clxxx BLOOD-VESSELS. 



may, after a time, be made to contract to their usual size by the reapplication of 

 stimuli. 



Termination of arteries. The only known termination of arteries is in veins, and 

 this takes place by means of capillary vessels of some of the forms above described, 

 unless in the maternal part of the placenta, and in the interior of erectile organs, 

 in which it has been supposed that small arteries open into wide venous cavities, 

 without the intervention of capillaries. Additional modes of termination have, 

 however, been assumed to exist. Thus, it was believed that branches of arteries 

 ended in exhalant vessels, which, in their turn, terminated by open orifices on the 

 skin, on the surface of different internal cavities, or in the areolar tissue ; other 

 arterial branches were supposed to be continued into the ducts of secreting glands, 

 and it was, moreover, imagined that, besides the red capillaries, there existed finer 

 vessels, which passed between the arteries and the veins, and from their small- 

 ness were able to convey only the colourless part of the blood. The existence of 

 these colourless or ll serous " vessels, as they were called (vasa serosa, vasa non 

 rubra), was held, by most authorities, to be universal ; by others it was assumed 

 as necessary, at least, in the colourless textures; but these views have now 

 been generally abandoned, although they long prevailed almost without question, 

 and were made the basis of not a few influential doctrines in pathology and 

 practical medicine. Of course it is not denied, that in growing parts there may be 

 capillaries in an incomplete state of development, which admit only the plasma of 

 the blood. 



Erectile, or cavernous tissue. By this term is understood a peculiar structure, 

 forming the principal part of certain organs which are capable of being rendered 

 turgid, or erected, by distension with blood. It consists of dilated and freely inter- 

 communicating branches of veins, into which arteries pour their blood, occupying the 

 areolae of a network formed by fibrous, elastic, and probably contractile bands, named 

 trabeculae, and inclosed in a distensible fibrous envelope. This peculiar arrangement 

 of the blood-vessels scarcely deserves to be regarded as constituting a distinct texture, 

 though reckoned as such by some writers ; it is restricted to a very few parts of the 

 body, and in these is not altogether uniform in character ; the details of its structure 

 will, therefore, be considered with the special description of the organs in which it 

 occurs. 



DEVELOPMENT OF BLOOD-VESSELS. 



The first vessels which appear are formed within the ovum, in the germinal mem- 

 brane, and the process subsequently goes on in growing parts of the animal body. 

 New vessels, also, are formed in the healing of wounds and sores, in the organisation 

 of effused lymph, in the restoration of lost parts, and in the production of adventi- 

 tious growths. The following may serve as an outline of the process. 



The network of vessels which form the vascular area in the germinal membrane of 

 the egg at an early stage of incubation (see page li.), consists of arteries and veins 

 communicating, without capillaries. These vessels are at first solid cylinders of 

 larger or smaller diameter, made up of formative cells cohering together. By 

 liquefaction of their substance in the interior, these cylinders become tubes, and 

 their central cells thus set free are the primitive blood-corpuscles. The uniformly 

 cellular substance forming the wall of the primitive vessels is then converted 

 into the different coats. It is probable that a similar mode of formation of arte- 

 ries and veins goes on within the body of the embryo as its organs and members 

 are progressively developed; but arteries and veins may also begin as capillaries, 

 which grow into larger vessels, as will presently be explained. 



The small vessels and capillaries originate from nucleated cells similar to those 

 which at first constitute the different parts of the embryo. The cell-wall, or envelope, 

 of these cells, shoots out into slender pointed processes, tending in different direc- 

 tions, so that they acquire an irregularly star-shaped or radiated figure. The pro- 

 longations from neighbouring cells encounter one another, and join together by their 

 ends, and the irregularly ramified or reticular cavities thus produced are the channels 

 of rudimentary capillaries. In growing parts, such as the tail of batrachian larvse, 

 where new vessels are formed in the vicinity of those already existing, as represented 

 in the adjoining figure (xcrx.) by Kolliker, not only do the processes of the stellate 

 cells join those of neighbouring cells, but some of them meet and join with similar 



