562 THE VASCULAR SYSTEMS 



thick and strong; the lateral margins are thin and translucent. These segments 

 are connected by their bases to the oval fibrous ring surrounding the auriculoven- 

 tricular orifice (annulus fibrosus dexter], and by their sides with one another, so 

 as to form a continuous annular membrane, which is attached around the margin 

 of the auriculoventricular opening; their free margins and ventricular surfaces 

 afford attachment to a number of delicate tendinous cords, the chordae tendineae. 

 The chordae tendineae are connected with the adjacent margins of the principal 

 segments of the valve, and are further attached to each segment in the following 

 manner: (1) Three or four reach the attached margin of each segment, where 

 they are continuous with the auriculoventricular tendinous ring. (2) Others, 

 four to six in number, are attached to the central thickened part of each segment. 

 (3) The most numerous and finest are connected with the marginal portion of 

 each segment. 



The columnae carneae (trabeculae carneae) are the rounded muscle columns 

 which project from nearly the whole of the inner surface of the ventricle, except- 

 ing near the opening of the pulmonary artery, where the wall is smooth. They 

 may be classified, according to their mode of connection with the ventricle, into 

 three sets. The first set merely forms prominent ridges on the inner surfaces of 

 the ventricle, being attached by their entire length on one side, as well as by their 

 extremities. The second set are attached by their two extremities, but are free 

 in the rest of their extent, forming arches; while a third set (musculi papillares) 

 are continuous by their bases with the wall of the ventricle, while their apices 

 give origin to the chordae tendineae, the papillary muscles. There are usually 

 two papillary muscles, the anterior and the posterior; of these, the anterior is the 

 larger; its chordae tendineae are connected with the right and left segments of 

 the tricuspid valve. The posterior sometimes consists of two or three muscle 

 columns; its chordae tendineae are connected with the posterior and the right 

 segments of the tricuspid valve. In addition to these, some few chordae spring 

 directly from the ventricular septum, or from small eminences on it, and pass to 

 the left and posterior segments. A fleshy band, well marked in the sheep and 

 some other animals, is frequently seen passing from the base of the anterior 

 papillary muscle to the interventricular septum. From its attachments it may 

 assist in preventing overdistention of the ventricle, and so has been named the 

 moderator band. 



The pulmonary valve (Fig. 418) consists of three semilunar segments (valvulae 

 semilunares a. pulmonalis), two of which are anterior and one of which is posterior, 

 formed by duplicatures of the lining membrane of the ventricle, strengthened by 

 fibrous tissue. They are attached, by their outer convex margins, to the wall of 

 the artery, at its junction with the ventricle, their inner borders being free, and 

 directed upward in the lumen of the vessel. The free and attached margins of 

 each are strengthened by a bundle of tendinous fibres, and the former presents, 

 at its middle, a small projecting thickened nodule, consisting of bundles of inter- 

 lacing connective-tissue fibres with branched connective-tissue cells and some few 

 elastic fibres. Such a nodule is called the corpus Arantii (iwduli valvularum semi- 

 lunarium [Arantii]}. From this nodule tendinous fibres radiate through the valve 

 to its attached margin, but are absent from two narrow crescentic portions, the 

 lunulae (lunulae valvularum semilunarium), placed one on either side of the nodule 

 immediately adjoining the free margin. 



The basal end of the pulmonary artery presents three dilatations opposite to 

 the valve. These are the pulmonary sinuses of Valsalva. Similar sinuses exist 

 between the semilunar valves and the commencement of the aorta; they are larger 

 than the pulmonary sinuses. 



In order to examine the interior of the left auricle, make an incision on the posterior surface 

 of the auricle from the pulmonary veins on one side to those on the other, the incision being 



