380 PHYSIOLOGICAL DEVELOPMENT. 



sufficiently manifest. That parts which are bent or strained 

 necessarily have their contained vessels squeezed, has been 

 before shown (§ 281) ; and whether the bend or strain is 

 caused, as in a plant, by an external force, or, as usually in an 

 animal, by an internal force, there must be a thrusting of the 

 liquids towards places of least resistance — that is, towards 

 places of greatest consumption. This which in animals with- 

 out hearts is a main agent of circulation, continues to further 

 it very considerably even among the highest animals. There 

 is experimental proof of the fact. The pressure in the jugu- 

 lar vein of a horse, which is about § of a pound per 

 square inch while the muscles are at rest, rises to 2J lbs. per 

 square inch when the muscles are contracted to raise the 

 head. Such, then, are the several forces we have to 



take into account in studying the genesis of the vascular 

 system. Let us now pass to the facts to be interpreted. 



Even in such simple ty^pes as the Hydrozoa, cavities in the 

 sarcode faintly indicate a structure that facilitates the transfer 

 of nutritive matters. These vacuoles, possibly caused by the 

 contraction of colloid substance in passing from the soluble 

 to the insoluble state, become reservoirs filled with the 

 plasma that slowly oozes through the sarcode ; and every 

 movement of the animal, accompanied as it must be by 

 changed pressures and tensions on these reservoirs, tends 

 here to fill them and there to squeeze out their contents in 

 that or the other direction — possibly aiding to produce, by 

 union of several vacuoles, those lacunae or irregular canals 

 which the sarcode in some cases presents. 



Irregular canals of this kind, not lined with any mem- 

 branes but being simply cavities running through the flesh, 

 mainly constitute the vascular system in Molluscoida and many 

 Mollusca. In the simplest of these types the nutritive liquid, 

 absorbed into the cavity of the peri-visceral sac, is thrust 

 hither and thither through this sac with every change in the 

 creature's attitude, and simultaneously fills some of the 

 sinuses which open out of this sac and run through the sub- 



