TISSUES, ORGANS, AND SYSTEMS. 93 



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 mas- 

 toid cells, and the conjunctiva. Among the glands, all the larger, pre- 

 sent in their excretory ducts, a distinct mucous membrane, as the lungs 

 from the glottis to the finest bronchice; 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 immediately 

 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 development support this view, 

 it would seem to be at any rate justifiable; yet every one is free, not- 

 withstanding, to look more to the differences which certainly do exist 

 between the parts in question, and to consider them as distinct struc- 

 tures. 



g. The membranes of the veins, lymphatics, the adventitious coat of 

 the arteries, and the endocardium, consist of a loose connective tissue 

 not altogether dissimilar to that of the fibrous membranes, and of finer 

 or coarser elastic fibrous networks, with' which in the veins smooth 

 muscles are also partly mixed. 



h. The so-called vascular membranes (tunica* vasculosce), to which 

 belong the pia mater, with the plexus choroidei, the choroid coat and the 

 iris, all contain very numerous vessels, which, however, appear to have less 

 reference to the membranes themselves than to the nutrition of other 

 organs. Supporting these vessels we have either a common connective 

 tissue, in which there are no elastic fibres (iris, pia mater), with paral- 

 lel, matted, and anastomosing bundles, or a homogeneous connective 

 tissue (plexus choroidei, choroidea), to which, as in the choroid, peculiar 

 elements, namely, anastomosing cells, generally filled with more or less 

 pigment, may be added. 



i. The homogeneous connective tissue. In many organs we find mem- 

 branes whose chemical nature agrees with that of connective tissue, but 

 which neither contain distinct bundles nor fibres, and appear to be more 

 homogeneous. Such is the homogeneous tissue which often invests the 

 bundles of the arachnoid singly, or in a number together ; the coats of 

 the Malpighian corpuscles of the spleen, and of the glandular follicles 

 of the intestine (tonsils, lingual-follicles, the solitary and Peyerian 



