28 STRUCTURAL COMPOSITION OF HUMAN BODY. 



In many, too, there is a kind of molecular current, exactly 

 resembling that which is seen in a vegetable cell. 



In the higher animals, phenomena such as these are so sub- 

 ordinate to the more complex manifestations of life that they 

 are apt to be overlooked ; but they exist nevertheless. The 

 mere nutrition of each part of the body in man or in the 

 higher animals, is performed after a fashion which is strictly 

 analogous to that which holds good in the case of a vegetable 

 cell, or a rhizopod ; or, in other words, the life of each anatomi- 

 cal element in a complex structure, like the human body, re- 

 sembles very closely the life of what in the lowest organisms 

 constitutes the whole being. For example, the thin scaly 

 covering or epidermis, which forms the outer part of a man's 

 skin, is made up of minute cells, which, when living, are com- 

 posed in part of protoplasm, and which are continually wear- 

 ing away and being replaced by new similar elements from 

 beneath ; and this process of quick waste and repair could only 

 take place under the very complex conditions of nutrition 

 which exist in man. One working part of the organism of an 

 animal is so inextricably interwoven with that of another, 

 that any want or defect in one, is soon or immediately felt by 

 the whole ; and the epidermis, which only subserves a mechani- 

 cal function, would be altered very soon by any defect in the 

 more essential parts concerned in circulation, respiration, &c. 

 But if we take simply the life history of one of the small cells 

 which constitute the epidermis, we find that it absorbs nour- 

 ishment from the parts around, grows, and develops in a 

 manner analogous to that which belongs to a cell which con- 

 stitutes part of a vegetable structure, or even a cell which 

 by itself forms an independent being. 



Remembering, however, the invariable presence of a living 

 albuminous matter or protoplasm of apparently identical com- 

 position in all living tissues, animal and vegetable, we must 

 not forget that its relations to the parts with which it is in- 

 corporated are still very doubtfully known ; and all theories 

 concerning it must be considered only tentative and of uncer- 

 tain stability. 



Among the anatomical elements of the human body, some 

 appear, even with the help of the best microscopic apparatus, 

 perfectly uniform and simple : they show no trace of struc- 

 ture, i. e., of being composed of definitely arranged dissimilar 

 parts. These are named simple, structureless, or amorphous 

 substances. Such is the simple membrane which forms the 

 walls of most primary cells, of the finest gland-ducts, and of 

 the sarcolemma of muscular fibre ; and such is the membrane 

 enveloping the vitreous humor of the eye. Such also, having 



