300 ESSENTIAL STRUCTURE OF SECRETING ORGANS.

(fig. 164); and thus the entire mass may be composed of numerous lobules, each having its own duct. Passing to still higher forms, we find all the ducts coalescing into a common trunk, so that the gland bears a strong resemblance to a bunch of grapes; as is seen in fig. 165, which represents the structure

Fig. 165.-INTIMATE STRUCTURE OF A COMPOSITE GLAND (THE PAROTID).

of the Parotid (one of the salivary glands) of Man. The main stalk is the duct into which all the others enter; from this pass off several branches, and these again give off smaller

twigs, the extremities of which enter the minute follicles in which the secretion is formed. These follicles are lined, as in their simple condition, with cells, I which are the essential instruments in the production of the secretion; the fluid which they separate is poured, by the giving-way of their walls, into the small canals proceeding from the follicles, thence into

Fig. 166.-PORTION OF ONE OF THE the larger branches, and finally

TUBULI

URINIFERI OF

HUMAN KIDNEY;

THE

into the main trunk, by

Showing its lining of flattened epithe- which it is carried into the situation where it is to be

lium cells.

employed or from which it is to pass out. The Liver will be seen to possess a structure exactly resembling this, in the

ESSENTIAL STRUCTURE OF SECRETING GLANDS.

301

Crustacea, by referring to fig. 47, fo; and in the Mollusca it is nearly the same (figs. 157, 7, and 149, f).

357. The required extent of secreting surface is not unfrequently given, however, by the prolongation of the follicles. into tubes, rather than by a great multiplication in their number. Of this we have a remarkable example in the Kidney of the higher animals (§ 368), which is entirely composed of such tubes, together with areolar tissue which binds them together, and the blood-vessels distributed amongst them. These tubes, like the follicles, are lined with epithelium-cells (fig. 166), which are the real instruments in the separation of their secreted product.

358. That there is nothing in the form of any secreting apparatus, however, which determines the peculiar nature of its secretion, is evident from this

fact, that, in glancing through the Animal series, we find the same secretion elaborated by glandular structures of every variety of form. Thus, we have seen that the bile is secreted, in the lowest animals in which we can distinguish it, by a number of distinct follicles, as simple in their structure as are those by which the mucous secretions are formed in the highest. Again, the bile is secreted in Insects, by a small number of long tubes, which open separately into the intestinal canal just below the stomach (fig. 112); and these tubes apparently differ in no respect from those that form the urinary secretion in the same animals, which open nearer the outlet of the intestinal canal. In fact, the distinct function of the latter was not known, until it was ascertained that uric acid is to be found in them. In fig. 167, which represents the digestive apparatus of the Cockchafer, it is seen that the biliary vessels are only four in number, but are very long; and that, for a good part of their length,

Fig. 167.

ALIMENTARY CANAL

AND HEPATIC TUBULES OF COCKCHAFER.

302 ESSENTIAL STRUCTURE OF SECRETING GLANDS.

they are beset with a series of short tubes opening from them, by which the extent of secreting surface is much increased. On the other hand, although the urinary secretion is generally formed by long tubes, yet in the Mollusca it is secreted by follicles, according to the general plan of their glandular structures.

359. The secreting cells not unfrequently possess the power of elaborating a peculiar colouring matter, either separately, or along with the substances which seem more characteristic of the secretion. Thus the ink of the Cuttle-fish is in reality its urine, charged with a quantity of black matter formed in the pigment-cells (resembling those of the interior of the eye, § 533) that line its ink-bag; and the corresponding secretion in other Mollusca is rendered purple by the same cause. The bile seems to be universally tinged with a yellow or greenish colouring matter, which may be regarded, therefore, as an essential part of the secretion; and the urine of Mammals is also tinged by a yellow pigment, which seems related in its nature to that of the bile. In all these pigments, carbon is the predominating ingredient; and their amount is increased when the respiratory process is insufficiently performed.

