SKIN, AND MUCOUS MEMBRANES. 125 



branches, the membranous linings of the ducts of the salivary glands, 

 pancreas, and liver ; these membranes proceed into all the subdivisions 

 of the ducts, and line the ultimate follicles or caeca in which they ter- 

 minate. Again, the Broncliio-ptilmonary mucous membrane commences 

 at the nose, and passes along the air-passages, down the trachea, through 

 the bronchi and their subdivisions, to line the ultimate air-cells of the 

 lungs ; communicating in its course with the gastro-intestinal. Another 

 mucous membrane of small extent commences at the puncta lachrymalia, 

 lines the lachrymal sac and the nasal duct, and becomes continuous with 

 the preceding. Another, which may be considered a kind of offset from 

 either of the first two, passes up from the pharynx along the Eustachian 

 tube, and lines the cavity of the tympanum. 



201. Near the opposite termination of the alimentary canal, more- 

 over, we have the G-enito-urinary mucous membranes ; these commence 

 in the male by a single external orifice, that of the urethra ; passing 

 backwards along the urethra, the genital division is given off", to line the 

 seminal ducts, the vesiculae seminales, the vasa deferentia, and the secre- 

 ting tubuli of the testis ; another division proceeds along the ducts of 

 the prostate gland, to line its ultimate follicles, and another along the 

 ducts of Cowper's glands ; whilst the urinary division lines the bladder, 

 passes up along the ureters to the kidney, and then becomes continuous 



dth the membrane of the tubuli uriniferi. In the female, the .urinary 

 division commences at once from the vulva; whilst the genital passes 

 ilong the vagina into the uterus, and thence along the Fallopian tubes 

 their fimbriated extremities, where it becomes continuous with the 



jrous lining of the abdominal cavity, the peritoneum. 



202. Besides the glandular prolongations here enumerated, there are 

 many others, both from the internal and external surface. Thus we 

 have the Mammary mucous membrane, commencing from the orifices of 

 the lactiferous ducts, passing inwards to line their subdivisions, and 

 forming the walls of the ultimate follicles. In the same manner the 

 Lachrymal mucous membrane is prolonged from the conjunctival mucous 

 membrane, which covers the eye and lines the eyelids, and which is con- 

 tinuous with the skin at their edges. There are several minute glands, 

 again, in the substance of the skin, and in the walls of the alimentary 

 canal, which need not be here enumerated ; but which contribute 

 immensely to the extension of the surface of the mucous membrane, a 

 prolongation of this being the essential constituent in every one. In 

 their simplest form, these glandulse are nothing more than little pits 

 or depressions of the surface ; these are found both in the Skin and 

 Mucous membrane, and are particularly destined for the prouduction of 

 their protective secretions, hereafter to be described. 



203. We have seen, then, that the essential character of the Mucous 

 membranes, as regards their arrangement, is altogether different from 

 that of the serous and synovial membranes. For whilst the latter form 

 shut sacs, the contents of which are destined to undergo little change, 

 the former constitute the walls of tubes or cavities, in which constant 

 change is taking place, and which have free outward communications. 

 Thus in the gastro-intestinal mucous membrane, we have an inlet for the 

 reception of the food, and a cavity for its solution, the walls of which 



