CONCERNING PROTOPLASM. 71 



we speak of the skin, for instance, we are merely indicating a layer of 

 microscopic cells. When we speak of brain tissue we are again dis- 

 coursing of cells (Fig. 22, c, d) ; and bone itself, in its essential and 

 living parts, is a true cellular tissue. In the human body, it is true, 

 there are muscular fibres, nerve fibres, tendon fibres, and other 

 structures of like nature ; but the physiologist points out that the 

 presence of these latter elements does not invalidate his previous 

 statement concerning the universal cellular composition of the frame. 

 For the body at first consists entirely of cells, and most of the fibres 

 of the body as, for example, the fibres of muscle by means of 

 which we move, or those of the crystalline lens of the eye can be 

 shown to be formed directly from cells by the elongation or modifica- 

 tion of the latter ; whilst the growth and renewal of all fibres take 

 place through the production of new cell-elements. It may be 

 assumed as an axiom of physiology that the bodies of all animals, 

 man included, are formed of cells, which become differentiated to 

 form cellular tissues in the one case, or still further modified to form 

 fibres in the other. 



Such information, all-important as it undoubtedly is, leaves us, 

 however, on the mere confines of our physiological and anatomical 

 study of the higher frame. To understand clearly the relations of 

 the primitive protoplasmic animalcule with the " lord of creation " 

 himself, it is needful to pay a little attention to some further details 

 of microscopical study. Suppose that we examine under the 

 microscope a transverse section of bone. In such research we shall 

 assuredly light upon some facts of interest as assisting our compre- 

 hension of the true typical structure of the most complicated organism 

 in nature. A cross section of bone shows us that the apparently solid 

 tissue is everywhere perforated by the minute "canals" to which 

 Clopton Havers gave his name, and which contain and protect the 

 blood-vessels that nourish the bone. Each Haversian canal of bone 

 is seen to be surrounded by concentric circles of bony matter. 

 When these circles are minutely examined, each is found to consist 

 of elongated spaces, called "lacunae," placed at irregular intervals, 

 and which communicate with each other by minute processes called 

 " canaliculi." Imagine a central lake to be surrounded by circles of 

 smaller lakes, the latter communicating with each other by a complex 

 series of branching rivers, and a fair idea will be gained respect- 

 ing the arrangement of the minute elements of a bone. In a 

 living bone the disposition of parts is not altered from that dis- 

 closed in its microscopic section. The blood-vessels ministering to the 

 nutrition of the bone traverse the Haversian canals already mentioned. 

 Each " lacuna " or lake, however, is occupied by a minute mass of 

 protoplasm, which in all essential respects might.be compared to an 

 Amoeba; and the protoplasm of one lacuna sends out minute pro- 



