531) TRANSACTIONS OF SECTION T. 



which -can. be attributed to the differences in these causes of onward motion. 

 Thus where growth is the leading cause of this progressive motion, as, for 

 example, in the development of bone, the blood-tissue is later found occupying 

 spaces that are cut off from the general mass except by lines of communication 

 too small to transfer a full share of pressure from the circulatory mechanism. 

 In this isolated space the blood-tissues preserve to a greater degree powers 

 of intrinsic growth than in those places where the tissue bears the brunt of new 

 forces. It is true that other factors induced by the new motion given to the 

 fluid core of this tissue complicate this matter. This notwithstanding, it is, 

 however, clear that certain definite differences in circumstance, and those princi- 

 pally of a purely mechanical kind, leave the blood-tissue in one district possessed 

 of aboriginal properties which are in a large degree lo6t elsewhere. 



As to that other tissue, which forms the circulatory system and embraces 

 the blood-tissue, there is here little room for doubt that the structures found 

 are the result of special local conditions acting upon originally similar cells, 

 and little room for the suggestion that samples of several different kinds of 

 special formative cells are driven into these positions by destiny and not by 

 mechanics. This is an old theme, well extended and illustrated by exact obser- 

 vation, especially by Thoma ; that in every blood-vessel the arrangement of 

 structures is an almost immediate guide to the conditions of pressure met with 

 in that vessel. Let us proceed through the structures in the walls of a small 

 artery, giving a definite mechanical origin to each tissue. The elastic tissue 

 first met with in the inner coat of the vessel is the result of periodical or 

 intermittent pressure. In the large, arteries, where intermittent pressure is 

 the main phenomenon and where its influence is felt right through the thickness 

 of the wall, this elastic tissue has the major share in forming the structure 

 of the wall. In the small artery, where the total quantity of the causative 

 phenomenon is small, the innermost structures are affected most. This inner 

 zone, formed under the influence of intermittent pressure, protects from inter- 

 mittency the tissue formed by constant pressure, involuntary muscle. Both with 

 regard to this tissue and with regard to the elastic tissue, it is to be remembered 

 that the conformation of the material embracing the cylindrical mass of blood- 

 tissue is such as to convert incidents of internal pressure into tension as well as 

 pressure. Thus we may say that elastic tissue varies in quantity with the value 

 of intermittent, involuntary muscle with the value of constant, pressure and 

 tension. On the outer surface of this case, still more protected by the mechanical 

 value of the structures internal to it, but submitted to the traction and friction 

 of surrounding tissues, comes white fibrous tissue. Again, when windows have 

 been cut in the outermost case of large vessels, leaving the inner case intact, and 

 thus destroying the tensile character of the mechanical conditions and permitting 

 the internal pressure to hammer through these windows, they have been found 

 closed in by plaques of cartilage, and even by true bone. It is true that the 

 explanation offered for such results has been different from that here inferred, 

 it being held that cells specially formative of cartilage and bone have been 

 admitted to this new situation by the brusque strokes of operating instruments. 

 True, too, that the complete ligature of vessels has been followed by develop- 

 ments of bone in unexpected places beyond the walls of the blood-vessels, 

 as in the pelvis of the kidney ; but how can you make a better internal hammer 

 and better provide for its constant use than, for example, by tying the renal 

 artery ? Let me state it as probable that white fibrous tissue, involuntary 

 muscle, and elastic tissue are produced by tension, whereas bone and cartilage 

 are formed by pressure. If we credit the main statement that they are first 

 formed from originally similar cells by circumstances special to each case, 

 and that the difference lies in the circumstances and not in the cells, together 

 with the statement illustrated in former paragraphs that modifications tend to 

 persist when once introduced, we shall probably get near to the truth of the 

 matter. Now it is impossible to leave this special case of the circulatory system 

 ■ — special because here there is no doubt that mechanical conditions are operative 

 from the earliest days of development and from the first beat of the heart — 

 without touching upon two points : the origination of the heart itself, and the 

 formation of valves. 



Picture the blood-tissue in its earliest form as a lacery of networks dis- 

 tributed in a layer throughout the embryo, protected better by the greater 



