STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 53 



The Vascular System is developed in the middle germinal 

 layer with the earliest stages of the connective tissue. The blood- 

 vessels, which are chiefly made up of connective tissues, soon 

 traverse all the parts of the body, and distribute the nutrient fluid 

 or blood. And even the blood may be considered as an outcome 

 of the connective tissues, since the cells of the blood are at first 

 formed from the mesoblast, and later from the connective tissue 

 corpuscles. 



An arrangement of special cells, such as epithelial or muscle 

 cells, with a special function, constitutes an organ. However, in 

 the higher animals and man, an organ is almost invariably a 

 complex structure, having various tissues entering into its con- 

 struction. Thus a skeletal muscle is made up of a quantity of 

 muscle fibres held together by sheets of connective tissue, and 

 attached to bones by connecting bands. It is further traversed 

 by many bloodvessels, and the fibres are in immediate relation 

 to certain nerves which terminate in them. The various secreting 

 organs are made up of epithelial cells, held together by connective 

 tissue and in close relation to bloodvessels and nerves, and are so 

 arranged that they pour their secretion into a duct. The bones, 

 which are the organs which give the body support, contain, in 

 addition to the bone tissue of which they are composed, a great 

 quantity of indifferent cells, fat cells, nerves, and bloodvessels. 

 They are covered on the outside with a tough vascular coat, which 

 gives them strength, assists their nutritive repair and reproduction, 

 and acts as a point of attachment for the muscles and ligaments. 

 Where the bones are in immediate relation at the joints, they are 

 commonly tipped with cartilage. 



If, then, we analyze anatomically the architecture of the human 

 body, we shall find that it is made up of a number of complex 

 parts, each adapted to some special function, and composed of such 

 an association of the simple tissues as the special part demands. 



The general arrangement of these organs and their modes of 

 action will be discussed in a future chapter. 



