326 THE HEAET. 



Each flap of the auriculo-ventricular valves is provided with 

 tendinous cords chordae tendinesb which are attached to 

 the free margin and under surface, so as to keep the valves 

 tense when closed, a condition which is produced by the 

 shortening of muscular pillars musculi papillares with 

 which the tendinous chords are connected. The inner sur- 

 face of the heart is covered by a serous membrane endo- 

 cardium, which is smooth and glistening, being firmly adhe- 

 rent to the muscular structure of the organ. 



The arterial openings, both on the right and left side, are 

 provided with three-flapped semilunar valves, to prevent the 

 regurgitation of blood. The veins opening into the auricles 

 are not capable of closure, but the posterior vena cava has an 

 imperfect flap eustachian valve at its aperture. There is 

 one more point that I must particularly notice in the con- 

 struction of the heart. In foetal life the two auricles com- 

 municate by an opening foramen ovale which becomes 

 closed immediately after the animal's birth. Its position can 

 be seen in the heart of the oldest animals from a depression 

 and circular fold which constitute the remnant or vestigium 

 foraminis ovalis. 



It is important to inquire into the arrangement of the 

 muscular fibres, whereby the peculiar rythmic action of the 

 heart for the propulsion of blood is produced. Until recently 

 the subject was involved in much mystery, but a distin- 

 guished student of the Edinburgh University Dr Pettigrew 

 has satisfactorily explained it, and was called upon, in 

 1860, to deliver the Croonian Lecture at the Eoyal Society 

 of London. I may be permitted to quote from the abstract 

 of the lecture published in the Royal Society's Proceedings: 



" The Lecturer began by referring to the descriptions of the arrange- 

 ment of the ventricular fibres of the heart given by previous inquirers, 

 more especially Lower, Senac, Wolff, Gerdy, Duncan, and Keid ; he 



