MUCOUS MEMBRANES. 



119 



contour. They are disposed around the central vessel in from two to five 

 layers, the individual cells being indistinctly outlined and composed of clear 

 protoplasm containing a relatively large and deeply-staining nucleus. Con- 

 cerning the mooted question as to the presence of chromaffine cells within 

 the coccygeal body, the testimony as to their absence seems convincing 

 The cells at no period exhibit the chrome-reaction, and have no histogenetic 



Connective tissue 



stroina 



Capsule 



Capillaries 



Cells 

 Blood-vessels 



FIG. 158. Section of coccygeal body of adult man. X 220. 



relation to the sympathetic system. On the other hand, the epithelial char- 

 acter of the cells, their intimate relation to the blood-vessels, and the absence 

 of excretory ducts, has suggested its inclusion as an organ of internal secre- 

 tion ; however, there is no evidence warranting such assumption. 



MUCOUS MEMBRANES AND GLANDS. 



The apertures of the digestive, respiratory and genito-urinary tracts on 

 the surface of the body mark localities at which the integument becomes 

 continuous with the walls of cavities and passages communicating with the 

 exterior. The linings of such spaces and tubes constitute mucous mem- 

 branes. The latter, however, not only form the free surface of the chief 

 tracts, but are continuous: with the ducts and tubes leading into the glands, 

 which are secretory appendages developed as outgrowths from the mucous 

 membranes. These membranes line two great tracts, the gastro-pulmonary 

 and the genito-urinary. 



THE MUCOUS MEMBRANES. 



Every mucous membrane comprises two distinct parts : the epithelium, 

 which forms the immediate free surface and protects the delicate subjacent 

 structures, and the tunica propria, a connective-tissue stroma which gives 

 place and support to the terminal branches of the blood-vessels and nerves 

 and the beginnings of the lymph-channels. A stratum of submucous tissue, 

 ordinarily loose and extensible, usually connects the mucous membrane with 

 the surrounding structures. 



