NORMAL HISTOLOGY. 53 



CHAPTEE Y. 

 TISSUES AND ORGANS. 



A tissue consists of intercellular substance and an aggregation of 

 cells of common origin which usually exhibit a common form, struc- 

 ture, and function. The intercellular substance may be very slight in 

 quantity (merely a delicate layer of cement between the cells, as in 

 epithelial tissue), or it may make up the bulk of the tissue, as illus- 

 trated in the calcareous deposit of osseous tissue. It is deposited by 

 the cells and is usually formed by their agency. In some tissues the 

 cells vary in form; for example, in epithelial tissue, the newly- 

 formed cells are almost spherical and are rich in protoplasm, while 

 the old cells are merely flattened scales devoid of protoplasm. In 

 osseous and nervous tissues the young cells may be spherical or 

 oval, while the older cells are provided with protoplasmic processes. 



An organ is a single tissue which exhibits a special function or a 

 group of tissues so associated as to accomplish some .definite pur- 

 pose in the plant-or animal economy. It is often the case that an 

 organ may serve several purposes; as, for example, the tongue, which 

 aids in mastication, deglutition, and articulation, and is an organ of 

 taste and secretion. There is usually, however, one function which 

 is preeminent. In the structure of an organ it is not the rule that 

 all its tissues are derived from the same source. The tissues of 

 the stomach, for example, are derived from the epiderm, mesoderm, 

 and hypoderm. 



As has been suggested in the preceding chapter, all the organs and 

 tissues of the animal body are derived from a single cell. This cell 

 is produced by the fusion of a sperm nucleus with a germ nucleus. 

 From this primordial cell, by a process of segmentation, there is 

 produced, first, the morula; then the gastrula, with its two 

 layers epiblast and hypoblast. From these layers is de- 

 rived a third layer, the mesoblast. We now have the blastoderm, 

 consisting of three distinct layers of cells epiblast, mesoblast, and 

 hypoblast. From these layers are produced the primitive layers of 

 the embryo epiderm from the epiblast, mesoderm from the meso- 



