THE LACTICIFEROUS VESSELS. 



29 



known that nature never progresses by bounds, but by gentle ascents, and that, not 



only does one fundamental structure run through the whole of vital_existences (whilst 



the anatomical characters of widely - separated 



classes are yet very distinct) ; yet that there are 



certain similarities which become, as it were, the 



larger links which unite them together. The 



structure now under consideration is the large 



link which binds vegetables and animals together. 



No other vegetable vascular tissue uniformly 



branches, and none has a pulsatory motion of its 



contents ; but both these conditions are universal 



in the animal kingdom. There is yet another 



similarity : The Lacticiferous or milk-bearing tissue 



(Fig. 72), is devoted to the maintenance of the 



vitality of the other vegetable structures, and not 



to any extraneous object whatever. If a stem be Figl : 7 l'~ M - iIk V 7 ess ^ ls from tbe . stipules 



J J of the Jftcus elastica, or India-rubber 



in great part cut through, the effect is to kill the fig-tree, showing the branched and 



plant-not so much by destroying its functions as S^S^SS^ofthd? Stints^ 



by pouring out the milky juice, which should 



maintain the life of all the structures in fact, by bleeding it to death. This is not the 



4. 



Fig. 73. The smallest vessels or capillaries of the frog's foot, as seen by the microscope, -whilst the 

 circulation is proceeding, a indicates a vessel of a larger sixe, which subdivides at b into c'iie 

 capillaries. The vessels anastomose with each other, and branch in every direction, and con- 

 tain the oval bodies, or blood globules, which correspond to the granules in Fig. 74. 



Fig. 74. Milk-vessels of a water-plant the| Limnocharis Hiimboldtii, showing their granular con- 

 tents ; and the walls apparently made up of a series of oblong cells of cellular tissue, aud the whole 

 inclosed in hexagonal cells, as shown at b. The arrows indicate the direction of the current. 



case with the woody tissue ; for if that were nearly drained of its contents the plant 

 would not necessarily perish ; but if the milky juice be withdrawn too -abundantly 



