COLUMNAR EPITHELIUM

This consists of a single layer of elongated nucleated cells. Such an epithelium lines the alimentary canal from the cardiac orifice of the stomach downwards, and also most of the ducts of secreting glands. When the columnar cells are short, the term cubical epithelium is employed.

The border of columnar cells is more strongly refracting than the body of the cell, and though there are no differences in its resistance to reagents, it no doubt consists of somewhat modified protoplasm. The body of the cell is often vacuolated, and often contains numerous fat globules; this is especially the case in the columnar epithelium of the small intestine, during the absorption of fat. These fat globules can be identified by the deep black colour they give with osmic acid (see Absorption).

Columnar cells often break down to form goblet cells, and their more superficial protoplasm is transformed into mucin, the chief constituent of mucus.

CILIATED EPITHELIUM

Ciliated epithelial cells are usually columnar in shape; the cilia are protoplasmic tapering processes; in the human subject 4 to 8 μ in length, but in many invertebrates, like the mussel, they are much larger.

Ciliary movement is independent of the circulatory and nervous system, but it is dependent on nutritional changes occurring in the cell with which the cilia are connected, as all movement ceases when they are severed from the cell. The conditions most favourable to ciliary action are a temperature a little above that of the body (40° C'.), and free access of oxygen.' The movement is retarded by cold, by heat a little over 40° C. (this coagulates the proteids of the protoplasm of which they are composed); weak acids and all strong chemical reagents also kill cilia. Carbonic acid, chloroform and ether stop ciliary action, but the cilia recover when the poisonous vapour is replaced by oxygen. Distilled water acts as a protoplasmic poison here as elsewhere.

If cilia are allowed to work after being removed from the body, they will in a varying time get languid and finally stop. If for instance a few bars of the gill of the sea mussel be mounted in a little

Cilia will, however, like muscle, continue to work some time in an atmosphere containing no oxygen. Their protoplasm, like that of muscle, is able to store up oxygen for future use (Sharpey: see Quain's Anat. vol. ii. p. 58).

sea water, and watched with the microscope, they will probably finally be brought to a standstill in about two hours. This is no doubt a condition resulting from fatigue; and fatigue in its turn is, as in muscular tissue, the result of the accumulation of the products of activity. In the case of muscle, sarcolactic acid appears to be a substance that especially tends to cause fatigue; probably in the case of cilia, an acid is also produced; at any rate a dilute alkali will set the cilia going again. A drop of dilute potash (1 part KHO to 1000 of water), passed under the cover-glass, will cause the cilia in the specimen just mentioned to work vigorously once more. If an acid is produced by the activity of the cilia, the potash no doubt neutralises this, and thus the activity of the cilia, which was hindered by the acid, is restored. The alkali may also act by increasing the amount of imbibition water (see p. 188).

In certain particulars, ciliary action resembles amoeboid action: it is for instance accelerated and hindered by the same reagents. On the other hand ciliary movement resembles muscular movement: it is not due to contractility occurring in all directions, but, as in muscular movement, in one direction only. Engelmann' has suggested that the contractile protoplasm is situated chiefly on the concave side of the curved cilium, so that on contraction the cilium will be brought downwards, and on the contractile motion ceasing, the cilium will be erected by the elastic recoil of the substance forming its convex border. Engelmann also states that cilia are doubly refracting.

Ciliated epithelium in man lines the greater part of the respiratory passages, the Fallopian tubes and uterus, some of the ducts of the testis (vasa efferentia and coni vasculosi), and the cerebrospinal canal. In some of the lower vertebrates, like the frog, the pharynx and œsophagus are lined by ciliated epithelium, and in the human subject there is an indication of the same state of things having existed, for the lining cells of the ducts of some of the minute glands opening into the pharynx retain the cilia which have been lost from the general surface of the mucous membrane. In most of the above cases, as the name mucous membrane suggests, many of the ciliated cells may, like columnar cells, break down into goblet cells with the formation of mucin.

MUCUS

Mucus is the name given to the slimy secretion of the surfaces of various internal cavities (alimentary canal, respiratory passages, bladder, uterus, &c.), and in certain lower animals, e.g. the snail, it is

Engelmann, article in Hermann's Handbuch, 1879.

poured out on the external surface of the animal. The membranes that line these cavities are called mucous membranes.

In some cases, certain of the lining epithelium cells yield mucin, the chief constituent of the secretion, by the formation of what are called goblet cells (fig. 71). The more superficial part of the cell protoplasm undergoes certain changes, which result in the formation of a highly refracting globule of mucin; the precursor of mucin within

FIG. 71.--Goblet cells. Highly magnified (Klein).

the cell is called mucinogen; after the mucin is expelled, the basal portion of the cell alone remains. This may once more grow into a normal epithelial cell, and may again undergo this mucoid degeneration.

