THE ESSENTIALS OF HISTOLOGY. 



INTRODUCTORY. 



ENUMERATION OF THE TISSUES AND THE GENERAL 

 STRUCTURE OF ANIMAL CELLS. 



Animal Histology l is the science which treats of the minute struc- 

 ture of the tissues and organs of the animal body ; it is studied with 

 the aid of the microscope, and is therefore also termed Microscopical 

 Anatomy. 



Every part or organ of the body, when separated into minute frag- 

 ments, or when examined in thin slices (sections), is found to consist of 

 certain textures or tissues, which differ in their arrangement in different 

 organs, but each of which exhibits characteristic structural features. 



The following is a list of the principal tissues which compose the body : 



1. Epithelial. 



2. Connective : Areolar, Fibrous, Elastic, Adipose, Lymphoid, Cartilage, Bone. 



3. Muscular: Voluntary, Involuntary or plain, Cardiac. 



4. Nervous. 



Some organs are formed of several of the above tissues, others contain only one 

 or two. 



It is convenient to include such fluids as the blood and lymph amongst the 

 tissues, because they are studied in the same manner and contain cellular elements 

 similar to those met with in some of the other tissues. 



The elements which compose the tissues are of the nature either 

 of fibres or cells. Some tissues are composed almost entirely of fibres 

 with relatively few cells interspersed amongst the fibres ; this is the 

 case with most of the connective tissues. Others, such as the epithelial 

 tissues, are composed entirely of cells, whilst nervous and muscular 

 tissues are formed of cells which are partly or wholly extended to form 

 fibres. 



Cells. A cell is a minute portion of living substance or protoplasm, 

 which is sometimes inclosed by a cell-membrane and always contains a 

 vesicle which is known as the nucleus. 



The protoplasm of a cell (fig. 1, p) is composed of albuminous sub- 

 stance, which is characterised in typical cells by possessing the property 



1 From torros, a web or texture. 

 A 



