40 TISSUES 



After fertilization the ovum divides and redivides 

 into numerous cells, forming an irregular mass termed 

 the mulberry mass or morula. The latter collection of 

 cells divides again into an outer and inner cell mass 

 called the blastula. The outer mass is supposed to dis- 

 appear, while the inner continues to develop and forms 

 two layers an outer, the ectoderm or epiblast, and an 

 inner, the entoderm or hypoblast. This is termed the 

 gastrula or diploblast. A third layer is developed from 

 the two former layers, each setting aside a few cells 

 which develop the third layer, termed the mesoderm 

 or mesoblast, that lies between the two layers. The 

 formation receives the name of blastodermic- vesicle or 

 triploblast. 



All tissues of the body are composed of cells arising 

 from the cells in the original three layers of the triplo- 

 blast or blastodermic vesicle. Tissues, which are always 

 studied microscopically, consist of cells held together 

 by an intercellular cement, and perform a definite 

 function; thus they may be supportive, as bone, etc., 

 or functional, as the liver, etc. All the tissues to be 

 seen and understood in their minute arrangement are 

 first treated by histologic methods in the laboratory 

 by hardening, sectioning, fixing, dehydrating, staining, 

 etc., and are then observed under the microscope. 

 This process is not essential to the nurse's knowledge, 

 but should she desire a complete understanding of 

 the subject she should refer to the standard works on 

 histology. 



Tissues are divided into epithelial, connective, 

 muscle, and nerve 



Epithelial Tissue or Epithelium. They may be pro- 

 tective, as the cells of the skin and conjunctiva of the 

 eye; secretive, as the cells of the pancreas, parotid 

 gland, etc.; excretive, as the cells of the kidneys; 

 to prevent friction, as those seen in the cells of the 

 synovial sacs between the articulating cartilages of 

 joints, peritoneum, and layers of pleura. Epithelial 



