PRESIDENTIAL ADDRESS. 537 



thickness of -material covering 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 deter- 

 mined 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 stimula- 

 tion 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 consistent, suppose that 

 here, too, the same force is in action. In this, however, there is no difficulty, 

 sine© 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, probably 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 I 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 T 

 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 lym- 

 phatic 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 con- 

 sider how it will affect the blood, again enveloped bv 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 become 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 everv place where valves are found in the blood-vessels, and see similar factors 

 at work. In the arterial system there is no projection forwards of interrupted 

 columns of blood, nor is this the case in any of those veins in which no valves 

 are found, as notably in those veins that are protected from partially distributed 

 results of external pressure bv the rigidity or by some other incident in the con- 

 formation of the framework in which they are found. 



And now let us turn to the main function of this developing system, which is 

 to drive the blood in continual sequence past tissues that contribute to it and 

 tissues that abstract from it certain chemical materials, and let us select the 

 main incident — namely, the carriage of oxygen from the lungs to other parts. 

 That this is a main incident is clearly shown by the fact that the red corpuscles 

 which form so important a feature in the structure of blood are formed in a 



