540 



On the Mechanism of the Cardiac Valves. 



them will still be active. The result is that closure finally occurs, and is 

 rendered absolute by the rapidly increasing pressure of blood in the ventricle 

 which now commences to contract. 



The function of the muscular slips now described may perhaps be regarded 

 as a double one, (a) to keep the flaps away from the heart walls, and thus 

 to ensure the provision of an adequate space between the flap and the 

 ventricular wall for the full development of the retrovalvular eddy, and 

 (b) to afford by its contraction actual mechanical assistance to the raising 

 of the flaps into the position of final closure. 



It is obvious that the anatomical arrangement is admirably adapted to the 

 carrying out of these functions. Placed at the base of the auricle and 

 deriving its stimulus thence, the muscle in the valves will contract, and 

 will relax, last of all the auricular tissue, and thus ensure, not only that the 

 mechanical assistance referred to under (b) shall become available at the 

 proper moment, but also that the work of the eddy shall be assisted up 

 to the very moment of final closure of the valve. 



There are other points of interest connected with the presence of muscular 

 tissue in the segments of the auriculo-ventricular valves which it is not 

 proposed to deal with in the present communication. 



I wish to record my indebtedness to Mr. E. B. Britton for permission to 

 make use of the specimen from which fig. 1 was prepared. 



The above research has been assisted by grants from the Government Grant 

 Committee of the Boyal Society, from the Besearch Fund of the University 

 Colston Society, and from the British Association for the Advancement of 

 Science. 



DESCRIPTION OF FIGUEES. 



Fig. 1.— Heart 141204 (Britton). Eat, 3 weeks old. Slide 53, Section 3. x 29. Tri- 

 cuspid Valve. 



The auricular muscle is seen to sweep uninterruptedly into the basal part of 

 the valve flap, of which it forms the upper portion ; it is inserted into the 

 connective tissue substance at the junction of the basal and middle thirds of the 

 flap as seen in the figure. 

 Fig. 2.— Heart 130614. Slide 252. Section 2. Cat, adult. Tricuspid Valve, x 29. 



The auricular muscle passes without change into the valve, of the thickness of 

 which it forms a considerable part. The muscle fibres are situated towards the 

 auricular surface of the flap, lying beneath the endocardium. 



A contraction of this muscle would lead to a powerful raising of the valve 

 segment. 



