38 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



is composed. This is a purely chemical process, is accompanied by the 

 liberation of force, and requires for its performance the free access of 

 oxygen. Gases, also, must therefore pass from the exterior to the 

 interior, — a process which the laws of absorption and diffusion of gases 

 are quite sufficient to explain. Again, the chemical processes concerned 

 in the assimilation of food result in the formation of carbon dioxide, 

 urea, kreatin, and other crystalline bodies which are no longer of use 

 and which must be removed ; or the cell activity may take on the form 

 • of a secretion, — that is, the cell-protoplasm may, from the matter sup- 

 plied to it, manufacture certain substances which in the higher organisms 

 have to be used elsewhere. In either case the products of the proto- 

 plasmic operations must be removed. Here, also, physical laws are all 

 sufficient. Gases are removed under the laws of gaseous diffusion and 

 absorption. All crystallized products are eliminated by equally 

 simple means. The absorption of fluid by the cell-contents througb imbi- 

 bition leads to increase of volume, and hence to increased pressure on the 

 cell-membrane. Filtration thus comes into operation. Or, if the pres- 

 sures within and without the cell are equal, interchange of matters in 

 solution may take place through diffusion and osmosis. All these proc- 

 esses nruoy occur equally well in membraneless cells ; for, in all cases, to 

 permit interchange the dividing membrane must be capable of absorbing 

 the fluids or solutions with which it is in contact. The conditions, then, 

 are analogous to those of a body containing fluid by imbibition in con- 

 tact with another fluid. 



We have now to consider some of the physical processes concerned 

 in these operations. The chemical processes will subsequently receive 

 attention. 



The explanation of the physical processes in cells is to be found in 

 the molecular forces which fluid molecules exert on each other and on 

 solids with which they may be in contact. 



1. Cohesion is the force which binds together adjacent molecules of 

 the same nature ; for example, two molecules of water or two molecules 

 of iron. This attractive force is strongly exerted in solids, less so in 

 liquid, and is absent in gases. It is measured by the force which is 

 required to tear a body asunder. The closer the molecules of a body 

 are together, the greater their cohesive force. 



Cohesion varies inversely with the square of the distance which 

 separates the molecules; anything, therefore, which drives the molecules 

 apart will tend to weaken their cohesive force. When bodies are heated 

 the expansion which they undergo is due to the separation of their mole- 

 cules. Hence, when solids are heated their molecules are further and 

 further separated until Anally their cohesive force is balanced by the 

 repulsive force, and the bodies pass from a solid to a liquid state. This 



