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PROFESSOR D. WATERSTON ON 
In the next specimen modelled (12’5 mm.) a considerable change lias occurred. 
The muscle coat of the bulbus can be traced quite definitely to a point where it 
abruptly becomes continuous with the fibrous coat of the truncus arteriosus, and at 
that level are found the commencing pulmonary and aortic valves. 
The terminal part of the bulbus has now become more horizontal, and there is a 
marked inequality in the length of the ventral and dorsal walls : the ventral 
wall has increased in length, while the dorsal wall has remained stationary or has 
even shortened. 
Hence the valves (pulmonary) of the more ventral vessel lie some distance above 
the heart, while the aortic valves are near to it. 
Within the bulbus the division of the lumen into two vessels now extends along 
practically its whole length, and there are two distinct channels, both of which, 
however, communicate with the right ventricle below. 
Perhaps the more striking change — it certainly directed my own attention 
specially to this region — is the dilatation of the channel leading to the pul- 
monary artery. 
Up to the 12 '5 -mm. stage the lumen was narrow and inconspicuous. Now it is 
large and dilated, and the orifice into the pulmonary artery is placed upon the dorsal 
wall, not at the termination (text-figs. 11. and 12). 
The ventral wall is lined with a reticulated formation of muscle continuous with 
that of the ventricle. Thus the “ infundibulum” of the right ventral is formed. 
The so-called congenital stenosis of the pulmonary artery is due to failure in the 
dilatation of this part of the right ventricle. 
The development of this region of the heart may be summarised : — - 
1 . The pulmonary artery of the adult is developed from a portion of the truncus 
arteriosus, the lumen of the truncus arteriosus being divided into : 
(a) The pulmonary artery, from which arise the sixth pair of arches, very close 
together. 
(b) The ascending aorta. 
2. The pulmonary and the aortic valves , are formed at the distal extremity of 
the bulbus cordis. The infundibulum of the right ventricle is formed from a 
portion of the bulbus cordis, and is at first merely a narrow channel, but becomes 
distended. 
3. The increasing obliquity and the consequent relative elongation of the ventral 
wall causes the pulmonary valves to lie some distance above the right ventricle, 
while the aortic valves lie near the base of the left ventricle. 
The septum of the bulbus is continued downwards in a spiral and oblique 
direction to join the upper margin of the septum of the ventricle, in such a way that 
the dorsally placed vessel (the aorta) is in continuity with the left ventricle, while 
the infundibulum communicates with the right ventricle only. 
