TISSUES. 51 



organs. Under the first denomination comes every constant 

 arrangement of the elementary parts always recurring in similar 

 modes in the same parts; under that of an organ, on the other 

 hand, a certain sum of elementary parts having a definite form, and 

 function. When several or many organs of a similar or different 

 kind are united into a higher unity, this is called a system. 



The tissues are of different kinds, according as structural 

 elements of one kind only occur in them, or as various 

 elements and even organs take part in their formation. We 

 can thence distinguish simple and complex tissues, which, 

 however, cannot be sharply separated from one auother, and 

 which may be most fittingly arranged in the following series : 



(a.) Simple tissues. 



1. Epidermic tissue. 



2. Cartilaginous tissue. 



3. Elastic tissue. 



4. Connective tissue. 



(b.) Complex tissues. 



5. Osseous tissue. 



6. Smooth muscular tissue. 



7. Transversely striated muscular tissue. 



8. Nervous tissue. 



9. The tissue of the blood-vascular glands. 

 10. The tissue of the true glands. 



The organs may be divided like the tissues, into simple 

 and complex. 



To the simple belong : 



(a.) Horny tissue, as the epidermis, the epithelia, hairs, nails, 

 and the lens, which consist solely and wholly of epithelial 

 cells of one kind or another. 



(6.) The true cartilages and the elastic cartilages, which in 

 their interior, with few exceptions, consist only of cartilaginous 

 tissue, though externally they possess a vascular and nervous 

 coat, the perichondrium. 



(c.) The elastic ligaments, consisting of elastic fibres, with 

 some connective tissue, and containing only at the surface a few- 

 vessels, and nerves. 



(d.) The tendons, ligaments, true fibrous membranes, andfibro- 

 cartilages, containing a preponderance of connective tissue, 



