GENEBAL CONSIDERATIONS. 5 



I 



Inorganic Substances. — These substances (gases and mineral matters) are 

 usually found in solution in the animal fluids. Their presence is indispensable 

 in the constitution of the living body. 



Sometimes the mineral matters are solid, amorphous, or crystallized. In this 

 condition they are rarely met with in healthy organs (iriternal ear) ; but they are 

 frequently found in diseased ones. 



Tissues. — The anatomical elements, by being grouped in different manners, 

 form tissues. Some tissues are constituted by one kind of element ; these are 

 simple tissues — for example, epithelial tissue. But the larger number are formed 

 by a combination of several different elements ; these are composite tissues — for 

 example, nerve tissue. 



The anatomical, physico-chemical, and physiological characters of the tissues, 

 are repeated, it need scarcely be remarked, in the anatomical, physico-chemical, 

 and physiological properties of the elements which enter into their composition. 



Only four simple types of fundamental tissue are described, and which are 

 based on the morphological, chemical, and physiological characters of the 

 constituent elements — 



1. In the first place, there are to be noted the tissues of connective substance 

 or framework, comprising the varieties of connective tissue, fibrous tissue, elastic 

 tissue, cartilaginous tissue, and bone tissue. 



2. Then there are the cellular tissues, formed entirely of persistent cells ; 

 they are the epithelial tissue proper, and the glandular tissue. 



The cells of the epithehal tissue are differently arranged. If they are 

 disposed in a single layer, they constitute a simple epithelium ; but if superposed 

 so as to form two or more layers, then it is a stratified epithelium. According to 

 the form of the superficial layer of cells, the epithelium is polyhedric, pavement., 

 cylindrical, spherical, or calciform. In certain parts the superficial cells are 

 furnished with vibratile cilia, and the epithehum is then called vibratile (or ciliated). 



3. The third type is represented by the muscular tissue, the agent of move- 

 ment, which is divided into striated and non-striated muscle. 



4. Lastly, there is the nerve tissue, which is present under two aspects — 

 white substance and grey substance. The first is formed entirely of nerve fibres 

 and a supporting tissue or neuroglia ; the second by fibres and nerve cells united 

 by neurogha. The grey substance belongs to the most important portions of the 

 nervous system — the brain, spinal cord, and gangha, where it co-exists with the 

 white substance. The latter entirely constitutes the peripheral ramifications of 

 the nervous system. 



Organs. — The term " organ " is given to any portion of the body having a 

 determinate form and a function to fulfil. A bone, a muscle, the stomach, the 

 liver, and the brain, are organs. 



General Arrangement of the Organs and Apparatus. — All animal 

 organs are enclosed between two membranes named limitary or tegumentary 

 membranes, which are continuous with one another at the margin of the natural 

 openings. These are the skin and the mucous membranes, in the composition of 

 which are included a layer of connective tissue and an epithelium 



Organs protected by these membranes are solid or hollow. 



Among the first, a certain number act as supports : such are the organs formed 

 by the connective tissue, and particularly the cartilages and bones. 



Others are destmed to produce movements : these are the muscles. The 

 action of the muscles is communicated directly to the organs that are to be 



