TISSUES, ORGANS, AND SYSTEMS. 73 



in the intestine is also called the tunica nervea. The for- 

 mer is of the same structure as the corium, only softer, and 

 • not unfrcquently poor in elastic tissue. The mucous mem- 

 branes are distinguished from the serous, in general by their 

 greater vascularity, their more considerable thickness, their 

 numerous glands, and the mucous secretion, which may be 

 especially ascribed to their soft epithelium ; though there 

 are mucous membranes which are as delicate and glandless 

 as serous membranes ; and, on the other hand, the synovial 

 capsules may approximate the mucous membranes in their 

 vascularity and the nature of their secretion. The mucous 

 membranes and the external skin are analogous in all their 

 principal components, whence the transitions between the 

 two, such as exist upon the lips, eyelids, and elsewhere, are 

 not surprising. To the mucous membranes belong the 

 innermost coat of the tractus intestinalis ; the lining of the 

 nasal passages, and of their secondary cavities ; the Eustachian 

 tube, the tympanum and mastoid cells, and the conjunctiva. 

 Among the glands, all the larger, present in their excretory 

 ducts, a distinct mucous membrane, as the lungs from the 

 glottis to the finest bronchia; the liver in the larger gall- 

 ducts and in the gall-bladder; the pancreas in the ductus 

 pancreaticus; the urinary and sexual organs ; in the urethra, 

 bladder, ureters, pelvis of the kidneys, vagina, uterus, and 

 oviducts ; and in the ducts and follicles of the mammary 

 gland ; in the seminal vesicles and in the vas deferens. In all 

 these glands the coats of the mucous membrane pass imme- 

 diately into the walls of the glandular tubes and vesicles, 

 which might thus be regarded as composed of a more delicate 

 mucous membrane. The same might be said of the smaller 

 glands, as those of the intestine, which are directly connected 

 with the larger mucous expansions, only in that case the 

 smaller glands of the skin must be regarded as attenuated 

 processes of it. Inasmuch as both physiology and develop- 

 ment support this view, it would seem to be at any rate 

 justifiable ; yet every one is free, notwithstanding, to look 

 more to the differences which certainly do exist between 

 the parts in question, and to consider them as distinct 

 structures. 



(j. The membranes of the veins, lymphatics, the adven- 



