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TEXT-BOOK OF EMBRYOLOGY. 



together and prevent the blood from flowing back into the veins. The left 

 valve and the septum spurium later atrophy to a certain extent and probably 

 unite with the septum atriorum to form part of the limbus fossa ovalis (Vieus- 

 senii) . The right valve is the larger and in addition to its assistance in prevent- 

 ing a backward flow of blood into the veins, it also serves to direct the flow to- 

 ward the foramen ovale. As the veins come to open separately, the cephalic 

 part of the right valve disappears; the greater part of the remainder becomes 

 the valvula vena cava inferioris (Eustachii) and during foetal life directs the 

 blood toward the foramen ovale. In the adult it becomes a structure of 

 variable size. A small part of the remainder of the right valve forms the valvula 

 sinus coronarii (Thebesii) which guards the opening of the coronary sinus. 



(b) and (c) The valves between the atrium and ventricle on each side 

 develop for the most part from the walls of the triangular atrio-ventricular 

 opening (ostium atrio-ventriculare) . Elevations or folds appear on the rims of 

 the openings and project into the cavities of the ventricles where they become 

 attached to the muscle trabecule of the ventricle walls (Figs. 205 and 206). 

 On the right side three of these folds appear, and develop into the valvula 



Valve 

 Cavity of ventricle 



Muscle trabecule 



Valve 



Chordse tendinese 



Papillary muscles 

 Trabecule carneae 



Fig. 205. — Diagrams representing the development of the atrio ventricular valves, chordae 

 tendinese, and papillary muscles. Gegenbaur. 



tricuspidalis which guards the right atrio-ventricular orifice. On the left side 

 only two folds appear, and these become the valvula biscuspidalis (mitralis) 

 which guards the left atrio-ventricular orifice. These valves, which are at first 

 muscular, soon change into dense connective tissue. The muscle trabecule to 

 which they are attached also undergo marked changes. Some become con- 

 densed at the ends which are attached to the valves into slender tendinous cords 

 — the chorda tendinea, while at their opposite ends they remain muscular as the 

 Mm. papillares; others remain muscular and lie in transverse planes in the 

 ventricles, or fuse with the more compact part of the muscular wall, or form 

 irregular, anastomosing bands and constitute the Irabecula carnea (Fig. 205). 

 (d) and (e) The valves of the pulmonary artery and aorta develop at the 

 point where originally the endothelial tube was constricted to form the fretum 

 Halleri (p. 226) where the ventricular portion of the heart joined the aortic 



