4 THE MECHANISM OF THE CIRCULATION. 



hearts of rabbits through the walls of the closed thorax, and watched the 

 movements of the needle heads. The needles acted as levers, the thoracic 

 wall formed the fulcrum, the movements of the needle heads gave the 

 reverse of the movements of the needle points within the heart. From 

 the result of these observations Brlicke maintained that all parts of the 

 ventricles move downwards in systole with the exception of the apex, 

 the amount of movement increasing from below upwards. The apex of 

 the heart undergoes less change of position than any other part. Its 

 upward movement is neutralised by the descent of the whole heart. 

 This descent is occasioned by the elastic recoil of the great vessels. 1 

 Haycraft 2 has likewise passed needles through the closed thorax into 

 the mammal's heart. The needles indicated the contraction of the 

 organ, both in the transverse and antero-posterior diameter. They did 

 not show any sign of that twisting of the heart as a whole, which 

 is seen when the chest is opened. Comparing these results with those 

 of Ludwig, it is clear that by opening the thorax, and allowing the 

 lungs to collapse, a material difference is made in the change of form 

 of the heart. This follows from the removal of those structures which 

 naturally support the heart. Thus Haycraft observed that if an animal 

 be suspended horizontally, with the belly downwards, and the chest be 

 opened, the heart moves downwards towards the sternum in diastole, and 

 springs towards the vertebral column during systole. While, if the animal 

 be placed on one side, the heart sinks towards that side during diastole, 

 and springs towards the other side during systole. In diastole the heart 

 always follows the pull of gravity, in systole the direction of the spring 

 of the apex depends upon the fulcrum, or, in other words, upon that 

 part of the heart which is supported by some neighbouring structure. 

 When the animal lies on its back, the fulcrum to the heart is afforded by 

 the fibrous structure of the pericardium. The base of the left ventricle 

 rests securely upon this strong membrane, upon which it is slung as 

 upon a hammock. 



In the closed thorax the falling back of the heart in diastole is 

 largely prevented by the resistance of the expanded lungs. Thus, when 

 a dog is placed symmetrically on its back, and the lungs are allowed to 

 collapse, the heart falls backwards and to the left during diastole, while 

 with each systole it springs upwards and towards the right, and the 

 twisting movement from left to right is observed. If the lungs be now- 

 inflated so that the heart is fully supported by the lungs on both sides, 

 then no rotation of the organ as a whole takes place. 



Ludwig and Hesse took casts of excised hearts, which had previously been 

 distended with blood at a pressure of 1 50 mm. Hg, so as to imitate the diastolic 

 form. Other casts were taken of empty hearts in a state of heat contraction 

 produced by the sudden immersion of the organs in a solution of potassium 

 bichromate heated to a temperature of 50 C. From the measurements of 

 these casts they determined that the heart is diminished in heat systole in all 

 its diameters except the vertical, while the areas of the auriculo-ventricular 

 orifices are diminished by one-half. 



As the musculature of the heart is, speaking generally, composed of an 

 inner and an outer longitudinal layer, and a middle circular layer, we must 

 suppose that in heat-rigor the contraction of these layers strikes a balance, so 

 that the vertical diameter does not shorten. 



1 Halford, "On the Movements and Sounds of the Heart," London, 1861, p. 12. 



2 Journ. PhysioL, Cambridge and London, 1891, vol. xii. p. 452. 



