30 INTERSTITIAL CHANNELS. 



4, the products resulting from changes occurring in 

 tissue long since dead. 



Interstitial Channels. But in every embryo and 

 in every normal tissue the equable distribution of nu- 

 trient matter is provided for and the flow regulated. 

 As the growing embryo increases in size, intercom- 

 municating spaces (lacunae) and channels are formed 

 here and there, and along these the fluids adapted 

 for nutrition freely pass and meander in all direc- 

 tions, bathing, it may be, uniformly the growing 

 bioplasts in every part, or distributing amongst them 

 a gradually diminishing quantity of nutrient matter 

 according as the distance from the nutrient surface 

 increases. See Lect. V, 117. 



46. Vessels. In the higher animals and in man 

 actual tubes (vessels) are, however, developed for the 

 purpose not only of conveying nutriment long distances 

 from the one part of the body where it is taken up, 

 and distributing it freely and equably to the several 

 tissues and organs, but for removing products re- 

 sulting from decay. Every part of the developing 

 mass receiving its proper supply, growth may be as 

 active or more active in those portions which are 

 situated at the greatest distance from the point of 

 supply than in those quite close to it. The vessels 

 form a system of closed tubes composed of deli- 

 cate membrane. These tubes are continually being 

 traversed as long as life lasts by fluids holding in so- 

 lution or in suspension everything that is required by 

 the growing tissues which differ so remarkably from 

 one another in structure, chemical composition, and 

 physical and physiological properties. These tubes 

 are everywhere so arranged that the contained fluid 

 may circulate very rapidly and very freely. In this 

 way every part of the growing mass is irrigated, but 

 the growing tissue is not in contact with the nutrient 

 fluid. The wall of the tube intervenes, but this is 

 usually so very thin that solutions will pass through 



