G8 



ARRANGEMENT OF THE CARDIAC MUSCULAR FIBRES. 



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-cartila- 

 ginous 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. 18, II). Similar fibres exist 

 around the four 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 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). 



v.p 



Fig. 18. 



I. Course of the muscular fibres on the left auricle Observe the outer transverse 

 and inner longitudinal fibres, the circular fibres on the pulmonary veins (v, p)\ 

 V, the left ventricle (John Reid). II. Arrangement of the striped muscular 

 fibres on the superior vena cava (Elischer) o, opening of vena azygos ; 

 v, auricle. 



Circular muscular fibres are found where the vena magna cordis 

 enters the heart, and in the valvula thelesii which guards it. 



From a pliy 'siological point of view the following facts are to be noted 

 as a result of the anatomical arrangement : 



(l.)'The auricles contract independently of the ventricles. This is 

 seen when the heart is about to die ; then there may be several 

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