360. It appears, then, that the different secreting cells have the power of elaborating a great variety of products; and that no essential differences can be discovered in the structure of the glands into whose composition they enter, which can account for that variety. We are entirely ignorant, therefore, of the reason why one set of cells should secrete biliary matter, another urea, another a colouring substance, and so on; but we are as ignorant of the reason why, in the parti-coloured petal of a flower, the cells of one portion should secrete a red substance, whilst those in immediate contact with it form a yellow or blue colouring matter; and we know as little of the cause, which occasions one set of the cells of which the embryo is composed to be converted into muscular tissue, another into cartilage, and so on.

361. One of the most curious points in the Physiology of Secretion, is the interchange which sometimes occurs in the functions of particular glands. When the operation of some one gland is checked or impaired by disease, it not unfrequently happens that another gland, or perhaps only a secreting surface, will perform its functions more or less perfectly; this

RECEPTACLES FOR SECRETED PRODUCTS.

303

happens most frequently in regard to the important Excretions, as if Nature had especially provided for their continued separation from the blood, that its purity may be unceasingly maintained. Thus the urinary secretion has been passed off from the surfaces of the skin, stomach, intestines, and nasal cavity, and also from the mammary gland; the colouring matter of the bile, when it accumulates in the blood (as in jaundice), is separated from it in the skin and conjunctival membrane of the eye (§ 537); and milk has been poured forth from pustules on the skin, and from the salivary glands, kidneys, &c. Such cases have been regarded as fabulous; but they rest upon good authority, and they are quite consistent with physiological principles.

362. Some of the main ducts or channels, through which the glands pour forth their secretions, are provided with enlargements or receptacles, which serve to retain and store up the fluid for a time, until it may be desirable or convenient that it should be discharged. Thus, in most of the higher animals, the duct which conveys into the intestinal tube the bile secreted by the liver, is also connected with a receptacle termed the gall-bladder; the bile, as it is secreted, passes into

b

C

d

this, and is retained there until it is wanted for assisting in the digestive process (§ 213); when it is pressed out into the intestinal canal. It is a curious fact, that in most persons who die of starvation, the gall-bladder is found distended with bile; showing that the secretion has continued, although it has not been poured into the intestine for want of the stimulus occasioned by the presence of food in the latter. In many quadrupeds, especially those of the Ruminant tribe, the milk-ducts are in like manner dilated into a large receptacle, the udder, which retains the secretion as it is formed, until the needed. In all Mammals, and in some Reptiles, Mollusks, and Insects, but not in Birds or Fishes, we find the ureters, which convey away the urinary excretion from the kidneys, dilated at their lower extremity into a

PARATUS.

a, kidneys; b, ureters; c, bladder; d, its canal, the

urethra.

period when it is

304

STRUCTURE OF THE LIVER.

bladder (fig. 168), which serves to retain all the fluid that is poured forth by the gland during a considerable length of time, and thus prevents the necessity for its being continually passed out of the body.

Characters of Particular Secretions.

363. In nearly all animals, the Liver holds the first rank among Glands or secreting organs, in regard both to its size and to the obvious importance of its function. The principal varieties of its plan of structure in the Invertebrated classes having been already noticed (§ 356), we shall here limit ourselves to a sketch of that peculiar arrangement of its elementary parts, which presents itself in Man and other Vertebrata. The position of this organ in the abdominal cavity is shown in fig. 30. It is chiefly composed of a mass of cells of a flattened spheroidal form (fig. 169, B), the diameter of which is usually from 1-800th to 1-1600th of an inch; each cell presents a distinct nucleus, which is surrounded by yellow biliary matter in a finely granular condition; and in the midst of this there are usually one or two large fatty globules, or five or six small ones. The quantity of fat in the liver is very liable to increase, however, when there is a large amount of oily or fatty matter in the food, or when the respiratory function is not performed with sufficient activity.

▲, Showing the manner in which the substance peculiar distribution of

of its lobules is disposed around the branches of the hepatic vein a; B, cells of which the lobules are composed, more highly magnified.

the" portal" vessels and of the hepatic ducts.

The Vena Porta, it will be remembered, collects the blood that has been distributed to the alimentary canal, and conveys