TISSUES OF THE BODY. 365 



processes presents a most peculiar appearance under the microscope. Be- 

 tween these cells, however, may be observed a greater or smaller number 

 of roundish marks of varying size, sometimes in the form of a dark spot 

 (fig. 358, a, a), sometimes in that of a ring (b). 



These have been hitherto held by many to be preformed openings or 

 "stomata," and to account for the exit of white and coloured blood cor- 

 puscles (p. 128). 



The recent investigations of Arnold also have confirmed the correctness 

 of this view. 



Tow, whilst we believe that in many parts of the body the whole of the 

 capillary vessel is represented in this cellular tube just mentioned, there 

 are some localities in which the latter is enveloped in a delicate homo- 

 geneous membrane, probably the first indication of the tunica intima. 

 There are again places in which the surrounding connective-tissue forms 

 an external envelope for all capillaries, even the most minute, in fact, 

 an adventitia capillcms, which may be regarded as equivalent to the 

 tunica cellulosa of larger trunks. Thus we find the capillaries of the 

 brain (fig. 359, a) enclosed in a homogeneous nucleated membrane, and 

 those of lymphoid organs (I) closely invested in reticular connective 

 substances. Again, other more considerable, but still capillary vessels, 

 may be enveloped loosely in a layer of connective-tissue (c), and the space 

 thus left between the latter and the vessel may serve the purpose of a 

 lymphatic passage. We shall refer again to these lymph sheaths, and 

 only stop to remark here, that every adventitial tissue of a blood-vessel 

 containing lymphoid cells, must not be regarded as one of the latter. 

 Another circumstance also may frequently give rise to the deceptive appear- 

 ance of such a sheathing, namely, that a blood-vessel is often bounded 

 on each side by lymphatic canals ; this is most commonly seen in 

 uninjected preparations. 



Now, although in those cases just described the capillary wall is easy 

 of recognition in it own individuality, there are others in which the cells 

 of the tube become so intimately united with the adjacent tissues, that 

 they are either totally inseparable from the latter, or only so with the 

 help of the stronger reagents at our disposal, although treatment with 

 nitrate of silver naturally renders them visible. This is the texture of 

 the capillary canal as found, for instance, in the membrana pupillaris of 

 the fcetal eye, the skin, and other strong fibrous structures. 



203. 



Passing on now from these finer forms to the larger trunks, we meet 

 again with those layers already known to us, namely, the epithelial, and 

 the intima enveloping it, and finally the external fibrous coat. The latter 

 appears in the form of longitudinally striated connective-tissue, with 

 vertically arranged nuclei or connective-tissue corpuscles. 



Very soon, however, even in extremely fine trunks, especially as we 

 pass towards the artery, between the two internal membranes and. the 

 external coat, a thin layer of transversely arranged contractile fibre cells 

 may be observed, whose nuclei we may easily detect. The latter are 

 spoken of as transversely oval. There can be now no doubt but that in 

 this we have before us the first rudiments of the middle or muscular coat 

 of the larger trunks. 



Once more to recapitulate : \re see first (a) the layer of flattened cells, 

 then (b) the longitudinally streaked internal coat, then (c) the transverse 



