It is easy to overlook this twofold compo sition of areolar tissue in specimens examined in water, but their discrimination is made easy by a trifling artifice. This consists in adding a drop of acetic acid, which instantly swells the white bands, and makes them transparent, but produces no change in the yellow fibrils. These effects of the acid may be watched, if the agent be made to spread gradually over the specimen ; and there can scarcely be conceived a more beautiful example of the aid chemistry will afford anatomy than that presented in the course of this interesting process.t The change produced in the white bands is such as to skew very clearly that they are not truly fasciculi, or aggregations of fibrillw. The action of the acid on these two elements is identical with that produced on the two tissues to which I have shewn them to be anatomically allied.
To these two elements of areolar tissue are to be attributed physical properties similar to those of white and yellow fibrous tissues, and these will vary greatly in different situations, accord ing to the proportion and mode of arrangement under which the two elements coexist.
Of the areolur tissue of glands.—There appears to be a very prevalent misconception with regard to the quantity of this tissue found in the interior of the large glands, as the liver and kidney. It is imagined that it pene trates into every interstice, mingles with the capillary rete, and envelopes the ultimate secreting tubules. It is, however, impossible in the most recent specimen of these organs to discover anything answering to this descrip tion. All that can usually be detected is a small quantity accompanying the larger vessels in their course within the organ, and forming septa between its coarse subdivisions. And it would be difficult to suppose a purpose which a more abundant supply could subserve. The ca pillary network and the secreting tubules by their mutual and intricate interlacement sufficiently sustain one arother ; no freedom of motion is required between them ; there is no force tending to separate them. I am far from saying, however, that the ultimate substance of these glands consists only of simple mucous membrane and bloodvessels. In the inter stices of these there are probably nerves and lymphatics, of the mode of termination of which we know nothing, but which seem much fewer than is commonly supposed. There is also more or less of an interstitial amorphous substance, hereafter to be described.
In these glands and in the substance of many compound mucous membranes there are to be seen here and there small bodies not unlike cellular tissue in an early stage of its development. They have a bulging nucleus
from which they taper to the extremities; and they are much longer and slenderer than the prismatic epithelium. With their nature and use 1 am at present quite unacquainted.
The lungs seem mainly to owe their extraordi nary elasticity to the yellow fibrous element of their submucous areolar tissue. This is spread in great abundance under the whole surface, and much predominates over the white. the trachea and bronchia it is besides largely deve loped in longitudinal bands visible through the mucous membrane. In the whole of this re gion its fibrils take a general longitudinal direc tion, but branch and inosculate at very frequent intervals, enclosing areolse of small dimensions. But this element does not cease with the tubes; it is prolonged in the form of branching, arching bands over the basement membrane of the air cells, which it renders elastic and firmly supports.
Where mucous membranes are not destined to move on the parts they cover, the areolar tissue beneath them is very scanty. This is the case in the nasal cavities, even in the por tions furnished with a great substratum of bloodvessels. But where much motion is re quired, as where a muscular lamina underlies the mucous, and the enclosed cavity is liable to vary in its dimensions, the areolar tissue is co pious, and very similar in its elements and in the size of its interstices to the ordinary forms. Examples of this are seen in the whole alimen tary tract.
But it is under the cutaneous part of the mu cous system that this tissue assumes its highest developement. Elsewhere its object is to pro mote freedom of movement, or to confer elasti city. Here it answers both these purposes, and in addition gives a great capacity of resistance against external pressure and violence. The former end is attained by the structure called subcutaneous fascia, which is a large quantity of this tissue in its ordinary form. The two latter are effected by that more condensed part to which the term of cutis has been given. This last is the structure to which the submucous areolar tissue of the intestinal canal mainly cor responds, as may be shown by an exarninatioh of the submucous tissue of the mouth, pharynx, and (esophagus, which holds an intermediate place. To describe its modifications in different situations would be to encroach too much on the province of another article (see Star), and a few general remarks must here suffice.