that the rods contain the precursor of visual purple, and this is acted upon by some other substance from the hexagonal cells; or it may be that the hexagonal cells withdraw the supposed substance from the rods and work it up into visual purple.

The subcutaneous injection of pilocarpine causes in the frog (not in the rabbit) a hastening in the regeneration of the visual purple (Dreser).

The physiological uses of visual purple.-The rays of light which are focussed on the rods and cones produce in those structures certain

FIG. 75.-Diagrams of Absorption Spectra 1, of visual purple; 2, of visual yellow; 3, of xanthophane in ether; 4, of rhodophane in turpentine; 5, of chlorophane in ether. This method of representing absorption spectra has been explained in connection with fig. 58, p. 276.

obscure chemical changes which no doubt are very similar to those produced by the action of light upon a sensitive photographic plate. The most tempting hypothesis suggested by the discovery of visual purple was, that that substance is itself the sensitive chemical material, the changes in which are indicated by the changes of colour it

undergoes. But further research has shown that this view cannot be adopted, and that probably the changes in the visual purple are merely accidental accompaniments of other chemical changes that are as yet undiscovered. This conclusion is derived from the consideration that vision occurs in the absence of visual purple altogether; in birds and reptiles, for instance, it is absent, and in man the part of the eye which is most sensitive to light-the fovea centralis contains no rods, and therefore no visual purple. It is altogether absent in the bat; but, on the other hand, it is present in the owl. Both these animals are nocturnal, so the habits of the bat will not explain its exceptional condition.

1

Chromophanes, the pigments of the cones.-In birds, reptiles, and fishes, the inner segment of the cones contains a coloured oil-globule, the colour varying greatly. By using large numbers of birds' eyes Kühne and Ayres succeeded in preparing these coloured fats in large quantities. The retina were first dehydrated by absolute alcohol and then extracted with ether; the ethereal solution of the fat was evaporated to dryness and the residue treated with caustic alkali; the coloured soaps so formed were freed from excess of alkali by washing with water, and were separated by means of their different solubilities in various reagents; petroleum ether dissolving out a green substance, ether a yellow, and benzene a red material. It was not found possible to obtain the pigments in a state of purity, nor in a crystalline form; when the pigments were in association with fats or fatty acids instead of soaps they showed no difference in their solubility; the soaps can be decomposed by means of glacial acetic acid. The pigments are called chromophanes the green one, chlorophane; the yellow one, xanthophane; the red one, rhodophane. These pigments belong to the class of pigments called lipochromes, and their spectroscopic appearances (fig. 75, spectra 3, 4, and 5) should be compared with those of other lipochromes we have considered before (compare serum-lutein, p. 254; tetronerythrin, p. 325, and lipochrin, p. 459). Chlorophane shows two absorption bands; xanthophane and rhodophane each show one. The position of these bands shifts a little according to the solvent used, but, as in all other lipochromes, the bands are towards the blue end of the spectrum. There is always, in addition to the bands, a considerable absorption of the violet extremity of the spectrum. Like other lipochromes the chromophanes, when evaporated to dryness, give the following colour reactions :

1 Kühne and Ayres, Journ. Physiol. i. 109.

* Xanthophane is not identical with lipochrin obtained from the frog's hexagonal epithelium; the two pigments differ in solubilities and spectroscopic appearances.

1. Concentrated sulphuric acid; the fragments undergo a series of colour changes, being first a dirty green, then bluish green, and lastly violet. Later still this fades and only a brownish colour is left.

2. Concentrated nitric acid; a distinct bluish green colour is developed which lasts only a few seconds; then the flakes become colourless.

3. Iodine dissolved in solution of potassium iodide; the solution used is of this composition, iodine 0-25 gramme, potassium iodide 0.5 gramme, and distilled water 100 c.c. (Capranica).' This gives no colour reaction at all with the solid pigment. After the saponification of the pigment by the addition of strong caustic soda to the alcoholic solution, the iodine solution gives a bluish-violet colour, which, like the nitric acid colour, is evanescent.

Although Kühne speaks of these colours as the stable colours of the retina, he expressly points out that the word 'stable' is used only in a comparative sense; they are ultimately bleached by light; and this we have seen occurs in all other lipochromes, but much more slowly than in visual purple. A solution of chlorophane in a 5 per cent. solution of bile salts fades most rapidly; then one of xanthophane; while solutions of rhodophane are but little affected. In all cases, however, several days' exposure to sunlight is necessary for the bleaching process to become apparent. Indeed, some other lipochromes not connected with the eye at all, such as, for instance, that occurring in yolk of egg, bleach more quickly than the chromophanes.

