GENERAL CONSIDEBATIONS. 5 



In the third place, is the muscular tissue, which may be divided into 

 striated and non-striated (or striped, and non-striped or srhooth) fibres. 



Lastly comes the nervous tissue, which offers two aspects : the white and 

 grey substance. The first is entirely formed by nerve fibres, and the second 

 by fibres and nerve cells. 



Organs. — The term organ is given to an agglomeration of tissues 

 possessing a determinate form, and having a function to fulfil. Organs are 

 therefore composed of tissues, as the tissues themselves are constituted by 

 anatomical elements. 



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 whose composition is included a layer of connective tissue 

 covered by an epithelium. 



Organs are distinguished into those which are solid, and those which are 

 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 destined to produce movements : these are the two kinds of 

 muscles. The action of the muscles is communicated directly to the organs 

 that are to be moved, or it is transmitted through the medium of other 

 organs, such as the tendons and aponeuroses. 



The central nervous organs, nerves properly so called, and the vascular 

 glands, belong to this group of solid organs. 



With regard to the hollow organs, they are everywhere covered by the 

 internal, tegumentary, or mucous membrane. Examples : — the lungs and 

 stomach. There must also be included the vessels formed by elastic and 

 contractile membranes arranged as canals, in which the blood and lymph- 

 circulate ; and, lastly, the serous membranes, which line the interior of the 

 splanchnic cavities, and cover the surface of the organs contained in them. 



Appakatus. — Organs are very numerous in the animal economy, and ii^ 

 order to study them profitably it is necessary to classify them in a methodi- 

 cal manner, according to their physiological afiinities. Consequently, there 

 have been collected into a single category all those organs which are destined 

 to achieve the same physiological finality, and to such a group has been given 

 the name of apparatus. 



An apparatus is, then, an assemblage of all those organs of an animal 

 which concur to the same end, and which serve for the accomplishment of 

 the same function. 



We will successively describe, in the following order, the different appa- 

 ratus of which the organism is composed : — 



1. Loeomotory Apparatus ; 



2. J)igestive Apparatus ; 



3. Sespiratory Apparatus ; 



4. Urinary Depurative Apparatus ; 



5. Circulatory Apparatus; 



6. Innervatory Apparatus ; 



7. Sensory Apparatus ; 



8. Generative Apparatus ; 



This description will be terminated by a brief exposition of the evolution 

 of the foetus and its appendages. 

 4 



