SECT. l8.] CELL-TISSUES. 3 1 



II. OF TISSUES, ORGANS, AND SYSTEMS. 



^> 18. The elementary parts of the simple and higher kinds are 

 not scattered without order in the body, but united according 

 to definite laws to the tissues and organs, as they are called. 

 Under the former name is designated every constant grouping of the 

 elementary parts always recurring in the same icay in analogous 

 parts; under that of an organ, on the other hand, is understood 

 a certain sum of elementary parts possessing a definite form and 

 function. When several or many organs of similar or different 

 kinds unite to form a higher unity, the latter is called a system. 



It is difficult to classify the tissues properly. When we only 

 consider the conditions, such as they are, found in the adult 

 organism, it is easy, indeed, to enumerate a gradually ascending 

 series of simple to more and more complex formations. But in 

 this way, formations which are closely related to each other would 

 be torn asunder, and conversely. Better results are attainable, 

 when, together with the fully developed form, we also take the 

 origin and the chemical and physiological conditions into account, 

 and from this point of view the following series of tissues may be 

 constructed : — 



1. Cell-Tissues. 



Epidermic tissue. Tissue of true glands, r 



2. Tissues of Connective Substance. 



Mucous tissue. Elastic tissue. 



Cartilage-tissue. Areolar tissue. 



Osseous tissue and Dentine. 



3. Muscular Tissues. 



Tissue of the smooth muscles. 



Tissue of the transversely striped muscles. 



4. Nerve-Tissue. 



5. Parenchymatous Tissues of Vascular Glands. 



The organs may be divided into simple and compound. 



1. Simple Organs. 



1. Epidermis, epithelia, hairs, nails, enamel, lens. 



2. Simple true glands. 



3. Vitreous body. 



4. Chorda dorsalis, true cartilage, and elastic cartilage. 



5. Elastic ligaments and membranes. 



