794 DR PETTIGREW ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



from analogy, for on watching the hearts of cold-blooded animals, they are 

 found towards the end of the contractile act to become quite pale, not, as Harvey 

 supposed, from the blood being pressed out of the parietes, but from the blood 

 in their cavities seen through their transparent sides being almost entirely 

 expelled." An important inference to be deduced from the spiral nature of 

 the ventricular fibres and ventricular cavities and the undouhted spiral action 

 of the auriculo-ventricular valves is the effect produced on the blood as it leaves 

 the ventricles, that fluid being unquestionably projected by a wringing or twist- 

 ing movement, which communicates to it a gliding sinral motion. This view is 

 favoured by the spiral inclination of the sinuses of Valsalva to each other, these 

 structures gradually introducing the blood so projected into the A^essels, How far 

 the rotatory movement referred to, extends into the arteries, is difficult to deter- 

 mine; but when the smooth cylindrical nature of the vessels, and the great 

 velocity and force with which the blood travels, is taken into account, there is 

 every reason to suppose that the distance is considerable. 



The Mechanical a7id Vital TJieories of the Action of the Mitral and Tricuspid Valves 



considered. 



That the theory which attributes the closure of the auriculo-ventricular 

 valves to the mechanical floating up of the segments from beneath by the blood, 

 forced by the auricles* into the ventricles, distending equally in all directions,! 

 is of itself inadequate to explain all the phenomena, is, I think, probable from 

 analogy and the nature of things ; for if a merely mechanical arrangement of parts 

 was sufficient for the closure of the gradually contracting auriculo-ventricular 

 orifices, then, it may be asked, why were these apertures in birds and mammals 

 not furnished with sigmoid or semilunar valves* similar in all respects to those 

 met with in the veins and arteries ? The answer to this question is no doubt to 

 be found in the nature of the structures in which the valves are situated, as well 

 as in the circulation itself. In the veins, as is well known, the movements of the 

 blood are sluggish — the contraction of the vessels being feeble, and not conse- 

 quently calculated to interfere to any great extent Avith the closure of the valves. 

 In the arteries, where the circulation is more vigorous, and the contractions of the 

 vessels more decided, the valves are surrounded by dense fibrous rings, which 

 protect them alike from the contractions of the ventricles, and the contractility 



* According to Harvey, Lower, Senac, Haller, and others, tlie auricles contract with a very 

 considerable degree of energy. 



f " In a quantity of fluid submitted to compression, the whole mass is equally affected and 

 similarly in all directions." — Hydrostatic Law. Dr George Britton Halford attributes the 

 closure of the auriculo-ventricular valves entirely to the pressure exercised by the auricles on the 

 blood forced by them into the ventricles. That, however, this is not the sole cause, will be shown 

 further on. 



