TISSUES OF THE BODY. 



229 



At certain points they contain no glands, but in most cases the latter 

 occur in such quantities that the groundwork of connective-tissue is 

 completely thrown into the background owing to their amount, as has 

 been already remarked. As an instance of such extreme richness in 

 glands, we may take the mucous membrane of the stomach (figs. 

 222 and 223). Attention has been recently directed to the occurrence 

 of smooth muscle fibres in many of these membranes, to which we 

 must ascribe considerable physiological importance. We shall refer to 

 this again. 



9. The so-called vascular membranes of the brain and eye, the pia 

 mater and plexus chorioidea, and choroid, also 



belong to the connective-tissues. In all these 

 we find a multitude of blood-vessels supported 

 by soft connective-tissue. The latter appears 

 under different forms. One of these, that of 

 the choroid of the eye, has been already de- 

 scribed (p. 219). The plexus chorioidei show 

 in the infant a completely homogeneous sub- 

 stance, in which round non-ramified cells are 

 imbedded. In the adult also the whole bears 

 in general the character of an undeveloped 

 streaky connective- tissue (Hdckel). In the^'a 

 mater, on the other hand, we have the fibrillated 

 form of the latter. 



10. Finally, connective-tissue tunics are 

 found widely distributed throughout the vascu- 

 lar system. The. endocardium may be reckoned 



among these, also the external coat of the vessels, or so-called tunica adven- 

 titia, and most of the middle and internal layers of the arteries, veins, and 

 lymphatics. Great variety is met with here, however. Together with 

 structures of fibrillated connective-tissue, with a larger or smaller propor- 

 tion of elastic fibres, are found, especially in arteries, membranes which, 

 without having any bundles of the former, present in a homogeneous 

 ground substance elastic networks alone, of either fine or coarse, or some- 

 times very thick fibres. They may also occur homogeneous without fibres. 

 Thus we find a gradual transition from connective-tissue membranes to 

 purely elastic ones. 



11. In other parts also we encounter a preponderance of elastic 

 elements, with a sometimes slight, sometimes great, decrease, and at other 

 times almost complete disappearance of the fibrillated interstitial connec- 

 tive-tissue. This is the case in the various ligaments and membranes of 

 the larynx, the trachea, the bronchi, and tissues of the lungs. Externally 

 also, around the oesophagus, is found a principally elastic layer, and 

 connecting the latter with the tubes of the respiratory apparatus. Beside 

 other more limited occurrences, we may reckon also to this category the 

 ligamenta flava of the spinal column and ligamentum nuchce of some 

 mammalian animals. 



137. 



All connective-tissues of the living body are, as has been already 

 remarked, saturated by small quantities of an animal fluid, in which we 

 may suppose the matters of nutrition and decomposition to be contained. 

 This, arriving by the blood-vessels and exuding from them, gives up its 



Fig. 223. Vertical section of the 

 mucous membrane of the hu- 

 man stomach, a, papillae of the 

 surface ; b, peptic glands. 



