40 HISTOLOGY. 



even part with a portion of that which is essential to its normal condition 

 If, however, it is supplied from the other side, it will continue to supply 

 the imbibing fluid and receive more from behind. Thus a current will be 

 established from the water on one side of the tissue to the fluid having a 

 high affinity for it on the other ; but this is not all : for the fluid alluded 

 to, not content with absorbing all the water which the animal tissue sup- 

 plies it with, in its turn transudes the tissue to get at and mix with the 

 water on the other side, and thus a counter-current is set up in an oppo- 

 site direction, which is slower, however, than the former one. These are 

 the currents which are termed endosmotic and exosmotic, and which con- 

 tinue until the difference between the two liquids ceases, and they are 

 equally saturated by each other. 



VITAL PROPERTIES. 



The most prominent vital property possessed by the tissues is the power 

 of assimilation, or of appropriating to themselves such of the organizable 

 substances presented to them as may suit their purposes. This power is 

 supposed to be partly due to chemical affinity, and partly to vital affinity. 

 It is most probable, however, that future researches will prove that the 

 power of assimilating is subject to the ordinary chemical laws, but under 

 modifying circumstances, which can only exist in a living body or tissue. 



To this may be added the power of reproducing, in its appropriate 

 place, a new portion of a tissue, when injured or destroyed. 



Another property which is essentially vital is contractility a phenome- 

 non which is made manifest by the visible shrinking or contraction of 3 

 living tissue when irritated, either by mechanical or chemical stimuli. 

 The muscular tissue exhibits this property in the highest degree. This 

 contractility must be distinguished from the permanent contraction or 

 crispation which a part suffers when exposed to a high temperature. 



A third vital property is sensibility, which, however, requires that the 

 tissues shall be united so as to form a continuous line from the part mani- 

 festing it to the brain. This property is enjoyed in very different degrees 

 by the different tissues, and constitutes an important distinction between 

 them, depending upon the presence and number of nerve-fibres mixed 

 with the tissue. 



DEVELOPMENT OF THE TISSUES. 



The tissues, however diversified in form, are all developed in the fol- 

 lowing manner: 



A nucleated cell attracts from the blastema in which it is formed, or 

 from the capillary vessels contiguous to it, certain elements which combine 

 in its interior, and either form a portion of the animal body, in which 

 case the remains of the cell, and particularly its nucleus, continue to exist 

 in the part, and can be made evident by chemical agents under the 

 microscope, or they become filled with a fluid, and bursting when ripe, 

 and mixing and flowing along with their former contents, are discharged 

 into a tube or duct, and constitute a secretion or a secreted fluid. The only 

 difference between these two kinds of cell, which we designate by the terms 

 formative and secreting, is that the former secrete a solid or semi-solid, 

 which remains in the body with the debris of the cell for an appreciable 

 period o r tiiri", whilst the latter secrete a fluid which escapes from the 



