AREOLAR TISSUE. 123 



through by transudation ; but that the albuminous matter will be for 

 the most part kept back, so that only a very small proportion of it is to 

 be found in the transuded liquid. This appears to be a sufficient ex- 

 planation of the presence of a weak serous fluid in the cavities of areolar 

 tissue ; and there is not any necessity, therefore, to imagine the exist- 

 ence of a secreting power, either in the areolar tissue itself, or in the 

 walls of the capillaries which traverse it. When there is a want of 

 firmness or tone in the walls of the vessels, producing (as we shall here- 

 after see, 609) an increased pressure of the contained fluid on their 

 walls, and diminished resistance, the watery part of the blood will have 

 an unusual tendency to transudation : and we accordingly find that it 

 then distends the areolae, and produces dropsy. The physical arrange- 

 ment of the parts of the tissue is so much altered, that its elasticity is 

 impaired ; and it consequently pits on pressure, that is, when pressure 

 has made an indentation in the surface, this is not immediately filled up 

 when the pressure is withdrawn, but a pit remains for some seconds or 

 even minutes. The free communication which exists among the inter- 

 stices, is shown by the influence of gravity upon the seat of the dropsi- 

 il effusion ; this always having the greatest tendency to manifest itself 



the most depending parts, a result, however, which is also due to 

 the increased delay that takes place in the circulation in such parts, 

 when the vessels are deficient in tone. This freedom of communication 

 is still more shown, however, by the fact, that either air or water may 

 ~>e made to pass by a moderate continued pressure, into almost every 

 >art of the body containing Areolar tissue ; although introduced at only 



single point. In this manner it is the habit of butchers to inflate 

 veal : and impostors have thus blown up the scalps and faces of their 

 children, in order to excite commiseration. The whole body has been 

 thus distended with air by emphysema in the lung; the air having 

 escaped from the air-cells into the surrounding areolar tissue, and thence, 

 by continuity of this tissue with that of the body in 

 general at the root or apex of the lungs, into the Flg ' 8> 



entire fabric. 



197. The structure of the Serous and Synovial 

 Membranes is essentially the same with that of Areolar 

 tissue. It is the peculiar character of these mem- 

 branes to form closed bags or sacs, having a very 

 smooth and glistening inner surface, and containing 

 a fluid more or less allied in composition to the 

 serum of the blood. The disposition of the Syno- 

 vial membranes may be understood by studying one 

 of the simpler forms of the joints, such as is repre- 

 sented in the accompanying diagram; but although Ideal8ectionofajoint; 

 originally continuous over the surfaces of the Articu- a > > the extremities of 



1 /~i , >i ,i ci i -i i the two articulated bones; 



Cartilages, the feynovial membrane does not con- b, b, the layers of carti- 

 tinue to be distinctly traceable after the joint has come *f ^^synov'aimem- 1 

 into play, and its vessels retreat from the portion over ciiatd^Turfa^? 6 a and 

 which the two surfaces are exposed to friction, but passing from one'to the 

 form a circle round its margin, from which the Carti- 0< 

 lage is nourished ( 278). The arrangement of the Serous membranes 



