102 THE NtJTIlITrVE FUNCTIONS. 



both animals and plants the Vital changes take place in the 

 cells situated amongst the meshes of the capillary net- 

 work, and that the fibro-vascular system in plants and the 

 arterial system in animals are merely the agents employed 

 in the transmission of the fluid to these cells, which are 

 the animal and vegetable laboratories. Therefbre the mo- 

 tion of the sap and blood must necessarily originate in the 

 forces generated by the nutritive processes carried en in 

 these cells, in the different parts of the body. There must 

 be some cause to induce the muscular contractions of the 

 heart, those successive impulses which drive the blood to 

 the remotest parts of the system, and it seems clear that 

 these phenomena have their origin in the vital processes 

 which are continually going on in those ceUs with which 

 the capillaries finally communicate. 



The motion of the blood in the capillary vessels is well 

 seen in the web of the frog's foot, when examined with" a 

 power of 250 diameters. The blood-globules are seen roll- 

 ing along in single and double file through the minute 

 capillary ramifications. 



The motion of the sap in the capillary vessels of plants 

 may be observed in those in which it is more or less dis- 

 colored, and rendered opaque and milky, by the presence 

 of floating particles of resin, caoutchouc, and other sub- 

 stances. It was first observed by M. Schultz, of Berlin, 

 in 1820, and was called by him cyclosis, in reference to 

 its circulatory character, and latex in allusion to its milky 

 appearance. 



The vessels which contain the latex or milk-sap, are 

 cylindrical tubes with thin, transparent walls, without any 

 appearance of transverse partitions. "They are either simple 

 or branched, and frequently anastomose among themselves, 

 forming a network with irregular and unequal meshes. 



