54: TISSUES AND ORGANS. 



blast, and hypoderm from the hypoblast. From these primitive tis- 

 sues are derived all the structures of the body. 



From the epiderm are developed the nervous system, the epithe- 

 lium, covering the surface of the body, the enamel, the nails, the 

 hair, the organs of special sense, and all glands except those which 

 open into the alimentary tract from the oesophagus downward. 



From the mesoderm are derived the blood, blood-vessels, all the 

 connective tissues (cartilage, bones, tendons, etc.), the muscles, the 

 dentine, and cementum. 



From the hypoderm are developed the epithelial lining of the 

 alimentary canal and the glands opening into it. 



Tissues may be classified into four groups : (1) Epithelial tissues, 

 (2) Connective tissues, (3) Muscular tissues, (4) Nervous tissues. 

 The organs of the body are constituted of these tissues. For ex- 

 ample, the tongue consists of epithelial, muscular, connective, and 

 nervous tissues. In the following studies tissues and organs are 

 treated in the order considered most convenient for practical work. 

 The first structure to be considered is the blood. 



MEMORANDA. 