In other cases the mucin is chiefly furnished by certain small racemose glands, situated beneath the general epithelial lining, with its duct opening on the surface. Here the cells of the acini of the gland undergo, as in the mucous salivary glands, the same transformation of the cell protoplasm into mucinogen, and this suspended in an alkaline liquid is expelled as mucin through the duct upon the surface of the mucous membrane.

The chief properties of mucin are its stickiness and viscosity and its solubility in dilute alkalis like lime water; from these solutions it is readily precipitated by acetic acid, in excess of which it is insoluble. In composition, it consists of a globulin in combination with a carbohydrate called animal gum. By treatment with dilute sulphuric acid, the animal gum is converted into a sugar, which, like grape sugar, reduces alkaline solutions of cupric hydrate.

It is probable that there are several different kinds of mucin, i.e. different proteids combined with animal gum; that obtained from the snail, for instance, is distinguished by Hammarsten into foot mucin (obtained from the foot), and mantle mucin (obtained from the mantle); the properties of these two substances are slightly different from one another, and from the mucin obtained from saliva, mucus, &c.; and these in turn differ from the mucin found in the ground substance of connective tissue. All mucins, however, are alike in the reactions that

1 Hammarsten, Pflüger's Archiv, xxxvi. 373.

have been already mentioned, viz. their tenacity, their precipitability by acetic acid, and the fact that a reducing sugar is obtainable from them. (Some more particulars concerning mucin will be found under the heading Connective Tissues, p. 476; see also p. 144.)

The pseudo-mucin of ovarian fluids differs from true mucin in not being precipitable by acetic acid; the same is the case with colloid material, formed in colloid degenerations of tumours, and contained within the vesicles of the thyroid body. Pseudomucin and colloid substance are probably identical (see p. 353).

Nucleo-albumins are like mucin in their physical characters, and in many of their reactions. The slimy material in bile was long mistaken for mucin; it, however, is not a compound of a proteid with a carbohydrate, but with the phosphorised substance known as nuclein. Such a nucleo-albumin we have already described as a constituent of the white blood corpuscles; similar substances occur in the other animal cells; Hammarsten, for instance, has described one in the cells of the submaxillary gland (which contain however true mucin in addition).

2

Such considerations show that all slimy substances do not necessarily contain mucin; and it is especially the nucleo-albumins that must be carefully distinguished from that material.

The chief solid constituent of mucus, then, is mucin; epithelial cells, and débris of such cells, and a few leucocytes are also present, and these are suspended in a liquid which is doubtless a transudation from the blood; it has an alkaline reaction, and contains a certain small proportion of proteids and extractives, as well as mineral salts like those in the blood itself. The following table gives a few analyses that have been made of mucus.3

1 These are the three characteristics of the mucin-group as defined by Hammarsten. Chem. Centralbl. 1884, p. 814.

2 Hammarsten, Zeit. physiol. Chem. xii. 163.

5 I am indebted to Charles's Physiological and Pathological Chemistry, p. 289, for

this table.

The mucus of various parts differs a little in appearance and in reaction. Charles' describes the varieties as follows:

The amount of mucus normally secreted is small, merely sufficient to lubricate the surface; in the case of the respiratory cavity, it entangles dust particles from the inspired air, and it together with this foreign matter is removed by the activity of the ciliated epithelium. It is stated that the mucus of the alimentary tract may aid digestion.

In cases of mild inflammation of the mucous membranes (catarıh), the amount of mucus secreted is increased. In more severe cases, the leucocytes become abundant, and the secretion is called muco-purulent, that is, a mixture of pus with mucus.

Sputum consists of the secretion of the mucous membrane of the respiratory tract mixed with a certain amount of saliva and occasionally nasal mucus. The following are some particulars concerning the different kinds of sputa in a few important diseases.

In

Quantity. This is very variable, especially in bronchitis. phthisis it may range from 80-150, in pneumonia from 26-300 grammes per diem.

Colour. In chronic inflammation of the bronchi it may be studded with black particles of carbon, especially in those living in a sooty atmosphere. In acute cases of bronchitis it is yellowish, owing to admixture with pus. In pneumonia the typical sputum is rusty, i.e. brown or yellowish-red, from the presence of altered blood pigment. As hepatisation proceeds, the sputum becomes greyish or purulent. In phthisis the expectoration may be tinged with bright blood.

Viscidity. The most viscid expectoration is that of pneumonia. The most watery expectoration occurs in the early and late stages of bronchitis.

Odour. In bronchiectasis and gangrene of the lung, the sputum has a putrid odour.

1 Charles, loc. cit. p. 288.