endotlielial connective tissuo to the organs which they invest. The endothelium of the Serous Membranes consists of irregular and squamous cells, the edges of which may be smooth or slightly serrated. The cells are closely adapted, to each other by their edges, so as to form a continuous smooth layer, which forms the free surface of the serous membrane. Scattered irre gularly over this surface are the minute orifices, or stomata, which open into lymphatic vessels. The cells which sur round the stomata differ in form and appearance from tliA rWlivmrv Mirlntriplinin Fla - 34. Kr.aothelial colls from i tne ordinary enaotnciium , pci . itom . al s . rous mcmbrnne- Th they are smaller, and are polyhedral, their contents are granular, and the nucleus is more distinct. The endothelium lining the Lymphatic Vessels consists of flattened cells, which, instead of having an irregular shape, are elongated spindles, slightly sinuous in outline. The endothelium of the lymphatics is continuous with that of the serous membranes through the stomata, so that the cavities of the serous membranes are now regarded as great lymph-sacs. the roe stomata may bo seen surrounded by polyhedral nucleated cells; the one to the left is closed. The light band marks the position of a vertical lym
phatic vessel. (After Klein.)
