62 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



intermolecular spaces of the salt. In solutions, however, the solids are 

 bound to the water; in tissues, the reverse. 



Since all fresh, organic, tissues contain water, their specific gravity 

 must be comparatively low; drying, by driving oft' the water,, while 

 decreasing their weight by the amount of water displaced, will increase 

 their specific gravity, though even then, like all organic bodies, they will 

 be specifically lighter than most minerals. The specific gravity of dif- 

 ferent tissues will also vary according to the nature of their special con- 

 stituents ; thus, adipose tissue will represent one extreme, bones and 

 teeth the other, and tissues which are rich in fat, like the nervous tissues, 

 will be of less specific gravity than those which contain inorganic 

 matters. 



As the specific gravity of the tissues depends upon their constitu- 

 ents, it will vary according to the relative proportions of those con- 

 stituents at different ages, in different individuals, and in different 

 nutritive states. No fixed figures can, therefore, be given to represent 

 the specific gravity of the different tissues, but, though not constant, the 

 following represents the average specific gravity of the most important 

 tissues of the human bocby : — 



Bones, . . . . . 1.656 



Elastic tissue and tendons, . 1.12 



Muscles, . 1.073 



Arteries, . . . . 1.096 



Veins, . . . 1.05 



Nerves, . .... 1.046 



1. Cohesion. — It follows from what has been said as to the freedom 

 of molecular movement in most organic tissues, as shown in their capa- 

 bility of imbibition, that their cohesion must be less than that of most > 

 inorganic solids. It is highest in the bones, lowest in glands and brain, 

 though it is comparatively high in nerves. Cohesion is there due to the 

 fibrous envelope (neurilemma) and not to the nerve-fibre; and as these 

 sheaths relatively increase as the nerve-trunks subdivide, the cohesion of 

 the fine nervous twigs of the skin is relatively higher than that of the 

 nerve-trunks. 



The greater the amount of water contained in a tissue the less its 

 cohesion, for the wider apart will be the molecules, and the molecular 

 attraction decreases as the square of the distance which separates them. 

 Consequently desiccation increases cohesion. The order of cohesiveness 

 is inversely as the quantity of water; thus, the following list is arranged 

 with tissues of greatest cohesion and least water first, and as water 

 increases cohesion decreases : — 



1. Bones. 4. Muscles. 7. Intestines. 



3. Jendons. 5. Veins. s. Glands 



3. Nerves. (i. Arteries. 9. Brain. 



