102 



NORMAL HISTOLOGY. 



Muscular tissue 



near their attached borders, while the fibre-elastic tissue of the chorda ten- 

 dinece is continuous with the middle layer. The semilunar valves guarding 

 the aorta and the pulmonary artery correspond in their general structure 

 with the other valves, although muscle in them is wanting. At the periphery 

 and in the central thickenings or noduli of the leaflets, the elastic tissue is 

 particularly rich. Within the folds guarding the orifices of the inferior vena 

 cava and of the coronary sinus, the interendothelial connective tissue is an 

 inconsiderable layer, which, in the case of the Eustachian valve, is sometimes 

 further reduced by absorption resulting in a fenestrated condition of the leaflet. 

 The Myocardium. The middle layer of the heart-wall, the myocar- 

 dium, is composed of a close elongated network of muscle-fibres, the inter- 

 muscular spaces of which are filled with connective tissue. The latter 

 corresponds to an endomysium and, within the ventricles, contains only 

 a small amount of elastic tissue, except around the openings of the valves. 



In these locations, dense plates 

 of fibrous tissue (annulifibrosi) 

 encircle the valvular orifices 

 and contain many elastic fibres, 

 as well as give attachment 

 to the strands of cardiac mus- 

 cle. The histology of cardiac 

 muscle has been described 

 elsewhere (page 55) and 

 need not here be considered. 

 Although as ordinarily seen 

 in microscopical preparations, 

 the fibro-muscular sheets that 

 compose the myocardium seem 

 to follow no particular arrange- 

 ment, it has been shown that 

 in the architecture of the heart 

 they are disposed according 

 to a definite but complex 

 plan. For a description of 

 this arrangement, however, the 

 reader must be referred to 

 the systematic text-books of 

 anatomy. 



In addition to the ordi- 

 nary fibres of cardiac muscle, 

 the subendocardial layer of the myocardium in many places, especially in the 

 ventricles, contains peculiar fibres, distinguished by their large size, pale color 

 and abundant sarcoplasm, with a corresponding lessening in the number of 

 contractile fibrillae. These have long been known as Purkinje fibres and 

 were regarded as immature and imperfectly differentiated muscle-elements. 

 Their significance, however, has only recently been recognized. They are 

 now regarded as the terminal part of an elaborate system of special muscle- 

 fibres, whose probable function is the coordinative connection of the atrial 

 and ventricular musculature, that otherwise are distinct and unconnected. 

 The most evident part of this system is the definite atrio-ventricular bundle, 

 which, beginning in the atrial wall in the vicinity of the coronary sinus, 

 passes from the interatrial septum, under the attachment of the posterior 

 leaflet of the tricuspid valve, into the pars membranacea septi; here dividing 



FIG. 139. Longitudinal section of leaflet of tricuspid 

 valve. X 20. 



