Archive image from page 1066 of Cunningham's Text-book of anatomy (1914). Cunningham's Text-book of anatomy cunninghamstextb00cunn Year: 1914 ( DEVELOPMENT OF THE HEAET. 1033 The Atrioventricular Canal.—In the early stages the atrio-ventricular canal opens through the dorsal wall of the ventricular chamber towards the left side, but, as the ventricle increases in size, the atrio-ventricular opening moves to the right till it occupies the middle part of the dorsal wall of the ventricle. Whilst the change in position is occurring Trachea (Esophagus the atrio-ventricular canal becomes Junction o
Archive image from page 1066 of Cunningham's Text-book of anatomy (1914). Cunningham's Text-book of anatomy cunninghamstextb00cunn Year: 1914 ( DEVELOPMENT OF THE HEAET. 1033 The Atrioventricular Canal.—In the early stages the atrio-ventricular canal opens through the dorsal wall of the ventricular chamber towards the left side, but, as the ventricle increases in size, the atrio-ventricular opening moves to the right till it occupies the middle part of the dorsal wall of the ventricle. Whilst the change in position is occurring Trachea (Esophagus the atrio-ventricular canal becomes Junction of right anterior and ! ! Left posterior j. u,x .Licj. ucwjxica posterior cardinal veins \ , cardinal vein compressed into a transverse cleft which is bounded by a cephalic (upper in adult position) and a caudal wall. On the middle part of each of these walls an endocardial thickening appears which is called an endocardial cushion. Each cushion is a flattened eminence, and when the two eminences meet and fuse the atrio-ventricular canal is divided into right and left portions, of which the right por- tion forms a passage from the right part of the still incompletely divided atrium into the right por- tion of the incompletely divided ventricle, and the left portion in a similar manner forms a channel of communication between the left part of the atrial chamber and the left part of the ventricle. These two parts of the primitively single atrio-ventricular canal become the right and left atrio-ventricular apertures of the adult heart, and the margins of the apertures take part in the formation of the cusps of the atrio-ventricular valves. The Division of the Atrium.—It has already been pointed out that as the tubular .heart bends on its long axis the atrial chamber forms part of the dorsal limb of the loop. It lies, therefore, dorsal to the truncus arteriosus and the bulbus cordis, and retains this position throughout all the later stages of development; consequently, it forms
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