THE HEART. 593 



superior and internal, and corresponds 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 distinguished 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-ventrkular zones do not completely suiTound the openings they 

 circumscribe. They are flat, brilliantly white tendons, laid one against the other 

 at 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 

 chordae tendinete 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 Ruminants, 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 (Fig, 355). — The muscular tissue of the heart is that of 

 organic life, as it contracts independently of the will. Nevertheless, it is formed of 

 red striped fibres, which differ from the striped muscles of the locomotory apparatus. 

 They are granular and dark under the microscope, and ramify and anastomose in 

 such a manner as to form an extremely fine network in the myocardium. From 

 the joining end to end of the segments of Weissmann, there result simple or 

 ramified prisms, the bases of which are notched like stairs. Each Weissmann 

 segment comprises : 1. In the centre, one or two nuclei with a nucleolus. 2. Con- 

 tractile cylinders of unequal length, around the nuclei. 3. A mass of protoplasm 

 enveloping the nucleus, extending 'between the contractile cyHnders, and forming 

 a kind of sarcolemma. Between the fibres of the myocardium there is very little 

 connective tissue, but there is a great number of blood-vessels and lymphatics. 



The arrangement of the muscular fasciculi of the heart has been the object 

 of numerous investigations, which have only complicated what was already 

 known on the subject. We will endeavour to sum up, as simply as possible, this 

 arrangement, in examining it successively in the ventricles and auricles. 



1. Fibres of the Ventricles. — According to the remark of Winslow, we 

 may compare the ventricles, in regard to the arrangement of the fibres composing 

 them, to " two muscular sacs included in a third ; " that is to say, each ventricle 

 is formed oi proper muscular fibres, covered externally by a layer of unitive fibres, 

 which envelop the two ventricles in common. 



a. Proper fibres of the ventricles. — Taken altogether, these fibres represent, 

 for each cavity, a hollow cone, open at both its extremities — at the superior 

 extremity, by the auriculo-ventricular and arterial openings ; and at the inferior 

 extremity, by an aperture which admits the reflected fibres of the common layer. 

 All form loops, attached by their extremities to the outline of the superior 

 orifices, on the fibrous zones, and are rolled, more or less obliquely, around the 



