CAVERNOUS VESSELS. VASCULAR PLEXUSES. 289 



all capillaries are not constructed alike, and that they are 

 not altogether intercellular tubes. Supposing that a nu- 

 cleated or a non-nucleated, and in the first instance solid, 

 process elevates itself from a capillary wall, gradually becomes 

 elongated and hollow, its cavity communicating with the 

 lumen of the capillary, this may, in favourable cases, be 

 regarded as a funnel-shaped outgrowth from a cell, but it is 

 not an intercellular passage. In many instances, as in tad- 

 poles, such outgrowths from capillaries are discoverable, which 

 present no trace of cellular structure when treated with nitrate 

 of silver, although in older vessels they can be readily brought 

 into view. Must we not consequently conclude that the 

 capillary wall thus beset with processes, is similarly composed 

 to the funnel-like projections, and that, as Strieker says, they 

 are composed of protoplasm, which has assumed a tubular form? 

 The capillary wall is contractile both in young and in adult 

 animals. Strieker* saw the capillaries, not only of tadpoles, 

 but of the nictitating membrane of frogs, contract to such an 

 extent, that not even a single file of blood corpuscles could 

 traverse them. Lastly, he observed small loop-like projections 

 raise themselves from the wall of the capillaries of the nicti- 

 tating membrane, and again become retracted. It is not im- 

 probable that it is by means of such contractions the corpuscles 

 are pressed into the capillary wall, and ultimately made to 

 traverse them. 



CAVERNOUS VESSELS, LACUNAR BLOOD PATHS, VASCULAR 



PLEXUSES. 



Cavernous vessels result from the unravelling of the vas- 

 cular wall, which becomes converted into a spongy tissue ; or 

 from its becoming fibrous and membranous towards the lumen 

 of the vessel, giving off processes that intercommunicate with 

 each other, and which either form a spongy layer on the inner 

 surface of the vascular wall, or a plexus traversing its entire 

 calibre. A similar result is obtained from the occurrence of 

 quickly consecutive anastomoses of vessels of various size. The 



* Wiener SitzungslericJite, Bande li. and Hi. 



