52 



AKUAXGEMENT OF THE CARDIAC MUSCULAR FIBRES. 



(1) The muscular fibres in the auricles are completely separated from the fibres 

 of the ventricles by the fibrous rings which surround the auriculo- ventricular orifices, 

 and which serve as an attachment for the auriculo-ventricular valves (fig. 27, I). 

 The auricles are much thinner than the ventricles, and their fibres are generally 

 arranged in two layers ; the outer transverse layer is continuous over both auricles, 

 whilst the inner one is directed longitudinally. The outer transverse fibres may be 

 traced from the openings of the venous trunks anteriorly and posteriorly over the 

 auricular walls. The longitudinal fibres are specially well marked where they are 

 inserted into the fibro-cartilaginous rings, while in some parts of the anterior 

 auricular wall they are not continuous. In the auricular septum, some fibres, 

 circularly disposed around the fossa ovalis (formerly the embryonic opening of the 

 foramen ovale), are well marked. Circular bands of striped muscle exist around 

 the veins where they open into the heart ; these are least marked on the inferior 

 vena cava, and are stronger and reach higher (2 -5 cm.) on the superior vena cava 

 (fig. 27, II). Similar fibres exist around the pulmonary veins, where they join the 

 left auricle, and these fibres (which are arranged as an inner circular and an outer 

 longitudinal layer) can be traced to the hilus of the lung in man and some 



Fig. 27. 

 I. Course of the muscular fibres on the left auricle with the outer transverse and inner longi- 

 tudinal fibres, the circular fibres on the pulmonary veins (v. p.) ; V, the left ventricle (John 

 lldd). II. Arrangement of the striped muscular fibres on the superior vena cava (Elischer) 

 a, opening of vena azygos ; r, auricle. 



mammals ; in the ape and rat they extend on the pulmonary veins right into the 

 lung. In the mouse and bat, again, the striped muscular fibres pass so far into 

 the lungs that the walls of the smaller veins are largely composed of striped muscle 

 (Stieda). 



Circular muscular fibres are found where the vena magna cordis enters the heart, 

 and in the Valvula T/iebesii which guards it. 



, Physiological Significance. (1) The auricles contract independently of the 

 ventricles. This is seen when the heart is about to die ; when there may be several 

 auricular contractions for one ventricular, and at last only the auricles pulsate. 

 The auricular portion of the right auricle beats longest ; hence it is called the 

 " ultimum moriens." Independent rhythmical contractions of the vena? cava? and 

 pulmonary veins are often noticed after the heart has ceased to beat. [This beating 

 can also be observed in those veins in a rabbit after the heart is cut out of the 

 body.] 



(2) The double arrangement of the fibres (transverse and longitudinal) produces 

 a simultaneous and uniform diminution of the auricular cavity (such as occurs in 

 most of the hollow viscera). 



(3) The contraction of the circular muscular fibres around the venous orifices, 

 and the subsequent contraction of the auricle, cause these veins to empty themselves 



