518 THE BLOOD-VASCULAR SYSTEM 



sinuses [sinus aortse] or sinuses of Valsalva. From the right and left sinuses the 

 right and left coronary arteries, respectively, arise. 



After ventricular systole the increased pressure in the great vessels distends the valves with 

 blood. The noduli meet in the centre and the lunulse, coming into mutual contact, produce a 

 tri-radiate line of contact between the valves. 



ARCHITECTURE OF THE MYOCARDIUM 



In the adult heart the myocardium of the atria is separate from that of the ventricles. 

 There is, between the atria and ventricles, a fibrous partition, the upper and lower surfaces ol 

 which give attachment to the muscle fibres of these cavities, respectively. 



The fibrous partition (fig. 431) is thickest in the triangle formed by the meeting of the aortic, 

 and right and left atrio-ventricular ostia. This interval is filled by a mass of fibrous tissue, 

 which in the angles between the aortic and the left atrio-ventricular ostium forms two thickened 

 triangular masses, the trigona fibrosa. The fibrous mass is continued to the pulmonary ostium 

 as the tendon of the conus. Below the point of junction of the trigona and the tendon of the 

 conus these structures blend with the septum membranaceum ventriculorum. The septum 

 membranaceum, tendon of the conus, and part of the trigona are derived from the aortic septum 

 (pp. 516, 527). The trigona give off laterally, on either side, atrio-ventricular rings which en- 

 circle the venous ostia and give attachment to the atrio-ventricular valves. There are also weak 



Fig. 431. — Base of a Well Developed Heart showing the Course of the Superficial 



Muscle Fibres. 



From X to jX'' around the front of the aorta indicates the course of the aortic septum. 



(Mall, f nat. size.) 



I 



rings surrounding the pulmonary and aortic orifices; the aortic and left atrio-ventricular rings 

 being partly confluent. The rings surrounding the arterial and venous ostia are the annuli 

 fibrosi. 



The atrial musculature is attached to the trigona and atrio-ventricular rings only. The 

 superficial fibres are attached to both rings and either encircle both atria in one loop, or enter the 

 septum and form a figure 8. The deeper fibres are attached to one ring and encircle one atrium 

 only; some fibres encircle only the auricle. 



The ventricular musculature is very complex and consists of numerous superimposed layers 

 distinguished from one another by the direction taken by the muscle fibres. In a general way, 

 the fibres of the deepest layer have a direction crossing those upon the surface of the same area 

 at a riglit angle. The intervening layers of fibres pass through all stages of obliquity. 



Recent work upon the origin anil distribution of the ventricular fil)res has resulted in the 

 recognition of a certain uniformity of behavior, thus: — 



1. Ail fil)res ari.se from, and arc inserted into, the fibrous partition at the base. The at- 

 tachment may l)e directly to the trigona or annuli, or indirectly to them by means of the chordae 

 tendinea; and atrio-vfuitricular valves. 



2. The more .suj)crfici;il fib^^s (fig. 432) tend to encircle the entire heart, passing over the 

 longitudinal sulci. If they enter the septum they do so by passing into the vortex or whorl 

 at the apex of the left ventricle. The.se fibres have always a <l(!fiiiite direction upon the sur- 

 face, i. c., from right to left upon the sterno-costal surface and from left to right on the dia- 

 phragmatic (fig. 431). 



3. The deeper filjrcs all enter the septum in a direction oblique or perpendicular to its 

 longitudinal axis. In addition they completely encircle one or both ventricles forming, in the 

 latter case, double loops (fig. 433). 



