634 SPECIAL PHYSIOLOGY. 



base is the opening from the auricle just mentioned, whilst above, and 

 in front of this, is the aperture leading into the pulmonary artery. 

 That portion of the ventricle conducting to the artery, forms a conical 

 prolongation, named the infundibulum or conus arteriosus. Both of 

 these openings are guarded by remarkable valves. The auriculo-ven- 

 tricular opening corresponds with the middle of the sternum, on a level 

 with the third intercostal spaces and fourth costal cartilages. It is 

 somewhat oval, and measures about 1J inch in diameter, in the male. 

 It is surrounded by a strong fibrous ring, and its valve being composed 

 of three pointed segments, is hence called the tricuspid valve. These 

 segments, of a trapezoidal shape, are formed by a doubling of the lining 

 membrane of the heart, inclosing bands of fibrous tissue, and, it is said, 

 a few muscular fibres; the segments are continuous at their base, and 

 are there fixed to the fibrous ring around the opening into the auricle 

 (Fig. 106 #, 2). Of the three segments, one corresponds to the front 

 of the ventricle, another to its posterior wall, and the third, the largest, 

 lies between the auriculo-ventricular opening and the pulmonary artery. 

 Each segment is thicker at its centre; whilst its margins are thinner, 

 more transparent, and indented. To the margins, and also to the 

 ventricular surfaces of the segments, are attached numerous fine ten- 

 dinous cords, the chordae tendinece (Fig. 105), the other ends of which 

 are connected either with certain muscular columns, to be presently 

 described, projecting from the walls of the ventricle, or with the inner 

 surface of that cavity, especially with the septum. The chordae ten- 

 dineae, proceeding from the adjacent margins of any two segments of 

 the valve, are connected with the same muscular column. Some of the 

 cords are inserted into the base of the segments, others are connected 

 with its central thicker part, whilst the finest and most numerous are 

 inserted into its thin marginal portion. 



The muscular bands just mentioned, named the columnce carnece, 

 are found in nearly every part of the inner surface of the ventricle. 

 They are of three kinds: first, some which form merely irregular, and 

 frequently reticulated, prominences on the sides of the cavity ; a sec- 

 ond kind are adherent at each end, though free in the middle ; lastly, 

 a third kind, considerably larger than the others, and named the mus- 

 culi papillares, form three or four bundles, which project upwards from 

 the walls of the ventricle, and are connected with some of the chordae 

 tendineae of the tricuspid valve. The internal surface of the infundib- 

 ulum is smooth. 



The orifice of the pulmonary artery, Fig. 105, 5, corresponds with 

 the upper border of the third left costal cartilage, and second inter- 

 costal space, close to the sternum. It is circular, arid measures, in 

 the male, a full inch in diameter. Its protecting valves consist of 

 three semicircular membranous folds, named semilunar valves (Fig. 

 105 *, Fig. 106, 4), attached, by their convex margins, to the sides of 

 the pulmonary artery at its line of junction with the ventricle, but 

 free at their straight borders, which are turned upwards in the direc- 

 tion of the artery. In the middle of the free border of each valve is a 

 small fibro-cartilaginous nodule, the corpus Aran'ii. When stretched 

 across the vessel, the borders meet each other, forming lines diverging 



