THE DEVELOPMENT OF THE VASCULAR SYSTEM 237 



(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 trabeculas of the ventricle walls (Figs. 214 

 and 215). On the right side three of these folds appear, and develop into the 

 vahula 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 trabeculae to which they are attached also undergo marked changes. 

 Some become condensed at the ends which are attached to the valves into 

 slender tendinous cords the chorda tendinece, while at their opposite ends 



Muscle trabeculae 



Trabeculae carneae 



FIG. 214. Diagrams representing the development of the atrio-ventricular valves, chordae, 

 tendinese, and papillary muscles. Gcgcnbaur. 



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 con- 

 stitute the trabecula carnea (Fig. 214). 



(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 

 f return Halleri (p. 231) where the ventricular portion of the heart joined 

 the aortic bulb. Before the aortic trunk and bulb are divided into the aortic 

 arch and pulmonary artery, four protuberances appear in the lumen (Fig. 

 213). The septum aorticum then divides the two which are opposite so that 

 each vessel receives three (Fig. 213). These then become concave on the 

 side away from the heart, in a manner which has not been fully determined, 

 and at the same time enlarge so that they close the lumen. Those in the 

 pulmonary artery are known as the valvula semilunares arterice pulmonalis, 

 those in the aorta as the valvula semilunares aorta. 



Changes after Birth. The migratory changes of the heart from its origi- 

 nal position in the cervical region to its final position in the thorax will be con- 



