1082 GENERAL ANATOMY OR HISTOLOGY. 



separated from the blood-corpuscles. In human blood the crystals are elongated 

 prisms (Fig- 601, A), and in the majority of animals belong to the rhombic system, 

 though in the squirrel hexagonal plates are met with. Small brown prismatic 

 crystals of hcemin (Fig. 601, B] may be obtained by mixing dried blood with common 

 salt and boiling with a few drops of glacial acetic acid. A drop of the mixture 

 on a slide will show the characteristic crystals on cooling. Ilcematoidin crystals 

 (Fig. 601, (7) occur sometimes in old blood-clots. 



LYMPH AND CHYLE. 



Lymph is a transparent, colorless or slightly yellow fluid, which is conveyed 

 by a set of vessels named lymphatics into the blood. These vessels arise in 

 nearly all parts of the body as lymph-capillaries. They take up the blood-plasma 

 which has exuded from the blood-capillaries into the tissue-spaces where it has 

 nourished the tissue-elements, and return it into the veins close to the heart, there 

 to be mixed with the mass of blood. The greater number of these lymphatics 

 empty themselves into one main duct, the thoracic duct, which passes upward along 

 the front of the spine and opens into the large veins on the left side of the root of 

 the neck. The remainder empty themselves into a smaller duct which terminates 

 in the corresponding veins on the right side of the neck. 



Lymph, as its name implies, is a watery fluid of sp. gr. about 1.015, closely 

 resembling the blood-plasma, but more dilute and containing only about 5 per cent, 

 of proteids and 1 per cent, of salts and extractives. When examined under the 

 microscope, leucocytes of the lymphocyte class are found floating in the trans- 

 parent fluid. They are always increased in number after the passage of the 

 lymph through lymphoid tissue, as in lymphatic glands. They are constantly 

 furnishing a fresh supply of colorless corpuscles to the blood. 



Chyle is an opaque, milky-white fluid, absorbed by the villi of the small 

 intestine from the food, and carried by a set of vessels similar to the lymphatics, 

 named lacteals, to the commencement of the thoracic duct, where it is inter- 

 mingled with the lymph and poured into the circulation through the same chan- 

 nels. It must be borne in mind that these two sets of vessels, lymphatics and 

 lacteals, though differing in name, are identical in structure, and that the character 

 of the fluid they convey is different only while digestion is going on. At other 

 times the lacteals convey a transparent, nearly colorless lymph. 



Chyle exactly resembles lymph in its physical and chemical properties, except 

 that it has, in addition to the other constituents of lymph, a quantity of finely 

 divided fatty particles, the so-called "molecular basis of chyle," to which the 

 milky appearance is due. It contains a little more proteid than lymph, but the 

 chief difference lies in the large quantity of fats, soaps, lecithin, and cholesterin 

 present in the former. Lymph and chyle, containing, as they do, fibrinogen in 

 solution and leucocytes, clot on removal from the body, the coagulum being free 

 from red cells, and presenting a clear or whitish jelly-like appearance. 



EPITHELIUM. 



All the surfaces of the body the external surface of the skin, the internal 

 surface of the digestive, respiratory, and genito-urinary tracts, the closed serous 

 cavities, the inner coat of the vessels, and the acini and ducts of all secreting and 

 excreting glands, the ventricles of the brain, and the central canal of the spinal 

 cord are covered by one or more layers of simple cells, called epithelium or 

 epithelial cells. These cells are also present in the terminal parts of the organs 

 of special sense, and in some other structures, as the pituitary and thyroid bodies. 

 They serve various purposes, forming in some cases a protective layer, in others 

 acting as agents in secretion and excretion, and again in others being con- 

 cerned in the elaboration of the organs of special sense. Thus, in the skin, 

 the main purpose served by the epithelium (here called the epidermis) is that of 

 protection. As the surface is worn away by the agency of friction or change of 



