510 Dynamic Theory. 



CHAPTER LIIL 



ACTION OF THE HEART. 



A study of the comparative development of the heart and vascular 

 system, shows that they have been built up by the circulation of the 

 blood. Movement of a nutritive fluid takes place before it can be called 

 blood, and before there are any vessels for its conveyance. The neces- 

 sity for the movement of nutritious matter begins as soon as an animal 

 is so large that some of its cells are interior, that is, when they no longer 

 form a part of the external surface. As the animal form increases in 

 size, and habitual activities are put upon it, the courses taken by nutri- 

 tive fluids to reach the interior parts, become definite and fixed, and are 

 formed into tubes. The original cause of the movement of the nutri- 

 tive fluid must be attributed to the chemical attraction of the molecules 

 of which the tissues are formed, for the new elements contained in the 

 fluid. This of itself would cause a circulation, the attracted particles 

 crowding past the discarded effete atoms, and thus pushing them to the 

 rear. Chemism and electricity are only two forms of one energy, and 

 when the former is in activity, the latter is invariably developed. In 

 all probability, the very earliest circulation is stimulated by electric dif- 

 ferences of potential, because electricity would be generated by differ- 

 ences of temperature in different parts of the circuit, by the chemical 

 exchanges, and by the mechanical movement of parts engaged. Elec- 

 tric tensions would tend to relieve themselves by the mechanical con- 

 traction of the parts, which would involve the tubes conducting the 

 fluid, and thus establish pulsations which would become periodic and 

 rythmical in proportion to the regularit}?" of the habits of the animal. 

 As the animal increases in size, and the parts to be supplied with nour- 

 ishment are of greater extent, the rythmical contraction of the vessels 

 becomes an important and finally a necessary factor in the propulsion 

 of the current. 



In the processes of differentiation, certain parts of the circulatory 

 canal become more contractile than others, and these parts constitute 

 the rudimentary form of the heart. 



The formation of channels, of greater or less consistency, through por- 

 ous tissues, takes place very readily. This is seen on a large scale in 

 the subsoil drainage of wet land. A subsoil ditch becomes the recep- 

 tacle of innumerable little underground veins, which are constantly be- 

 ing extended by water pouring into their inner extremities, and branched 

 into innumerable ramifications by veinlets pouring into their sides. As 

 the water percolates from the surface after every shower, it falls into 



