508 THE CIRCULATORY APPARATUS. 



serous membrane covering it, and which must be at once removed. The furrows should 

 then be cleared of their vessels and fat ; this renders the superficial muscular fibres 

 very apparent. The same result may be attained by immersing the heart in vinegar or 

 dilute hydrochloric acid. To isolate the ventricles and unitive fibres from each other, 

 the following procedure may be adopted : After removing the auricular mass and 

 dissecting the fibrous rings, the unitive fibres around these 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 through the walls of 

 the heart must be cut through. The same course is followed in the substance of the 

 interventricular septum, in order to separate the two sacs formed by the proper fibres. 



(It will be found that the simplest and best way to prepare the heart for an examina- 

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

 serous membrane, and the 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, 

 disposed in rings around the aurictilo-ventricular and arterial openings ; it 

 receives 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 annular framework, muscular tissue proper, 

 vessels and nerves, and serous tunics such are the elements entering into 

 the organisation of the heart. 



A. FIBROUS KINGS. 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 con- 

 cavities superior and internal, and which correspond to the insertions of 

 the three sigmoid valves. These zones are continuous, by their superior and 

 external contour, with the walls of the arteries, from which they are only dis- 

 tinguished by their whitish-grey colour and slight elasticity, the arterial 

 tissue being yellow and very elastic.' Their internal and inferior outline 

 sends three thin prolongations into the serous duplicatures of the sigmoid 

 valves. 



The auriculo-ventricular zones do not completely surround the openings 

 they circumscribe. They are flattened, brilliant-white tendons, laid one 

 against the other at the level of the ventricular septum, and against 

 the aortic ring ; they turn to the right and left around the auriculo- 

 ventricular openings, but without joining at their extremities, which are 

 dispersed as fibrillse in the muscular tissue of the ventricles. Above, these 

 zones give attachment to the muscular fibres of the auricles ; below, to the 

 ventricular fasciculi. Their internal and inferior border is prolonged into 

 the mitral and tricuspid valves, and is continuous, through these valves, . 

 with the tendinous cords fixed to the walls of the ventricles. Some of these 

 cords, generally the strongest, are even directly inserted into the auriculo- 

 ventricular zones. 



It must be noted that, in Solipeds, there is constantly found, at the 

 point where the aortic and auriculo-ventricular zones lie against each other, 

 a more or less developed cartilaginous body, which, in the larger Euminants, 

 is transformed into true bone. (Lavocat speaks of two cartilaginous points, 

 one to the right, at the junction of the aortic with the left auriculo- 

 ventricular ring and the cardiac septum; the other, less developed, on the 

 left, at the origin of the left ventricular groove.) 



B. MUSCULAR TISSUE. The muscular tissue composing the mass of the 