Kühne was unable by exposing the retina of living birds to the sun for several hours to produce any change in the colours of the coneglobules. He, however, considers it possible that a slight change may be produced, so slight as to escape observation, but sufficient to act as a visual excitation. He is inclined, indeed, to class the chromophanes with visual purple, and certain colourless substances not yet separated, as visual substances or visual excitants; and, alluding to Hering's theory of the existence of three visual substances concerned in colour vision, he points out that the coloured oil-globules represent half the spectral colours, i.e. from the red to the yellowish-green, so that with their complementary colours they yield all the colours of the spectrum (Kühne).2

1 These first two colour reactions were established as general tests for lipochromes by Capranica,Physiologisch-chemische Untersuchungen über die farbigen Substanzen der Retina,' Arch. f. Anat. und Physiol. 1877, p. 283; and the iodine test by Schwalbe, 'Handbuch der gesammten Augenheilkunde,' Gräfe u. Sämisch, Bd. i. Th. i. Cap. iv. p. 414, Leipzig, 1874. The sulphuric acid and iodine reactions were first described for lutein by Piccolo and Lieben, 'Studii sul corpo luteo della vacca,' Giornale di scienze naturali ed economiche, anno ii. vol. ii. p. 258, Palermo, 1866. And the nitric acid reaction was first pointed out as being possessed by lutein by Thudichum, 'Ueber das Lutein &c.' Centralbl. f. d. med. Wissenschaft, Bd. vii. 1869, p. 1.

2 Journal of Physiology, i. 189.

H H

CHAPTER XXII

THE CONNECTIVE TISSUES

INTRODUCTORY

A LARGE number of tissues are grouped together under the heading Connective Tissue; they may be classified as follows:

At first sight these tissues appear to form a heterogeneous group, but farther examination shows that the resemblances are sufficient to justify a grouping of them together. The similarities may be tabulated in the following way :

1. Embryological all are derived from the mesoblast.

2. Functional all have a connecting or supporting function.

3. Histological: there are many points of structure in common. The histological elements are a ground substance or stroma containing cells and fibres; the latter may, however, be absent, as in hyaline cartilage; and the structure may be, as in bone and dentine, masked by calcareous deposit.

4. Chemical all varieties of connective-tissue that contain white fibres yield gelatin, and the substance called chondrin yielded by cartilage is very similar.

The histological elements of connective tissue are four in

number :

1. Cells connective tissue corpuscles of various kinds, cartilage cells, and cells of various kinds found in bone and dentine.

2. White fibres : immeasurably fine fibres which do not branch, which run in bundles; these bundles have a wavy outline. The fibres

are not elastic; they are rendered transparent by acetic acid, and the substance of which they are composed (collagen) is converted into gelatin by the action of boiling water.

3. Elastic fibres: these are much thicker than the white fibres. They are angular in transverse section; they run singly, branching and anastomosing with one another; as their name implies, they are elastic; when massed together in large numbers they have a yellow colour, hence they are sometimes called yellow fibres. The substance of which they are composed is called elastin, which is a very insoluble material and but little affected by acids; hence, in a microscopic preparation, if the white fibres are rendered transparent by a little 1 per cent. acetic acid, the yellow or elastic fibres remain unaltered. The elastic fibres, in addition, may be distinguished by the readiness with which they are stained by magenta.

4. Ground substance, or intercellular substance. This is the material in which cells and fibres are imbedded; it has a jelly-like consistency, and its chief constituent is mucin. Like the cement substance of epithelium it is stained black by silver nitrate.

These different histological elements are differently distributed in the different connective tissues, and hence we are able to obtain a quantity of any one sufficient for chemical investigation from the tissue where that particular anatomical element preponderates in amount over the others; thus, elastin would be prepared from elastic tissue; collagen from fibrous tissue, and so on.

Keeping in view the enumeration of the histological elements found in the connective tissues, the following facts may be added concerning each individual tissue in our list :

1. Areolar tissue. This forms the subcutaneous tissue, the submucous and subserous tissue, the investing sheaths of organs, binding parts of organs to one another and different organs one to another. It is thus universally distributed. It contains all four histological elements, cells, and both kinds of fibres imbedded in ground substance.

2. Fibrous tissue. This is the tissue that is found in tendons and ligaments, in the true skin, and in the denser fasciæ. It consists chiefly of bundles of white fibres arranged parallel one to another, giving to fibrous tissue in this way its great strength. Cells, ground substance, and a few elastic fibres are also present, but are relatively less important quantitatively.

3. Elastic tissue.—The ligamentum nuchæ of quadrupeds and the ligamenta subflava are composed of this tissue. Elastic structures are also found in the walls of the trachea and its branches, and in blood vessels. It consists chiefly of large elastic fibres; the other connective-tissue elements are relatively unimportant.

4. Jelly-like connective tissue is found in the vitreous humour of the eye, and in the Whartonian jelly of the umbilical cord. It consists almost entirely of