GENERAL STRUCTURE 



Fig, 6. — Detached 

 Squamous Cells from 

 THE Mouth. (Lewis and 

 Stohr.) 



The study of any vertebrate reveals the presence of numer- 

 ous organs, each of which is for the performance of a particular 

 function. Thus the heart is the organ for the propulsion of 

 the blood, the kidney for the elimination 

 of the nitrogenous waste. Several organs 

 combined for a common purpose consti- 

 tute a system. The heart, with the various 

 vessels for conveying the blood, forms the 

 circulatory system. The following eight 

 systems are found in all Mammalia: 

 Osseous or bony, muscular, digestive, 

 respiratory, vascular, excretory, repro- 

 ductive, and nervous. 



The relative locations of the various systems are represented 



diagrammatically in Fig. 6i. The 

 organs have the same arrangement 

 throughout all the orders of mammals. 

 Moreover, the minute structure of the 

 same organ is so similar in the different 

 species that in many cases even the 

 microscope will not enable one to tell 

 from which of several kinds of mam- 

 mals the organ has been taken. 



Organs are made up of simpler ele- 

 ments, the tissues. Tissues are formed 

 of cells and intercellular substance, 

 sometimes the one and sometimes the 

 Fig. 7.— Stratified other is more in evidence. 



Epithelium from the 



CEsopHAGus OF A Child. THc ccll has been called the struc- 



ewts an to r.) tural Unit of the body. Typically, 



it is a mass of living matter, protoplasm, containing a central 



denser body, the nucleus. The nuclear substance is very 



17 



