8O THE TISSUES. 



and the epithelium of the membranous labyrinth of the ear ; and 

 finally, the entire nervous system, central and peripheral. 



From the entoderm : 



The epithelium lining the digestive tract, and all glands in con- 

 nection with it, including the liver and pancreas ; the epithelium of 

 the respiratory tract and its glands ; the epithelium of the bladder 

 and urethra (in the male, only the prostatic portion, the remainder 

 being of ectodermal origin). 



The cells of the mesoderm are early differentiated into three 

 groups (Minot, 99) : 



(a) Mesothelium. The mesothelial cells retain the character of 

 epithelial cells. They form the lining of the pleural, pericardial, 

 and peritoneal cavities, and give origin to the epithelium of the uro- 

 genital organs (with the exception of the bladder and urethra), and 

 striated and heart muscle tissue. 



(b) Mesenchyme, from which are derived all the fibrous connective 

 tissues, cartilage, and bone, involuntary muscle tissue, the^spleen, 

 lymph-glands, and bone -marrow ; and cells of an epithelioid charac- 

 ter, lining the blood and lymph-vessels and lymph-spaces, known 

 as endothelial cells. 



(c) Mesameboid cells, comprising all red and white blood-cells. 



It would be extremely difficult to attempt a classification of tis- 

 sues according to their histogenesis, as identical tissue elements owe 

 their origin to different germinal layers. The classification adopted 

 by us is based rather on the structure of the tissues in their adult 

 stage. 



We distinguish : 

 yA. Epithelial tissues with their derivatives. 



B. Connective tissues ; adipose tissue ; supporting tissues (car- 

 tilage, bone). 



C. Muscular tissue. 



D. Nervous tissue. 



E. Blood and lymph. 



A. EPITHELIAL TISSUES. 



Epithelial tissues are nonvascular, and composed almost wholly 

 of epithelial cells, united into continuous membranes by a substance 

 known as intercellular cement. They serve to protect exposed 

 surfaces, and perform the functions of absorption, secretion, and 

 excretion. 



The epithelia are developed from all of the three layers of the 

 blastoderm. 



They secrete the cement-substance found between their contigu- 

 ous surfaces. This takes the form of thin lamellae between the cells, 

 gluing them firmly together. In certain regions the epithelial cells 

 develop short lateral processes (prickles), which meet like structures 



