CIRCULATION OF THE BLOOD. 



501 



physiological characteristics of the' different cardiac cavities, this physio- 

 logical difference depending upon the anatomical structure of the different 

 parts of the heart, as has been clearly traced out by Landois whose 

 description of this subject has been mainly followed. In the auricles 

 the fibrous auriculo-ventricular rings serve to separate the auricular 

 muscular fibres from the ventricles, and, therefore, serves to explain the 

 fact that the auricles are enabled to contract independently of the ven- 

 tricles. 



In the auricles the muscular fibres are arranged in an external trans- 

 verse layer continuous over both auricles and an internal longitudinal 

 layer. This double arrangement of the fibres enables them in their 

 contraction to produce a uniform diminution of the auricular cavities 

 (Pig. 195). 



Fig. 196.— Course of the Ventricular Muscular Fibres. (Landois.) 



A, on the anterior surface ; B, view of the apex with the vortex (Hm le) ; C, scheme of the course of the fibres 

 within the ventricular wall; D, fibres passing into a papillary muscle {C. Li/thcir/). 



At the openings of the large veins into the auricles there is a special 

 development of circular bands of striped muscular fibre. The contrac- 

 tion of these bands enables the veins to empty themselves into the 

 auricle and also serves to prevent, to a certain degree, backward reflux 

 of the blood from the auricles into the veins when the auricles contract, 

 even though no valves are present in these vessels. In the ventricles the 

 fibres are arranged in a number of separate layers so as to form figure of 

 8 loops. First, there is an outer longitudinal layer which is in the 

 form of single bundles on the right ventricle and forms a complete layer 



