HISTOLOGY. 657 



In organised beings, the way in which nature works out 

 her most secret processes is by far too minute for observa- 

 tion by unassisted vision ; even with the aid of the improved 

 microscope, comparatively a very small portion has, up to 

 this time, been revealed to us. To point out in detail the 

 discoveries made through the employment of this instru- 

 ment, as regards physiology, would be to give a history of 

 modern biological science ; for there is no department in 

 this study which is not more or less grounded upon the 

 facts and teachings of the microscope. 



To the casual observer, the brain and nerves appear to 

 be composed of fibres. The microscope, however, reveals 

 to us, as was first pointed out by Ehrenberg, that these 

 supposed fibres do not exist, or rather, that they all consist 

 of numerous tubes, the walls of which are distinct, and 

 contain a fluid which may be seen to flow from their broken 

 extremities on pressure. In looking at a muscle, it appears 

 to be made up of fine longitudinal fibres only. The micro- 

 scope tells us that each of these supposed fine fibres is com- 

 posed of numerous smaller ones, and that these are crossed 

 by lines which have received the name of transverse striae ; 

 that muscular contraction, the cause of motion in animals, 

 is produced by the relaxation or approximation of these 

 transverse striae. 



The microscope has shown us that a distinct network of 

 vessels lies between the arteries and veins, partaking of 

 the properties of neither, and possessed of others peculiar 

 to themselves. These have been denominated intermediary 

 vessels by Berres, and, serving to connect the arterial with 

 the venous system, are commonly known as capdlaries. 



On regarding with the naked eye the different glands 

 in which the secretions are formed, how complex they 

 appear, how various in conformation ! The microscope 

 teaches us that they are all formed on one type ; that the 



substance. In both plants and animals there is but one histological element — 

 the endoplast — which does nothing but grow and vegetatively repeat itself; 

 the other element — the periplastic substance — being the subject of all the che- 

 mical and morphological metamorphoses in consequence of which specific 

 tissues arise. The differences between the two kingdoms are mainly, first. 

 That in the plant the endoplast grows, and the primordial utricle attains a 

 large comparative size, while in the animal the endoplast remains small, the 

 principal bulk of its tissues being formed by the periplastic substance ; and 

 secondly, In the nature of the chemical changes which take place in the peri' 

 plastic substance in each case. 



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