5 8 CONNECTIVE TISSUE. [sect. 26. 



membranes which are just as thin and glandless as serous mem- 

 branes ; and, on the other hand, the synovial capsules approach 

 the mucous membranes by their richness in blood and the nature 

 of their secretion. The mucous membranes and the external skin 

 correspond in all their principal parts, so that the transitions oc- 

 curring between them, as at the lips, eyelids, and elsewhere, are 

 not surprising. To the mucous membranes belong the innermost 

 coat of the intestinal canal, the lining of the nasal fossa; and ac- 

 cessory cavities, the lining of the Eustachian tube, tympanum 

 and mastoid cells, and the conjunctiva. All the larger glands 

 possess a distinct mucous membrane in their excretory ducts : thus, 

 the lungs are provided with such a membrane from the glottis to 

 the finest bronchia • the liver, in the larger hepatic ducts and in the 

 gall-bladder ; the pancreas, in the pancreatic duct ; the urinary and 

 sexual organs, in the urethra, bladder, ureter, pelvis of the 

 kidneys, vagina, uterus, and Fallopian tubes — in the mammary 

 ducts and sacs— and in the seminal vesicles and vas deferens. 

 In all these glands, the mucous lining ultimately passes into the 

 membrane of the gland-tubes and vesicles; so that these may 

 be regarded as being formed by an attenuated mucous mem- 

 brane. The same may be said of the smaller glands, as those of 

 the intestine, which are directly continuous with large expansions 

 of mucous membrane ; only in this case, we should have to regard 

 the small glands of the skin also as being formed by processes 

 of the latter. Since both development and physiology, to some 

 extent, support this view, so far, at least, it appears warranted. 

 Still it must not be forgotten, that many memlrance propria are 

 not of the nature of connective tissue, but appear as excretions 

 of the cells of the gland-tubes, so that the secerning glandular 

 elements still preserve a certain independence. 



g. The coats of the veins and lymphatics, the adventitious coat of 

 the arteries, and the endocardium, consist of a connective tissue not 

 unlike that of the fibrous membranes, and of finer and coarser 

 elastic fibrous networks, with which, in the veins, smooth muscular 

 fibres may be mixed. 



h. The so-called vascular tunics, to which belong the pia mater 

 with the choroid plexus, the choroid and iris. All contain nu- 

 merous vessels, which, however, are destined less for these tissues 

 themselves than for the nourishment of other organs. For sup- 

 porting the vessels either ordinary connective tissue is employed, 

 destitute of elastic fibres (iris, pia mater), with parallel, matted or 

 anastomosing bundles, or a homogeneous connective tissue (choroid 



