IOO THE TISSUES. 



Elastic Fibers. These are homogeneous, highly refractive, dis- 

 tinctly contoured fibers, varying in size from I // to 6//, and in some 

 animals are even larger. They branch and anastomose, and are not 

 cemented into bundles. When extended, they appear straight ; 

 when relaxed, they show broad, bold curves, or are arranged in 

 the form of a spiral. The broken ends of the fibers are bent in the 

 form of a hook. F. P. Mall has shown that elastic fibers are com- 

 posed of two distinct substances an outer delicate sheath which does 

 not stain in magenta, and an interior substance which is intensely 

 colored in this stain. The interior substance is highly refractive. 

 Elastic fibers are not affected by acetic acid, but are readily digested 

 in pancreatin and less readily in pepsin. They yield elastin on 

 boiling. 



J. MUCOUS CONNECTIVE TISSUE. 



Mucous connective tissue is a purely embryonal type, and 

 scarcely represented in the adult human body. It consists of 

 branched, anastomosing cells imbedded in a ground-substance which 

 gives a reaction for mucus and contains a varying number of 

 white fibrous tissue fibers which are developed from a syncytial 

 protoplasm. The latter as well as the mucous matrix are, directly 

 or indirectly, the products of the cells. During the development 

 of the embryo this tissue is found in large quantities in the umbilical 

 cord, and is here known as Whartori s jelly. Mucous connective 

 tissue is merely another name for embryonic connective tissue and is 

 found as such wherever connective tissue develops. In the adult 

 it occurs in the posterior chamber of the eye as the vitreous humor. 



2. RETICULAR CONNECTIVE TISSUE. 



Reticular connective tissue is a fibrous connective tissue in which 

 the intercellular substance has disappeared. The tissue is often 

 described as being composed of anastomosing branched cells, ar- 

 ranged in the form of a network with open spaces. The obser- 

 vations of Ranvier and Bizzozero, and more recently those of Mall, 

 have shown that the framework of reticular tissue is composed of 

 very fine fibrils or bundles of fibrils. These interlace in all planes 

 to form a most intricate network, surrounding spaces of varying size 

 and shape. According to F. P. Mall, the fibrils of reticular tissue 

 differ chemically from both the white and elastic fibers, although their 

 composition has not been fully determined. Like white fibrous 

 tissue, reticular tissue is not digested by pancreatin, but, unlike 

 white fibrous tissue, it does not appear to yield gelatin upon boiling 

 in water, but a mixture of gelatin and reticulin, a substance identi- 

 fied by Siegfried. 



The cells of reticular connective tissue, which are flattened and 

 often variously branched, lie on the reticular network, being often 

 wrapped about the bundles of fibrils. Unless they are removed, the 



