as to whether the nitrogen excreted is increased by work; it so happened in the actual experiment that the nitrogen excreted was lessened as compared with the periods before and after the muscular exertion, but the conditions under which the experimenters worked were not sufficiently rigorous to admit of accurate comparisons being drawn. The question as to the influence that work has on the increase or decrease of the nitrogen excreted has been investigated by Parkes on soldiers; he found a slight increase during work; by Flint2 and Pavy3 on the pedestrian Weston, who arrived at contradictory results; and by North, who experimented on himself. The last-named experiments are by far the most thorough that have been made; the following is a résumé of the methods employed and the results obtained.

Each experin eat lasted nine days; four days of ordinary occupation, one day's work, and a second period of four days of ordinary occupation. Reserve nitrogen' was got rid of by thirty-six hours' abstention from food, or by severe labour before the commencement of the experiment. Observations were made twice daily on the pulse, rate of respiration, temperature of body, and bodyweight. The food, carefully analysed, weighed, and cooked by the experimenter himself, was taken in four meals, and consisted of bread specially made by Mr. North himself, dried meat-powder, desiccated potato,' 'dried julienne,' condensed milk, cocoa, American evaporated apples,' 'Australian beef marrow,' sugar, salt, tartaric acid, and sodium carbonate (for raising the bread).

None of these articles of food presented any difficulties as regards analysis. The food, of which there was an unlimited supply, was of constant composition, so that for the first time in such experimentation a food, the chemical composition of which was absolutely known, was used. The fæces and urine were collected in specially-prepared bottles, carried in a knapsack during walking, which was the special form of work selected. The nitrogen (estimated by combustion with soda-lime), chlorides, sulphates, and phosphates were estimated both in food and excreta. For full particulars, the original memoir must be consulted, but the following summary of the full tables from one of the experiments will serve as an example to illustrate the general results obtained.

The experiment lasted from June 7-15, 1882; June 11 was the day on which work was done. The work was a walk of 32 miles in seven hours, the load carried being 27·75 lbs., and the loss of body-weight after the walk, 4-5 lbs.

1 Parkes, Proc. Roy. Soc. xi. 339; xvi. 44.

2 Flint, Journal of Anat. and Physiol. xii. 91.

3 Pavy, Lancet, 1876 (numerous papers).

4 North, Journal of Physiol. i. 171. Proc. Roy. Soc. xxxix. 443.

The weights in the foregoing tables are expressed in grammes. The results seen are as follows: nitrogen-obvious increase on the day of work continued on the days following it. The reserve at the end of the experiment was only 1:54 gramme less than on the day before the work: phosphoric acid-the excess of PO, excreted over that ingested (0-5 gramme) is probably within the limits of experimental error: sulphuric acid the increase after the work is undoubted, and proportional to the increase of nitrogenous material excreted; the amount of sulphates in the food was insignificant, and that in the urine was therefore derived from proteid metabolism.

These results confirm those of Parkes, but the disturbance produced by very severe labour was much more immediate and of greater intensity than that which Parkes observed, probably because the exertion he imposed on the soldiers under observation was inadequate. As in Parkes's experiments, where retention of nitrogen followed the diminution of nitrogen stored in the body, produced by privation of nitrogenous food, so after the disturbance of nutrition produced by severe labour, the immediate effect of which is to diminish the store of nitrogenous material in the system, there follows a corresponding diminution of discharge, the output being less than the intake. This store of nitrogen is more constantly operative than has been hitherto supposed; thus accumulation took place when the daily supply of nitrogen was not more than 176 grammes, no extra work being imposed; this amount cannot be regarded as more than an adequate supply for the normal needs of the body. The retention following starvation or exercise is a mere exaggeration of the normal tendency.

Muscular contraction thus enlarges the total excretion of nitrogen, but the increase is very small and is out of all proportion to the work done or the body-weight lost during the exercise. No doubt the nitrogen eliminated is derived ultimately from the muscles; but, as

1

North's experiments show, it is rather from what may be called reservenitrogen that the increased output is derived, not from the muscular nitrogen direct. As Gamgee points out, the effete nitrogen may leave the muscle not as urea or any intermediate substance in the formation of urea, such as creatine, but as proteid, and this proteid may be oxidised to form urea somewhere else.

APPENDIX

ELECTRICAL ORGANS

About fifty species of fishes are believed to possess electrical organs; the best known of these are the torpedo ray, the common skate, the electric eel (Gymnotus), and the Malapterurus. Many interesting physiological observations have been made upon these organs by Du Bois Reymond and by Burdon-Sanderson, Gotch, and Ewart with regard to their histology, development, and electromotive phenomena. In Malapterurus the organ appears to be epithelial in origin, but in other cases the organ appears to be analogous to muscle or to be developed from embryonic structures which elsewhere lengthen into muscular fibres; the nerve terminations, which in muscle form the comparatively small end-plates, in the electric organ form more extensive expansions.

But very little chemico-physiological work has been done at this subject; the observers speak of a mucoid fluid in the spaces between the electrical plates; and from the torpedo organ Weyl has extracted a substance which gives the reactions of mucin, except that no reducing sugar can be obtained from it on treatment with dilute acids; he calls it torpedo-mucin. A small quantity of gelatin and of a globulin (coagulated by heat at 55°-60°) was also obtained. The heat-coagulation temperature of the globulin is the same as that of myosinogen;✦ and it is also interesting to note, when comparing the organ with muscle, that, like muscle, it becomes acid after death (Boll), and much less transparent.

