DEVELOPMENT OF THE HUMAN HEART 277 



cle with the extension of the crista supraventricularis. 22 In the 

 ox, pig and sheep, however, it retains its original position and 

 contains the right limb of the atrio-ventricular bundle. 



It is thus seen that the medial cusp of the tricuspid valve is 

 attached in front by the medial tendon (or muscle), and behind 

 by the large papillary muscle, and the inequality of these two 

 structures accounts for the double appearance of the lateral valve. 

 In reality no true tricuspid valve is present and correctly speaking 

 there is no tricuspid valve. Both are bicuspid with medial and 

 lateral cusps. Both are tied down by two muscles, the two papil- 

 lary muscles on the left side and the large papillary muscle and 

 the median tendon on the right side. 



The atrio-ventricular cushions expand not only by their own 

 growth but to a greater extent by a process of undermining the 

 ventricle wall all around the venous ostia. To what extent this 

 burrowing has taken place is marked by the attachment of the 

 valves to the muscle walls of the heart. The tendons nearest the 

 tips of the valves were the first to form, while those nearer the 

 bases of the valves were formed subsequently. This has all been 

 demonstrated by His. By this process of undermining the attach- 

 ment of the base of the valve to the wall of the ventricle, the atrial 

 portion is telescoped into the ostia. The muscle of the atrium 



22 Toldt's 'crista' (I.e., fig. 946) ends in the medial papillary muscle (anterior), 

 which according to its development is correct. Spalteholz (I.e., fig. 424) extends 

 the crista past the anterior tendon down to the base of the large papillary muscle 

 (posterior) . This connecting band contains the right limb of the atrio-ventricular 

 band and is pictured by Tawara on his plate 7. Retzer (J. H. H. Bull., vol. 20, 

 1909, and Anat. Rec, vol. 6, 1912) associates the moderator band with the crista, 

 in fact he says that when absent it is represented by the crista. He recognizes 

 fully the meaning of the anterior (medial) papillary muscle, but when it is recalled 

 that the right limb of the atrio-ventricular band passes on the posterior side of 

 this muscle and the crista on its anterior, the identity of these two structures is 

 disproved. In fact in the embryo the crista reaches to the medial muscle and the 

 moderator band is considerably below it. In the adult heart this band is pushed 

 to the base of the large (posterior) papillary muscle as pictured in Spalteholz and 

 it contains the right limb of the atrio-ventricular bundle as shown by Tawara; 

 this I have been able to confirm. His, who introduces this term 'crista' (Beitrage 

 zur Anat. d. Mensch. Herzens, Leipzig, 1886, S. 9) is not clear in his description of 

 its attachment to the septum. According to its development it should end in the 

 septum aorto pulmonale, that is, at the point of origin of the medial papillary 

 muscle. 



