HISTOLOGY. 39 



articular coverings for the ends of the bones. It is also found existing 

 transitorily in the process of osteo-genesis. 



The Osseous tissue constitutes the skeleton, and, with some modification, 

 the ivory of the teeth or dentine. 



The Petrous tissue presents the extreme of departure from the animal 

 organization, consisting almost entirely of crystals, which are chiefly com- 

 posed of phosphate of lime. It is found in the enamel of the teeth, in the 

 otoconites, and in the concretions of the pineal gland. 



The Muscular Jibrous tissue constitutes the apparatus of motion, and is 

 divided into two distinct varieties, striped, or the muscular fibre of ani- 

 mal life ; and unstriped, or the muscular fibre of organic life : the former 

 acting in obedience to the will, and the latter being wholly independent 

 of it. 



The Nerve fibre is the conducting portion of the nervous system, and is 

 like the muscular compound, being formed of two distinct substances, the 

 one containing and the other contained. 



The Binary tissues are merely the intermixture or co-existence of two 

 elementary tissues, the proportions of which vary according to the exigen- 

 cies of the part of the body in which they are found. 



The grey or vesicular nervous matter consists of secreting cells, which 

 disappear and are reproduced. 



Adipose tissue consists also of secreting cells, which retain their con- 

 tents under certain circumstances for a great length of time, while under 

 others they rapidly disappear. 



The vascular, mucous, and serous tissues of older writers, consist of 

 aggregations of areolar, muscular, and epithelial tissues, variously modified. 



PROPERTIES OF THE TISSUES. 



The CHEMICAL PROPERTIES have been given in the early portion of 

 this chapter. 



PHYSICAL PROPERTIES OF THE TISSUES. 



The tissues, like other forms of matter, possess certain physical proper- 

 ties, such as colour, consistency, and density, which it is necessary to 

 describe under their respective heads. One property, however, is enjoyed 

 by every tissue, and this seems to play a most important part in the main- 

 tenance of the functions of life. I allude to the transudation of the solids 

 by the fluids, which is known by the title of endosmosis and exosmosis, 

 names by which the process was designated by Dutrochet, its discoverer. 

 All the tissues contain a certain quantity of water, and in some cases this 

 amounts to four-fifths of their weight, as may be proved by drying them ; 

 and this water is essential, not only to their vitality, but confers upon them 

 their organic properties pliability and elasticity. As the tissues imbibe 

 water in certain quantity, it becomes a subject of study to discover the 

 manner by which the quantity may be increased. It is well understood 

 how pressure from without would produce this effect, but even this would 

 be aided by the natural tendency to imbibe and retain an additional quan- 

 tity of water under favourable circumstances, which is strongly exhibited 

 by the softer tissues; Such a tissue saturated with water placed in contact 

 with another tissue or a lid having a higher affinity for water than it has, 

 will part with its superabundance, and if not supplied from behind will 



