538 STRUCTURE OF THE HEART. 



The Semilunar valves are placed around the commencement of the 

 aorta, like those of the pulmonary artery ; they are similar in struc- 

 ture, and are attached to the scalloped border by which the aorta is 

 connected with the ventricle. The tubercle in the centre of each fold 

 is larger than those in the pulmonary valves, and it was these that 

 Arantius particularly described ; but the term " corpora Arantii" is 

 now applied indiscriminately to both. The fossae between the semi- 

 lunar valves and the cylinder of the artery are larger than those of the 

 pulmonary artery ; they are called the " simis aortici" (sinuses of 

 Valsalva). 



STRUCTURE OF THE HEART. 



The arrangement of the fibres of the heart has been made the 

 subject of careful and accurate investigation by Mr. Searle, to whose 

 excellent article, " Fibres of the Heart," in the Cyclopaedia of Anatomy 

 and Physiology, I am indebted for the following summary of their 

 distribution : 



For the sake of clearness of description the fibres of the ventricles 

 have been divided into three layers, superficial, middle, and internal, 

 all of which are disposed in a spiral direction around the cavities 

 of the ventricles. The mode of formation of these three layers will 

 be best understood by adopting the plan pursued by Mr. Searle in 

 tracing the course of the fibres from the centre of the heart towards 

 its periphery. 



The left surface of the septum ventriculorum is formed by a broad 

 and thick layer of fibres, which proceed backwards in a spiral direction 

 around the posterior aspect of the left ventricle, and become aug- 

 mented on the left side of that ventricle by other fibres derived from 

 the bases of the two columnae papillares. The broad and thick band 

 formed by the fibres from these two sources, curves around the apex 

 and lower third of the left ventricle to the anterior border of the 

 septum, where it divides into two bands, a short or apicial band, and 

 a long or basial band. 



The Short or apicial band is increased in thickness at this point by 

 receiving a layer of fibres (derived from the root of the aorta and 

 carneae columnae) upon its internal surface, from the right surface of 

 the septum ventriculorum ; it is then continued onwards in a spiral 

 direction from left to right, around the lower third of the anterior 

 surface, and the middle third of the posterior surface of the right 

 ventricle to the posterior border of the septum. From the latter point 

 the short band is prolonged around the posterior and outer border of 

 the left ventricle to the anterior surface of the base of that ventricle, 

 and is inserted into the anterior border of the left auriculo-ventricular 

 ring, and the anterior part of the root of the aorta and pulmonary 

 artery. 



The Long or basial band, at the anterior border of the septum, passes 



