'LI 



MICROSCOPIC ANATOMY OF VERTEBIIATES. 



CHAPTER I. 



INTRODITTTOX. 



In courses of animal biology, or zoology, students usually begin 

 by studying the Protozoa where a single cell constitutes an indi- 

 vidual possessing a complete organization and carrying on all 

 functions essential to life. This conception of cellular independence 

 and self-sufficiency is modified upon consideration of simple multi- 

 cellular animals, such as are found among the Co'lenterates. In such 

 animals a large number of cells are interdependent, their functions 

 finding expression in the activity of the individual they constitute. 

 Cellular specialization becomes evident; certain cells become par- 

 ticularly active in secretion or in absorption; others serve for pro- 

 tection; still others function in contraction or conduction of stimuli. 

 Among the higher groups of animals cellular difi'erentiation and 

 interdependence becomes more marked, so that within the individual 

 animal body there are organizations of cells having the same struc- 

 tural and functional features. Such organizations of cells, struc- 

 turally similar, and their products, if any, are calleil tissues. Five 

 different types of tissues are recognized on the basis of the structural 

 and functional characteristics of the cells and cellular products com- 

 posing them. These are epithelium, connecti\'e tissue, muscle, 

 nerve and blood. 



Histology concerns itself with the study of the structural charac- 

 teristics of tissues and their interrelationships with one another. 

 The five types of tissue do not exist independently, but are asso- 

 ciated in the formation and function of various organs. A better 

 appreciation of how organs function is possible after a study of their 

 tissue composition or microscopic anatomy. Histology and micro- 

 scopic anatomy are but continuations of anatomy which concerns 

 itself with the internal and external structures of the animal body 

 as determined by dissection. The close association of structure 

 and function obliges a student of anatomy to consider various 



