the case of proteids we are not acquainted with its rational formula. The following five analyses show, however, discrepancies in the percentage of sulphur present; there is a remarkably large percentage of sulphur, and most of it is very loosely combined; no doubt the different methods adopted for the estimation of the sulphur employed by the various investigators will sufficiently account for the different results obtained, sulphur being a difficult substance to estimate correctly when occurring in organic compounds.

The term melanin has been applied to a large number of black pigments occurring in the body; thus we have already noted a black pigment in the blood of persons affected by malaria and other diseases (Melanæmia, see p. 310). This, no doubt, is derived from hæmoglobin; and perhaps the other black pigments of the body occurring in the retina, in the skin, and in melanotic sarcomata may ultimately have the same origin. There is no doubt that these pigments are, however, not all identical; elementary analyses show this; for instance, iron is present in some, absent in others. In the tissues of the lungs and bronchial glands the black pigment that occurs there simply consists of particles of carbon breathed in with the atmospheric air.

4

The black pigment of the skin and of the hair has been examined by Sieber, who made some few percentage estimations of the elementary composition of the substance, but obtained very discordant results. The elements present are carbon, nitrogen, hydrogen, and oxygen. It has never been crystallised; it is soluble in water, alcohol, ether, acids, and strong alkalis; the brown solution produced by dissolving it in hot potash is decolorised by chlorine water.

(The subject of melanin will be more fully dealt with under the Retina (p. 457) and Melanotic Sarcoma, chap. xxiii.).

1 The first four analyses are quoted from Hoppe-Seyler's Physiol. Chem. p. 90. 'Hoppe-Seyler, Physiol. Chem. p. 91. 3 Zeit. Biol. xxvi. 291.

• See also Hodgkinson and Sorby, Journ. Chem. Soc. 1877, p. 427.

Turacin

This is the only one of the many pigments in birds' plumage that has been satisfactorily examined. It is obtained from the touracon, or plantain-eater, of the Gambia. It is of a crimson colour; it is not crystalline; it shows two absorption bands between D and E, and is remarkable as being one of the few animal compounds that contain copper (Church 1).

Skeletins

The term skeletin2 has been applied to a number of nitrogenous but sulphur-free substances found in the skeletal tissues of invertebrates. They appear to be intermediate between carbohydrates and proteids, and Krukenberg considers that they are amido-derivatives of carbohydrates. The skeletal tissues of invertebrate animals are, as a rule, epidermal (not mesodermal, as in vertebrates). The skeletins appear to take the place of keratin in invertebrate animals. The principal skeletins are chitin, conchiolin, cornein, spongin, and fibroin.

Chitin. This substance has a very wide distribution among the invertebrate groups. It is in the arthropoda that it is found to the greatest extent; it forms the membrane of the ovum, the cuticle of the adult, with its appendages, the supporting substance in the trachea of insects, &c. It is also found in mollusca (jaws and odontophore); in worms (e.g. the set of annelids). It forms the membrane of the ova in other groups, and the cyst-wall in encysted forms of protozoa; but its presence in these and a few other situations where it has been described has not been fully proved.3

Chitin is frequently impregnated with mineral salts, calcareous salts in the crustacea, silica in the lingual ribbon of certain molluscs.

Preparation.-Chitin may be prepared in large quantities from the shell of the crab or other crustaceans, and in smaller quantities from the wing-cases of the cockroach or other insects; in the case of the crab and lobster calcareous salts must be first dissolved out by hydrochloric acid; this operation is not necessary if insects' or beetles' wings be used; the substance is boiled with solution of caustic soda ; this leaves the chitin undissolved: the residue is then dissolved in

1 Phil. Trans. vol. clix. (1870), p. 627. A number of observations on the pigments of birds' feathers will be found in Krukenberg's Vergleich. Physiol. Studien. 2 Krukenberg, Zeit. Biol. xxii. 241.

3 Chitin is not wholly epiblastic, however; it is found, for instance, in mesoblastic structures, e.g. the cartilages of sepia and limulus (see Invertebrate Cartilage, chap. xxii.). In those animals which possess chitin instead of keratin, the neurokeratin of the nerves is replaced by neurochitin,

concentrated hydrochloric acid, and precipitated from this solution by excess of water. This is once more dissolved in acid and reprecipitated by water, and to obtain the substance in a state of purity the operation should be again repeated.

Properties.-Chitin is thus, like keratin, a very insoluble substance, but differs from keratin in its insolubility in concentrated solutions of the caustic alkalis. Concentrated mineral acids will alone dissolve it. It is colourless and amorphous; it has the formula C15H26N2O10 ; and when heated with concentrated mineral acids it yieids not a fermentable sugar as Berthelot' supposed, but a nitrogenous substance, glucosamine, which reduces alkaline solutions of cupric salts as sugar does (Ledderhose 2). The equation which represents the reaction is as follows:

2C15H26N2O10+2H2O=4C6H12NO5+3C2H ̧O2

Glucosamine is an amido-derivative of dextrose; if a molecule of hydroxyl (HO) in dextrose be replaced by one of NH2, the formula of glucosamine will be obtained. If chitin be boiled with concentrated hydrochloric acid for about an hour, the solution becomes black or brown, and on evaporation rhombic crystals of the hydrochloride of glucosamine crystallise out. The formation of these crystals is a very convenient test for chitin. On a small scale the crystallisation can be carried out on a microscope slide, but on a large scale crystals an inch or more in length are obtainable; these are brownish at first, but after two or three recrystallisations may be obtained perfectly colourless. It is dextro-rotatory; a=+70-6° (Gamgee3). The sulphate of glucosamine may be obtained by treating chitin with sulphuric acid; by acting on this with barium hydrate the pure base which is also crystalline may be obtained. By the action of hydrochloric acid on chitin, not only is the hydrochloride of glucosamine formed, but also a dextrinlike carbohydrate (Krukenberg).

