114 TISSUES IN GENERAL. 



is a product of the mesoblast; from it proceed the tissues termed 

 connective tissue, which alone contains blood and lymph- vessels, 

 muscles, and nerves, the latter arising from the uppermost portions 

 of the mesoblast. 



DEFINITION. In comparing the earliest formations of the germ 

 with a single plastid formerly called a " uni-cellular organism" 

 or a "protoplasmic body," such as the amoeba, valuable hints 

 may be obtained as to the significance of the three germinal 

 layers. The amoeba is covered by an extremely thin layer of 

 living matter. If the amceba be flattened out and bent, its cover 

 will represent the upper and under thin layer of the germ, which 

 exclusively serves as an investing layer of both the outer surface 

 and all cavities of the body, being directly or indirectly con- 

 nected with the outer world. The main bulk of the amoeba is 

 living matter in reticular arrangement, with thickened points of 

 intersection of the threads of the net- work; this matter, retaining 

 in the mesoblast and its derivations its reticular shape, furnishes 

 in higher organisms the tissues, as a result of a sort of division, 

 of labor. The nature of the tissues is determined, first, by the 

 manner in which the living matter is distributed, and, secondly, 

 by the chemical changes of the fluid contained in the meshes of 

 the reticulum. 



Tissues are complex formations of living matter in a net- work 

 arrangement. The meshes of the net-work contain a liquid which 

 allows the living matter to exhibit contractility in a high degree, 

 as in muscles and nerves, or the net- work contains a more or less 

 solidified basis- substance, which limits its contractility, as in the 

 connective tissue. The latter, on account of the presence of this 

 basis-substance, mainly serves as a support for the more active 

 tissues (muscles and nerves), and as a carrier of liquids in closed 

 spaces. 



DIVISION. According to this view there are but four element- 

 ary tissues in the animal body. All these are interconnected 

 and built upon one and the same plan. 



1. Connective tissue. In this the reticulum of living matter con- 

 tains in its meshes a more or less solid, nitrogenous (glue-yield- 

 ing) basis-substance j while points of intersection rich in living 

 matter, suspended in a liquid, represent the connective tissue 

 corpuscles. Of all tissues only the connective tissue carries in 

 closed vessels the liquids which serve for nutrition, such as blood 

 and lymph. Aside from this, and acting as support for other 

 tissues, its physiological activity is relatively small. , 



