September 14, 191 1] 



NATURE 



367 



ing 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-tissue preserves 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 not- 

 withstanding, it is, however, clear that certain definite 

 differences in circumstance, and those principally of a 

 purely mechanical kind, leave the blood-tissue in one dis- 

 trict possessed of aboriginal properties which are in a large 

 degree lost 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 

 observation, especially by Thoma ; that in every blood- 

 vessel the arrangement of structures is an almost imme- 

 diate 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 struc- 

 tures are affected most. This inner zone, formed under 

 the influence of intermittent pressure, protects from inter- 

 mittency the tissue formed by constant pressure, involun- 

 tary muscle. Both with regard to this tissue and with l 

 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 j 

 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 trac- I 

 tion 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 I 

 to hammer through these windows, they have been found 

 closed in by plaques of cartilage, and even by true bone. I 

 It is true that the explanation offered for such results j 

 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 developments of 

 bone in unexpected places bevond 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 arterv? 

 Let me state it as probable that white fibrous tissue, in- 

 voluntary muscle, and elastic tissue are produced bv 

 tension, whereas bone and cartilage are formed bv 

 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 state- 

 ment illustrated in former paragraphs that modifications 

 tend to p rsist when once introduced, we shall probablv 

 get near to the truth of the matter. Now it is impossible 

 to leave this special rase of the circulatory system — special 

 because here there is no doubt that mechanical condition^ 

 are operative from the earliest days of development and 

 fmm the first beat of the heart — without touching upon 

 two noints : the origination of the heart itself and the I 

 formation of valves. 



Picture the blood-tissue in its earliest form as a lacery ' 

 NO. 2185, VOL. 87] 



of networks distributed in a layer throughout the embryo, 

 protected better by the greater thickness of material cover- 

 ing the central longitudinal axis than at the edges. In 

 the absence of this protection the peripheral parts are 

 subjected to incidents of compression which set pressure- 

 waves travelling along the meshes of this blood-tissue in 

 all directions from the point primarily affected. Since 

 these waves will tend to be reflected within the tissue, we 

 can think of the disturbance caused by them as possessed 

 of a certain periodic recurrence of rhythm determined in 

 its time-relations by the dimensions of the tissue, and as 

 undergoing a tendency to modification as these dimensions 

 are increased. In the earliest stages, whilst the distance 

 from edge to edge is less than one millimetre, giving these 

 waves the very slow rate of one metre per second, we can 

 imagine these periodic changes in pressure exerting their 

 influence upon the tissues enveloping the blood with a 

 frequency of one thousand per second. It is again not 

 difficult to imagine that the protection afforded to the 

 central axial portion, through which each wave must pass 

 in transit from edge to edge, allows us to think of the 

 tissue there as more pressed upon than pressing, so that 

 in this place our attention is directed to the enveloping 

 tissue-cells receiving this rapidly recurring stimulation and 

 being especially affected in the process into a formation 

 of cardiac muscle. Since cardiac muscle resembles so 

 closely in many minute particulars skeletal muscle, which 

 is developed mainly under the influence of electrical dis- 

 charge from the central nervous system, we must, if con- 

 sistent, suppose that here, too, the same force is in action. 

 In this, however, there is no difficulty, since it is a simple 

 matter to explain how mechanical pressure may give rise 

 to electrical change, as, for instance, when a nerve is 

 excited by mechanical pressure. There is, however, prob- 

 ably this distinction between skeletal and cardiac muscle, 

 namely, that the electrical stimulus provocative of the 

 latter is of a high frequency and approximates nearer to 

 what .1 might describe as a constant electrical current. 

 The heart is not by any means the only site of formation 

 of rhythmical contractile tissues, and in these other cases, 

 so far as I am acquainted with them, a similar state of 

 formative conditions may be described. Thus at those 

 points where the conical apices of that second network, 

 the lymphatic system, are forced by pressure of external 

 parts to flow towards certain points in this blood-tissue, 

 rhythmical lymph-hearts are described as developed in these 

 protected sites prior to the final penetration of the blood- 

 tissues, and the forced commingling of lymph with the 

 fluid core of the blood. 



Now give the agency that I have described a certain 

 direction, crediting it with a graduated qualitative influence 

 in different parts in correspondence with the date of their 

 formation and with the altering dimensions of the blood- 

 tissue as a whole, and the peristaltic character of the 

 movement subsequently performed by the contractile tissue 

 may be completely explained. Let us, then, suppose that 

 such a peristaltic contractile mass is formed in these 

 enveloping tissues, and consider how it will affect the 

 blood, again enveloped by its own endothelial cells. When 

 driven forward away through this site, the endothelial 

 covering, which at first will slip upon the enclosing heart, 

 later will acquire some attachment by the precipitation of 

 fibrous tissue due to repeated friction. The movement of 

 the endothelial cells is now only partial. They have be- 

 come describable no longer completely as the surface cells 

 of blood-tissue, and are in a measure the internal covering 

 of the heart, its "tunica intima." With each pulsation 

 this intima is dragged onwards to some slight degree 

 behind the blood column to which it originally belonged. 

 There is no difficulty whatever in thinking that valves are 

 necessarily formed at every point where the conditions are 

 such as tend to break up the blood column into separate 

 parts. Indeed, we may look particularly at every place 

 where valves are found in the blood-vessels and see similar 

 factors at work. In the arterial system there is no pro- 

 tection forwards of interrupted columns of blood, nor is 

 this the case in anv of tho«e veins in which no valves 

 are found, as notably in those veins that are protected 

 from partially distributed results of external pressure by 

 the rigidity or by some other incident in the conformation 

 of the framework in which thev are found. 



