CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



the veins in the lungs and trachea; the stylo-hyoid, thyro-hyoid, and crico- 

 thyroid ligaments; in the true vocal cords; and in areolar tissue. 



Structure. Elastic tissue occurs in various forms, from a structureless, 

 elastic membrane to a tissue whose chief constituents are bundles of fibers 

 crossing each other at different angles; when seen in bundles elastic fibers are 



yellowish in color, but individual fibers are not 

 so distinctly colored. The varieties of the tissue 

 may be classified as follows: 



a. Fine elastic fibrils, which branch and 

 anastomose to form a network. This variety 

 of elastic tissue occurs chiefly in the skin and 

 mucous membranes, in subcutaneous and sub- 

 mucous tissue, in the lungs and true vocal 

 cords. 



b. Thick fibers, sometimes cylindrical, some- 

 times flattened, which branch, anastomose and 

 form a network: these are seen most typically in 

 the ligamenta subflava and also in the ligamen- 

 tum nuchae of such animals as the ox and horse, 

 in which that ligament is largely developed, 

 figure 40. 



A certain number of connective-tissue cells 

 are found in the ground substance between the 

 elastic fibers which make up this variety of connective tissue, page 33. 



Areolar Tissue. This variety of fibrous tissue has a very wide dis- 

 tribution and constitutes the subcutaneous, subserous, ' and submucous tis- 

 sue. It is found in the mucous membranes, in the true skin, and in the outer 

 sheaths of the blood-vessels. It forms sheaths for muscles, nerves, glands, 

 and the internal organs, and, penetrating into their interior, supports and con- 

 nects the finest parts. 



Structure. To the naked eye it appears, when stretched out, as a fleecy, 

 white, and soft meshwork of fine fibrils, with here and there wider films joining 

 in it, the whole tissue being evidently elastic. The openness of the meshwork 

 varies with the locality from which the specimen is taken. Under the micro- 

 scope it is found to be made up of fine white fibers, which interlace in a most 

 irregular manner, together with a variable number of elastic fibers. On the 

 addition of acetic acid, the white fibers swell up, and become gelatinous in 

 appearance; but as the elastic fibers resist the action of the acid, they may still 

 be seen arranged in various directions, sometimes appearing to pass in a more 

 or less circular or spiral manner round a small gelatinous mass of changed 

 white fiber. The cells of areolar tissues are connective-tissue corpuscles. 



Gelatinous Tissue. Gelatinous connective tissue forms the chief 

 part of the bodies of such marine animals as the jelly-fish. It is found in 



FIG. 40. Elastic Fibers from 

 the Ligamenta Subflava. X 200. 

 (Sharpey.) 



