GLAND, COMPARATIVE ANATOMY OF. Neither a definition nor a classification of glands has yet been agreed on by comparative anatomists. Since the word itself offers no clew to its real mean ing, we must attempt to define it from universally accepted examples. Among these may be men tioned salivary glands, lachrymal glands, sweat glands, and poison-glands. All of these are organs which produce some particular substance from the blood with which they are supplied; furthermore, this substance is not cellular nor living, but ,is a mere chemical product. These facts give us a clew to our definition, and we may say that a gland is any cell or group of cells whose function is the production of a chemical substance, usually fluid, peculiar to itself. Such a definition will not include all those organs to which the name gland is given, but it will include all to which it ought to be applied. As an ex ample of the incorrect use of the term, we may refer to 'reproductive' or `genital' gland, as ap plied to the testis or ovary. These organs are not in any true sense glands, for they do not produce any chemical substance peculiar to themselves, but are simply the portions of the body where those cells are formed from which the next gen eration arises. So also the use of `gland' in con nection with the suprarenal capsule, the pitui tary body, and the pineal body is incorrect and confusing.
Various classifications for glands have been proposed based on their structure, whether uni cellular or multicellular, simple or branched, etc.; but it is perhaps as natural and certainly as con venient to arrange them according to their func tion. Thus we may class those which open on the surface of the body and are developed in the skin as tegumentary glands; those connected with the process of digestion as digestive glands; those connected with the blood system as vascular glands; those associated with the respiration as respiratory glands; and those connected with the reproductive organs as reproductive glands. The tegumentary glands may well be grouped accord ing to their structure, as unicellular and multi cellular glands, and the same classification is often applied to all glands.
Unicellular glands are everywhere abundant in invertebrates, but in vertebrates they are con fined almost entirely to the lower forms. Multi cellular glands are to be regarded as aggrega tions of unicellular glands in one region. Soon the multicellular gland differentiates into a se cretory and an efferent part, or duct. Multicellu lar glands with ducts are tubular or aeinous. Globular glands also occur in the epidermis of Amphibia. The tubular glands may be simple, branched, or anastomosing. Tubular glands first occur in the Amphibia in a few cases, but they are very abundant as sweat-glands in mammals. Acinous glands first appear in birds as uropygial glands, where they occur on the rudimentary tail and produce the oil for oiling the feathers. Among mammals acinous glands are highly developed and are of two kinds: namely, sebaceous glands and milk-glands. The sebaceous glands open on the rim of the eyelid, and on other special parts of the body, in intimate relation with hairs. In
addition to oiling the hair, these glands probably have a sexual function in mammals, since their secretion has a decided odor.
Milk-glands, characteristic of the mammalia, have probably a common ancestry with sebaceous glands, or they may even consist, indeed, of a group of highly specialized sebaceous glands. The glands of Echidna occur in the mammary pocket. Into this pocket the immature hatched young are placed, and there they are nourished by a se creted substance which is not like the milk of other mammals. The glands are in connection with hair-follicles. Among marsupials there is a larger number of mammary pockets; but these pockets have lost their protective function, which latter function is assumed by the marsupium. In Ornithodelphia the nutritive function is sub served by the secretions of sweat-glands. The re gion of the integument, at which the glands open, is usually modified for the purpose of transmit ting to the young the secretion of the glands. There are three types of these openings. The athelous type is found only in the lowest mam mals; here the glands open diffusely on the sur face without a true nipple. In the second typo the glands open at the base of an elevated crater, and in the third the region of their opening is elevated to the apex of a cone. To the second type belong the glands of the carnivores and un gulates, and to the third class belong those of marsupials and primates. The number of these glands varies in different kinds of animals and with the number of young produced in a litter. In man two is the normal number, but in a number of cases supernumerary nipples have been recorded both for males and females. Such cases are atavistic and point back to the condi tion which existed in man's ancestor.
The. liver and pancreas are recognized as the largest of the digestive glands, to which group also belong the salivary glands, gastric glands, and intestinal glands. The poison-glands of snakes, as modified salivary glands, also belong here. Of vascular glands the kidneys are decided ly the most important, while respiratory glands include the arytenoid and tracheal. Of reproduc tive glands we find a great variety among both invertebrates and vertebrates, such as the yolk and shell glands, and in mammals the Cowper's and prostate glands.
Any attempt to classify glands according to the substances they produce is unsatisfactory. Thus, if we attempt to separate secreting from excreting glands, we find that while the kidneys clearly belong in the latter class, the liver belongs to both, though chiefly secretory, and sebaceous glands are also difficult to classify. Perhaps the most obscure organs of this class are the so called 'ductless' glands, the spleen, thyroid, and thymus. They seem to produce some substance of great importance to the well-being of the body; but what it is, and how it affects the organism, are still involved in difficulties. However, as they are all closely associated with the blood system, they may well be called vascular glands.