1390 



VEIN. 



dilatations of the veins at the base of the 

 brain ; the large amount of veins in the 

 parenchymatous viscera ; the venous plexuses 

 of the spinal canal, together with the venous 

 cells and tubes in the cancellated tissue of all 

 bones, form a series of diverticula and reser- 

 voirs, equal to all emergencies*, where the 

 Wood may slowly flow, or recede, or stagnate. 

 But the most striking examples of venous re- 

 servoirs are furnished by comparative anatomy. 

 All those animals which have diving habits 

 subject themselves to peculiar disturbance of 

 circulation. The prolonged stoppage of re- 

 spiration, but still more the immense pressure 

 of water to which they are exposed, when 

 deeply submerged, mechanically empties all the 

 superficial veins and produces much deep- 

 seated and visceral congestion. This is com- 

 pensated for by a special arrangement. In 

 diving birds there is great dilatation of the 

 posterior vena cava. In the Grebes and Divers 

 (Podiceps and Colymbus) the posterior cava is 

 largely dilated in the liver and below, extend- 

 ing to opposite the kidneys. In the Cetacea, 

 which are subject to the same influence, 

 enormous dilatable venous plexuses, placed 

 within the abdominal cavity, perform the same 

 office. 



Venous absorption probably occurs wherever 

 veins exist, but especially in the alimentary 

 canal, where, in conjunction with the lacteals, 

 they perform the important office of extracting 

 from the food the different materials which 

 enter the circulation and nourish the various 

 tissues of the body. 



It is not my intention to enter upon any of 

 the elaborate arguments or disquisitions on the 

 evidence of venous absorption. 1 shall content 

 myself with a brief summary of the most im- 

 portant salient points. 



That absorption by the veins takes place 

 independently of the lacteals has been proved 

 by a series of conclusive experiments conducted 

 by Tiedemann and Gmelinf, who administered 

 to a number of animals various substances of 

 distinct and easily recognisable physical and 

 chemical properties, and afterwards analysed 

 the venous blood, the secretions, and the 

 chyle. Substances having deep colours, such 

 p.s cochineal, indigo, litmus, gamboge ; sub- 

 stances having strong odours turpentine, 

 assafoetida, garlic, musk and saline sub- 

 stances, such as sulphate of iron, chloride of 

 barium, ferro-cyanide and sulpho-cyanide of 

 potassium, were all in turn submitted to ex- 

 periment. Almost all of these matters were 

 found in the venous blood, several of them in 

 the urine, and a few only of the latter (the sa- 

 line substances) in the chyle. Therefore it ap- 

 pears that absorption did take place, in many 

 instances, independently of the lacteals, and 



* Not literally equal to all emergencies : the 

 great deep-seated congestion caused in the cold 

 stage of an ague-fit has ruptured the vena cava : 

 the same has occurred in violent muscular exertion. 



f Versuche liber d. Wege auf welchem Sub- 

 stanzen aus dem Magen u. Darmkanal ins Blut 

 g'elangen. Heidelberg. 1820. 



by the veins. This conclusion is fortified bv 

 other experiments conveying both positive 

 and negative evidence, which have been 

 performed by Majendie, Blake, Deltlle, Sega- 

 las, &c. 



Majendie and Delille in conjunction per- 

 formed the following experiment. They 

 severed all the parts of the posterior 

 extremity of a dog, excepting the arterv and 

 vein by which the circulation of the' limb 

 was kept up. They then introduced some 

 upas poison into the foot ; and in ten minutes 

 the animal died. It cannot be imagined here 

 that the lymphatics absorbed the poison, as 

 they were divided; but it may be objected 

 that the matter was mechanically introduced 

 into the divided extremities of the vessels in 

 the wound. This difficulty has been over- 

 come in another experiment devised by Ma- 

 jendie. He found that death was caused by 

 the introduction of nux vomica into the intes- 

 tine of an animal in which the lacteals had 

 been tied, where the veins, by which alone 

 absorption could here take place, remained 

 entire. An experiment exactly the reverse 

 of the last was performed by Segalas, which 

 completely bore out the idea of venous ab- 

 sorption. Instead of tying the lacteals and 

 leaving the veins untied, Segalas tied the 

 veins, the lacteals remaining unimpaired. The 

 result *vas, that when poison was applied to 

 the intestine, no absorption took place, and 

 no result followed. It was further found by 

 Mr. Blake, that ligature of the vena porta 

 prevented the action of poisons introduced 

 into the stomach. 



V. DEVELOPMENT OF VEINS. Are the 

 veins, large and small, originally developed as 

 capillaries, the succeeding changes by which 

 their texture and size are modified taking 

 place subsequently to their being permeable 

 sanguiferous channels ? Whether this is en- 

 tirely the case is doubtful : that it is so to 

 some extent is certain; and therefore the 

 development of veins would involve two con- 

 siderations first, the development of capil- 

 laries, and secondly, those after-changes by 

 which the vessels cease to be capillaries, and 

 become, in texture and volume, true veins. 



The arguments in favour of the view that 

 veins are developed primarily as capillaries, 

 are that veins, arteries, and capillaries con- 

 stitute one continuous system, which renders 

 a community of development antecedently 

 probable ; that arteries and veins are so 

 similar in structure as evidently to possess a 

 common development, which heightens the 

 probability of their development being the 

 same as their connecting link; that the part 

 possessed by capillaries basement mem- 

 brane is that which we see in other parts 

 capable of giving rise to fresh structures ; 

 that we see all stages between the largest 

 veins and capillaries filled up without a gap; 

 that structures which in an early stage are 

 furnished only with capillaries, and vessels 

 a little removed from them, are found shortly 



