672 NUTRITION To the group of serous glands belong the parotid of man and the majority of animals, the submaxillary gland of the rabbit, and some of the glands of the tongue ; to that of mucous glands belong the submaxillary and sublingual glands in most animals, some of the glands of the tongue, and the oesophageal glands. Glands belonging to the former of these classes secrete a fluid containing some, though it may be only a small quantity, of a proteid coagul- able by heat, and resembling, if not identical with, serum- albumin ; the mucous glands, on the other hand, as their name implies, secrete a liquid free from albumin, but con taining more or less mucin. In the serous glands the epithelium lining the acini is composed of comparatively small, rounded, or polygonal cells, of which the outlines are not very distinct until acted upon by certain reagents. No cell -wall is present; the protoplasm, which is not coloured by carmine, presents many dark granules, and surrounds an irregularly saccular or rounded nucleus which is coloured by carmine. In the mucous glands the characteristic (mucous) cells of the alveoli are large and clear, very faintly granular, with a rounded or oval nucleus near their periphery sur rounded by a trace of protoplasm. They possess a cell- wall, and a strongly refracting process which springs from the cell in the neighbourhood of the nucleus. In addition to the characteristic mucous cells there are found in the alveoli of most mucous salivary glands when examined in a state of rest, situated at some parts of the periphery, i.e., lying more internal than, or nearer to, the membrana propria than the mucous cells, half-moon-shaped aggregations of small cells, possessed of a round nucleus easily stained with carmine, and containing albumin ; to these aggregations the term of demilunes or lunulas of Gianuzzi has been applied. In some cases we find alveoli in which these small cells are not arranged in demilunes, but form a row of cells lying external to the mucous cells, and completely encircling them (see fig. 3). As has been said, in certain mucous glands the mucous cells are supplemented by the cells of the demilunes, though there are other mucous glands, as those of the tongue, where the typical mucous cells alone occur. There are glands, and the submaxillary of man is an example, which are termed mixed glands, inasmuch as some of the acini have all the characters of serous, others of mucous glands. The submaxillary gland of the guinea-pig possesses, accord ing to Klein, a structure undistinguishable from that of the pancreas. We shall not enter, in this place, into a detailed descrip tion of the innervation of any one of the salivary glands, but shall confine ourselves to the following statements. Each salivary gland is supplied by at least three classes of fibres, viz., secretory, vase-constrictor, and vaso-dilator fibres, of which the first and the third are conveyed to the glands in branches of cerebral nerves ; these are, the chorda tympani for the submaxillary and sublingual, and the auric ulo- temporal (which, however, derives them through communications with the otic ganglion from the glosso- pharyngeal nerve) for the parotid. The second class, the vaso-constrictor or vaso-motor fibres, run in sympathetic trunks. When, therefore, one of the cranial branches supplying a gland is stimulated there occur two acts, viz., secretion and simultaneous dilatation of blood-vessels ; that these two acts are not absolutely interdependent is proved by the fact that certain drugs paralyse the one set of fibres, but leave the other intact. When, on the other hand, the sympathetic filaments supplying the gland are stimulated, the blood-vessels of the gland contract, and there is produced a small quantity of saliva differing in physical characters and chemical composition from that obtained under the circumstances first referred to. Accord ing to Heidenhain, however, in each of the two kinds of nerves supplying a salivary gland there exist, besides the vascular nerve-fibres, secretory and trophic fibres, though the number of one or other of these classes may be insigni ficant, the secretory predominating in the cranial nerve branches, the trophic in the sympathetic. Stimulation of secretory fibres leads, according to Heidenhain, to an FIG. 3. Sections of a Mucous Gland of the Dog. A, during rest; B, after activity. (From Quaiu s Anatomy, fig. 505.) increased flow of water, stimulation of the trophic to an increased secretion of specific substances and to an in creased production of protoplasm. When a salivary gland passes from the state of rest into that of activity it is at once the seat of an increased blood flow, which is associated with the dilatation of the blood vessels of the organ. Under these circumstances, the blood leaving the gland presents a florid arterial colour, instead of the venous colour which characterizes the blood of the organ Avhen at rest. This vascular dilatation is explained by the coming into action of the before-mentioned vaso dilator fibres ; it is independent of the act of secretion. As was shown in a now classical investigation of Ludwig, when the salivary glands are thrown into activity there is a rise in temperature, so that the temperature of the saliva leaving the submaxillary gland may exceed by 1 5 C. that of the blood flowing to the gland. This rise in temperature cannot be explained by a study of the chemical characters of the salivary secretion, but is doubtless the result of the increased metabolic changes which necessarily accompany the act of secretion in the gland-cells, and which chiefly affect their protoplasm. That the secretion of saliva (and indeed secretion in general) is not a mere act of filtration was proved by Carl Ludwig when he showed that saliva can be secreted
