70 THE TISSUES 



(4) Epithelium of tympanum and of Eustachian tube. 



(5) Epithelium of thyreoid and parathyreoid, reticulum of 

 thymus, and Hassall's corpuscles. 



Mesoderm. — (i) All the connective or supporting tissues except 

 neuroglia. 



(2) Lymphatic organs with the exception of Hassall's corpuscles 

 and reticulum of the thymus. 



(3) Blood cells and bone-marrow. 



(4) Striated, cardiac and smooth muscle (with the possible excep- 

 tion of smooth muscle of sweat glands and of iris). 



(5) Endothelium lining blood-vessels and lymphatics. 



(6) Mesothelium lining serous membranes — pleura, pericardium, 

 and peritoneum. 



(7) Epithelium of genito-urinary system with the exception of the 

 urethra and a large part of the bladder. 



(8) Epithelium of the suprarenal cortex. 



In all but the lowest forms of animal life the body consists of an 

 orderly arrangement of many kinds of cells. From the cells is de- 

 veloped a substance which lies outside the cells and is known as inter- 

 cellular substance. This may be small in amount, just sufficient to 

 unite the cells, as in epithelium, or it may so predominate as to deter- 

 mine the character of the tissue, as in some forms of connective tissue. 

 It does not always completely separate the cells which may be con- 

 nected across the intercellular substance by extensions of their 

 protoplasm, as in the "intercellular bridges" of epithelium or the 

 anastomosing processes of connective-tissue cells. Less commonly 

 cells are united in such a multinuclear continuum that their bound- 

 aries are almost or wholly lost. Such a structure is known as a syncy- 

 tium. The association of a particular type of cell with a particular 

 type of intercellular substance is known as a tissue. The character 

 of a tissue depends upon the character of its cells, of its intercellular 

 substance, and their relations to each other. Further differentiation 

 of cells and intercellular substance within a particular tissue gives 

 rise to various sub-groups of the tissue. The association of tissues 

 to form a definite structure for the performance of a particular 

 function is known as an organ. The physiological association of 

 organs constitutes a system. The fact that chemical changes take 

 place in intercellular substance as well as in cells has led to the sug- 

 gestion that the intercellular substance is endowed with the same 



