210 THE VITAL FUNCTIONS. 



conveying the blood to those organs; and also the 

 set of veins (likewise either pulmonary or branchial) 

 which collect it from the respiratory organs for the 

 purpose of being afterwards distributed to every 

 part of the body. The systemic is often spoken of 

 as the greater, and the respiratory as the lesser 

 circulation. 



After premising this outline of the plan of the 

 channels of circulation, we have next to consider 

 the mechanical agents which move the blood, and 

 direct it in its proper course. The fluids, which, in 

 the ruder and less perfect conditions of this function, 

 are seen to meander in irregular streams through 

 the free spaces provided as reservoirs of nutriment, 

 appear, in many instances, to be driven in their 

 course by the action of vibratory cilia, situated on 

 the internal surface of those cavities.* But when 

 confined in cylindrical tubes, and consequently re- 

 stricted to more definite and concentrated streams, 

 the blood may be impelled in particular directions 

 either by the contractions of the vessel itself, or by 

 pressure applied externally. Thus we find that in 

 many of the simpler kinds of Annelida, the circu- 

 lation is carried on through a system of ramified 

 vessels by their action alone. As we ascend in 

 the scale of animals, we observe that the collection 

 of the blood into the central receptacles constituting 

 the stems of the nutrient or oxygenising trees, 



* In the Beroe ovatus the nutrient fluids circulate with great 

 regularity in a double system of vessels, which are much developed, 

 but of which no part seems capable of performing the office of a 

 heart; and their currents are determined, according to the recent 

 observations of Milne Edwards, by the action of vibratory cilia which 

 line the internal surface of the vessels at one extremity of the 

 system. Comptes Rendus, x. 408. 



