Chap. VIII] 



SUMMARY 



137 



secretions from the blood or lymph, the precise nature of their 

 composition or function is not known, beyond the fact that most 

 of them are essential to metabolism and they contain chemical 

 substances called hormones. The term hormone is suggestive, 

 because it means messenger and is applied to a substance which 

 is produced in one organ and on being carried by the blood to an- 

 other organ stimulates this latter to functional activity. 



Excretion. — An excretion resembles a secretion, except that 

 whereas the secretion is intended for use in the body, the excretion 

 is generally formed to be thrown out of the body. It therefore 

 follows that all excretions are first secretions, and some substances 

 are made use of before they are eliminated. For instance, bile 

 serves several purposes before it is eliminated, so that it is first 

 a secretion and then an excretion. Urine, on the other hand, is 

 a secretion, but is formed only to be eliminated. 



Definition — 



Varieties 



SUMMARY 



I Any thin expansion of tissue. 

 I An enveloping or lining tissue. 



1. Serous membranes. 



2. Synovial membranes. 



3. Mucous membranes. 



4. Cutaneous membranes. 



Consist of 



1 . Endothelium — a single layer of flat cells. 



2. Corium — ■ a thin layer of fibrous tissue. 

 Found — lining closed cavities or passages that do not communicate 



with the exterior. They are moistened by serum. 



' Pleurae — cover the lungs 



and line the chest. 

 Pericardium — covers 

 the heart and lines the 

 outer fibrous pericar- 

 dium. 

 Peritoneum — covers the 

 abdominal and the 

 top of some of the pel- 

 Three Classes { vie organs, lines the 



abdominal cavity. 



Lining membrane of the J r>i j i 



^ , \ Blood-vessels, 



vascular system I y , ,. 



•' "> Lymphatics. 



' Serous membranes proper - 



