CHAPTER II.

PASSIVE TISSUES.

Mucous Tissue-Retiform, White Fibrous, Yellow Elastic, Areolar Tissues-and Fat.

1. MUCOUS TISSUE.

This tissue, when examined under the microscope, is seen to consist of stellate protoplasmic cells, which possess processes that unite with one another, and are imbedded in a clear, semi-fluid matrix. In the human body it is met with in the jelly of Wharton in the foetus, and in the vitreous humour of the eye; that in the vitreous humour is modified in structure, most of the bodies of the cells have disappeared, their processes alone remaining dispersed throughout the matrix. It also occurs in many parts of the embryo.

2. ADENOID OR RETIFORM

TISSUE.

This tissue has also been called reticular and glandular tissue. It consists of stellate cells, whose processes unite; the interstices between

the cells being filled up with glandular material, as in the spleen pulp, or with cells, as in lymphoid tissue. A very fine variety of this tissue is met with in the spinal cord and brain, passing in between the bundles of nerve fibres, and supporting them; it is called neuroglia.

3. WHITE FIBROUS TISSUE.

Microscopical Appearances.-When examined under the microscope this tissue is seen to consist of fibres which have an ill-defined, wavy outline, and are usually parallel to one another. These fibres do not anastomose, and do not curl up at their ends (P. I. Fig. 3). They are very frequently bound together to make bundles, and in these bundles, oval, branched cells may occasionally be seen. These cells are similar in almost every respect to the ordinary stellate corpuscles, which are met with for the most part throughout the whole of the passive tissue group. They possess a delicate protoplasmic body, which is thick in the centre but bevelled off towards the sides, and contain a round or oval nucleus with several nucleoli.

Properties. This tissue is tough, inelastic,

strong, and unyielding. It swells up and dissolves on the addition of weak acids, and yields gelatin on boiling.

Uses of white fibrous tissue;

It forms ligaments, tendons, and aponeuroses for the attachment of muscles. It occurs, also, in supporting capsules, of which the best examples are the tunica albuginea of the testicle and the sclerotic coat of the eye. It enters into the formation of the external coat of large arteries, where it serves to protect them and to prevent their over-distention. Lastly, it enters largely into the formation of fascia, and forms a constituent part of areolar tissue.

4. YELLOW ELASTIC TISSUE.

Microscopical Appearances.-It consists of fibres which have a hard, well-defined outline, and which anastomose freely, and curl up at their ends (P. I. Fig. 2).

Properties. It is elastic and yielding, it is not affected by weak acids, and yields elastin on boiling.

Uses. It is useful in forming ligaments where adaptability is required, as, for instance, in the ligamenta subflava, in the true vocal

cords, and the suspensory ligament of the penis. It forms a part of the middle coat of blood-vessels, where it is of use in converting a jetting stream into a continuous flow. It is useful in the lungs, allowing for their expansion and recoil during respiration. It is found in the capsule of the spleen, where it permits of the enlargement of the gland, which occurs during digestion, and adapts itself to the diminished size of the gland when digestion is completed. Lastly, it is found throughout the skin generally, where it is of use in allowing it to stretch and to adapt itself to the various movements of the body. Finally, together with white fibrous tissue, it enters into the formation of areolar tissue.

5. AREOLAR TISSUE.

Microscopical Appearances.—If a portion of areolar tissue is examined under the microscope it is seen to consist of yellow elastic, and white fibres, which are associated together to form bundles, which cross and recross in all directions. Though the bundles of fibres thus freely interlace, the individual fibres of the bundles are, in most instances, parallel to one another. Between the bundles there is a cer

tain amount of granular ground substance which seems to cement them together; and in this are situate numerous stellate protoplasmic cells, which send their processes into the bundles of fibres.

Blood-vessels traverse the tissue on their way to other parts, but only a few have been seen to terminate in it.

Nerves also, in like manner, pass through it, but have not been seen to end in it.

Lymphatics.-A close connexion exists between the cells of areolar tissue and the commencement of the lymphatics; for the flattened cells, which enter into the formation of the walls of the lymphatic vessels, become continuous with the processes of the stellate cells of the areolar tissue.

Thus the lymphatics may be said to take origin, as it were, in a network from the cell spaces of the tissue. Absorption by the lymphatics readily takes place from these spaces.

Not only do large lymphatics traverse this tissue on their way to or from distant parts, but in many cases large lymphatic networks may be observed beneath the skin, and beneath mucous and serous membranes.