536 ELEMENTARY PHYSIOLOGY less. 



ultimate analysis, the investigation of the vital properties 

 of the histological units of which the body is composed. 

 And even the elements of physiology cannot be thoroughly 

 comprehended without a clear apprehension of the nature 

 and properties of the principal tissues. 



A good deal may be learned about the tissues without 

 other aid than that of tlie ordinary methods of anatomy, 

 and it is extremely desirable that the student should 

 acquire this knowledge as a preliminary to further 

 inquiry. But the chief part of modern histology is the 

 product of the application of the microscope to the 

 elucidation of the minute structu.j of the tissues ; and 

 this has had the remarkable result of proving that these 

 tissues themselves are made up of extremely small 

 homoio^nera, or similar parts, which are primitively alike 

 in form in all the tissues. 



Every tissue therefore is a compound structure : a 

 multiple of histological units, or an aggregation of his- 

 tological elements ; and the properties of the tissue are 

 the sum of the properties of its components. The dis- 

 tinctive character of every fully formed tissue depends on 

 the structure, mode of union, and vital properties of its 

 histological elements wlien tliey are fully formed. 



2. The Primitive Tissues. — Each tissue can be traced 

 backtoayouu^ or embryonic condition, in which it has no 

 chanuteristic properties, and in which its histological 

 elements are so similar in structure, mode of union, and 

 vital properties to those of every other embryonic tissue, 

 that our present means of investigation do not enable us 

 to discover any difference among them. 



These embryonic, undifferentiated, histological elements, 

 of which every tissue is primitively composed, or, as it 

 would be more correct to say, which, in the embryonic 

 condition, occupy the place of the tissues, are technically 

 named nucleated cells. The colourless blood corpuscle 

 (Lesson III., p. 100) is a typical representative of such a cell. 

 And it is substantially correct to say (1) that the histolo- 



