S92 CIRCULATORY APPARATUS. 



are divided with the point of the scalpel, care being taken not to injure the proper fibres. Then, 

 with the aid of the finger-nail or handle of the scalpel, follow the more or less artificial limit of 

 these two series of muscular planes in a spiral manner ; the vessels passing thiough the walls 

 of the heart must be cut through- The same course is fullowed in the substance of the inter- 

 ventricular septum, in order to separate tlie two sacs fiurmed by the proper fibres. 



(It will be found that the simplest and best way to prepare the lieart for an examination 

 of its fibres, is to steep it in a very weak dilution of hydrochloric acid. Remove the serous 

 membrane, and tlie fibres can then be traced, layer by layer, from their origin to their 

 termination.) 



The muscular tissue composing the heart rests on a fibrous framework, dis- 

 posed in rings around the auriculo-ventricular and arterial openings ; it receives 



Fig. 354. 



ArRICULO-VENTRICCLAR FIBRO-CARTILAGINOUS RINGS. 



1, Pulnionary artery ; 2, superior border of the infundibulum ; 3, aorta ; 4, sigmoid or semilunar 

 valves; 5, bulgings of the aorta corresponding to the semilunar valves; ii, left auriclo-ventricular 

 opening; 7, right ditto ; 8, left fibro-cartilaginous ring ; 9, right ditto ; 10, the two rings meeting 

 together in the middle line. 



vessels and nerves, and while covered in the internal cavities by two independent 

 serous membranes, it is enveloped, externally, by another membrane of the same 

 kind. An annvlar frameivork, muscular tissue proper, vessels and nerves, and 

 serous tunics — such are the elements entering into the structure of the heart. 



A. Fibrous Rings, or Tendinous Rings of Lower. — These are also 

 named the fibrous zones of the heart, and are four in number : one for each of 

 the openings at the base of the ventricles. 



The two arterial zones (the pulmonary and aortic) constitute two complete 

 rings, which are not disposed in a circular manner around the pulmonary aortic 

 openings, but are divided into three regular festoons with their concavities 