Conchiolin. This is the skeletin that forms the organic bases of the shells of gastropods. The shells are first macerated in hydrochloric acid, and the residue boiled with caustic soda. Like chitin, conchiolin remains undissolved. Its formula is CHINO. Like chitin, it dissolves in hot concentrated mineral acids. It does not give the xanthoproteic, Millon's, nor the Adamkiewicz colour reactions. On decomposition it yields leucine, a small and doubtful quantity of glycocine, but no tyrosine, and no glucosamine. Its formula and its decomposition products clearly distinguish chitin from conchiolin.

The cement substance between the eggs of various molluscs, whose shells and

1 Comptes rendus, xlvii. 227.

2 Zeit. physiol. Chem. ii. 213. Ibid. iv. 139.

3 Physiol. Chem. p. 301.

4 Zeit. Biol. xxii. 480.

egg-capsules contain conchiolin, is coloured red by heating with Millon's reagent, and contains a substance allied to keratin (Krukenberg, loc. cit.).

Cornein.—This is the skeletin originally described by Valenciennes and Frémy in certain corals (Gorgonia, Antipathes, &c.). It has been more fully examined by Krukenberg. Its formula is CHAN,O,. It is thus nearly allied to conchiolin. On decomposition it yields leucine and glycocine. It gives, moreover, a red colour with Millon's reagent. The mineral matter in corals is chiefly calcium carbonate.

Spongin, the skeletin of sponges, like the two substances just described, yields on decomposition leucine and glycocine. It gives none of the three colour reactions just enumerated; and, like conchiolin, yields on digestion peptone-like substances, which differ from true peptones and albumoses by not giving the colour reactions in question: thus they both differ from keratin, which is not digestible. The mineral matter in many sponges consists of siliceous spicules.

Fibroin, the substance of which spiders' webs are composed, is nearly allied to these substances. It is an insoluble substance, except in concentrated mineral acids and alkalis. It yields on decomposition leucine, tyrosine, and glycocine · and gives all the colour reactions just enumerated like a proteid.

Silk is a very nearly allied substance, but is considered by Weyl to be a true proteid. Its percentage of nitrogen is lower than that of fibroin.

Tunicin

Cellulose is found in very few places in the animal kingdom; it composes the tunic of the ascidians or tunicates, and it is there called tunicin. It is also found in the zoocytium or mucilaginous investing matrix of the Ophrydium versatile, and perhaps in some allied protozoa. In the case of the Ophrydium it is interesting to note that chlorophyll, another vegetable product, is present also.

Animal cellulose (C6H1005) may be purified by washing with water, dilute hydrochloric acid, dilute caustic potash, alcohol, and ether. By the prolonged action of sulphuric acid it is converted into a fermentable sugar which reduces Fehling's solution. According to Berthelot," this change is not effected in tunicin until after some weeks' boiling with the acid. Tunicin gives a blue colour, the cellulose from Ophrydium a brown colour with iodine and sulphuric acid.

THE RETINA

This is the innermost lining of the eye, and consists of both nervous and epithelial structures. The nerve-fibres passing through the various complex innermost layers terminate in a nerve-epithelium, which is

1 Berichte d. chem. Gesellsch. Berlin, xvii. p. 1848.

Zalocostas (Compt. rend. cvii. 252) found also traces of tyrosine, butalamine, and glycalanine (C5H12N2O4). Its constitution resembles that found in proteids by Schützenberger (see p. 115).

3 Weyl, Ber. d. chem. Gesellsch. Berlin, xxi. 1407, 1529.

4 Halliburton, Quart. Journ. Mic. Science, July 1885.

5 Berthelot, Ann. de chimie et de phys. série 3, tome lvi. p. 153.

called the layer of rods and cones; and it has been satisfactorily proved that it is this layer upon which the images of external objects are focussed by the refractive apparatus in front of it. The impressions of light affect the rods and cones, and thence they are propagated as nervous impulses via the optic nerve to the brain. External to the layer of the rods and cones is a layer of hexagonal epithelium cells containing a black pigment. It is these two layers that we have to describe in detail.

The retina as a whole gives indications of its twofold structure, nervous and epithelial. Its reaction is stated to be acid; and, like most animal tissues, it becomes opaque after death. Water dissolves out from it proteids, gelatin, and mucin, the two last-named substances being probably derived from the supporting connective tissue it contains. Alcohol dissolves lecithin from its nerve-fibres and cells. Other reagents are employed to dissolve out other constituents, such as the pigments, from the rods and cones. 1 Cahn gives the following quantitative

A résumé of the chief facts

Most of our knowledge of the chemistry of the retina is the result of the labours of Kühne and his pupils. will be found in Kühne's article in Herrmann's Physiologie' (1879), vol. i. p. 235.

Handbuch der

The Hexagonal Pigment Cells of the Retina

The pigmentary layer of the retina was at one time supposed to be a part of the choroid or vascular coat of the eye, but the facts of embryology have shown that it is in reality part of the retina, and is developed like the rods and cones from the epiblast, whereas the choroid is developed from mesoblastic tissue. The choroid, however, contains branched cells in which is pigment identical with the black pigment of the retina.

1 Hoppe-Seyler's Physiol. Chem. p. 699.

2 Three in number-one resembling myosin, coagulating at 55° C., another like mucin, and a third like serum-albumin.