6

Weyl found the percentage of water in the muscles of torpedo to be 775, in the electrical organ 89. He was also able to separate a number of organic substances from the electrical organ similar to those occurring in muscle and nerve, such as creatine, xanthine, lecithin, fat, cholesterin, fatty acids, and inosite. Frerichs and Städeler found urea.

In another research Weyl found that excitation of the organ produced an increased formation of phosphoric acid in it.

1 Physiol. Chem. p. 409.

2 McKendrick's Physiology, vol. i. chap. xx. 1888, contains a résumé of these researches, with bibliography. Du Bois Reymond's papers are translated in the Oxford Biological Memoirs, vol. i. 1887. See especially on the Chemical Reaction of the Electrical Organ of Malapterurus, p. 412. (It becomes acid on activity.)

5 Weyl, Zeit. physiol. Chem. xi. 525.

4 Krukenberg (Weitere Untersuch. zur vergleich. Muskelchemie,' Vergleich. physiol. Studien, 2 Reihe, 1 Abth. pp. 143-147) states, however, that he was unable to obtain myosin from the electrical organ of torpedo.

5 Arch. für Anat. und Physiol. 1873, p. 99.

6 Monatsber. d. königl. Akad. d. Wissensch. zu Berlin, April 1881.

7 Du Bois Reymond's Archiv, physiol. Abth. 1884, pp. 316-324.

CHAPTER XXI

EPITHELIUM

EPITHELIUM may be defined as a tissue which consists entirely of cells united by a small amount of cementing substance. As a rule an epithelium is spread out to form a membrane, lining a cavity or covering a surface. But in certain cases the tissue is not spread out in this way; for instance, the liver is an organ which may be said to consist of a mass of epithelial cells, and the various forms of cancer are also epithelial growths.

Epithelia may be classified from the histological standpoint into those which consist of one layer of cells only, called simple epithelia, and those which consist of more than one layer, which are termed compound.

The simple epithelia may be again subdivided into pavement (or endothelium), columnar, cubical, and ciliated, according to the shape of the component cells.

The compound epithelia comprise the transitional epithelium of the bladder and ureters, and the stratified epithelium, such as that lining the mouth, or covering the whole of the external surface of the body, where it is called the epidermis.

Separated from these various forms of epithelium on account of their specialised functions, the two following must be mentioned: secreting epithelium, such as occurs in the alveoli of the salivary glands, or the uriniferous tubules; and nerve-epithelium, the various forms of modified epithelial cells which are connected to the terminations of various sensory nerves, and form the receptive end organs for sensations of different kinds; instances of nerve-epithelium are the rods and cones of the retina, the auditory hair-cells, the olfactorial cells, &c.

Very little or nothing is known chemically with regard to a great number of the varieties of epithelium just enumerated. Microscopic research shows that the constituent cells are protoplasmic and contain nuclei, and we conclude that in their essential characteristics the protoplasm of these cells resembles that of other cells which we have better opportunities of examining chemically. With regard to the

structure of the nuclei, there is nothing to add to what has already been said relating to cell nuclei generally.

On the other hand, various specialised varieties of epithelium differ considerably from ordinary protoplasm. The tegumentary epithelium loses near the surface its protoplasmic character, and the cells become filled with horny material or keratin; this is exaggerated in certain parts like the nails and hair. Other forms of epithelial growth become calcareous, as in the enamel of the teeth; and in the invertebrate subkingdoms, the exoskeletons and shells are found to be composed of chitin, spongin, conchiolin, and other forms of albuminoid material, more or less permeated with calcareous deposit; and in a few cases, as in the ascidians, a carbohydrate material akin to cellulose is secreted by the epidermal structures.

In the various forms of secreting epithelium there are many points of chemical interest to be noted. It will be more convenient to reserve a detailed study of each until we actually deal with the secretions themselves. At the same time this will afford us an opportunity of glancing at secretion as a whole, the formation of ferments within cells, and the precursors of ferments or zymogens.

In connection with nerve-epithelia, the only one which we shall discuss is the retina with its various pigments.

PAVEMENT EPITHELIUM (ENDOTHELIUM)

This form of epithelium, which consists of a single layer of flattened cells, fitting together like the stones in a mosaic pavement, is found lining the interior of the heart and vessels, and of the serous membranes. In the serous membranes, openings exist between the cells, and from these stomata, as they are termed, capillary lymphatic vessels lead. The pulmonary alveoli are lined by flattened epithelial cells, very like those found in the vascular system; from the point of view of embryology they are however different, being hypoblastic, while endothelium is mesoblastic. These cells are extensible and elastic, and (as proper is seen in the capillaries, the walls of which consist only of endothelium) contractile also.

The outlines between these cells can in all cases be rendered visible by staining with silver nitrate. The cement between the cells has the power of forming a compound with this salt, which is reduced by light, and minute granules of metallic silver are thus deposited in it, marking out in black or brown lines the contours of the cells. The cement substance of epithelium is thus similar in this particular to the ground substance of connective tissue, and doubtless both have a similar composition, consisting chiefly of mucin.