DEVELOPMENT OF THE HEAET. 



1033 



The Atrio-ventricular 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 (Esophagus 



the atrio-ventricular canal becomes 

 compressed into a transverse cleft 

 which is bounded by a cranial 

 (upper in adult position) and a 

 caudal wall. On the middle part 

 of each of those walls an endocardial 

 thickening appears which is called Duct of Cuvier 



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. The 

 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 that position 

 throughout all the later stages of development ; consequently, it forms the dorsal part or base of 

 the fully developed heart. It has been stated also that, as development proceeds and the atrial 

 chamber expands, the right horn of the sinus venosus is absorbed into the right segment of the 

 dorsal portion of the expanding atrium. Whilst this process of absorption of the sinus venosus is 

 proceeding two other events are taking place : (1) The right and left margins of the atrium grow 



ventrally round the sides 



Truncus arteriosus 



Right horn of sinus venosus 



Secondary foramen ovale 



Left venous valve 



Right venous valve 

 Septum primum 

 Pericardial cavit; 

 Primary foramen ov 



Cushion of atrio- 

 ventricular canal 



Interventricular septum - 



Interventricular sulcus 



FIG. 829. SECTION OF THE HEART OF A HUMAN EMBRYO. 

 (Edinburgh. University collection.) 



6th aortic archesx.'. 



__ Septum primum 



Atrio-ventricular 

 orifice 



Bulbus cordis 



of the truncus arteriosus 

 and the adjacent part of the 

 bulbus cordis ; and (2) the 

 primitive atrium is divided 

 into the right and left per- 

 manent atria by the forma- 

 tion of two interatrial septa. 

 By the ventral growth 

 of the right and left margins 

 of the atrium the auricles 

 of the right and left per- 

 manent atria are formed, 

 and by the formation and 

 fusion of the two interatrial 

 septa the permanent inter- 

 atrial septum of the fully de- 

 veloped heart is established. 

 The septa by which the 

 atrial chamber is divided 

 are the septum primum and 

 the septum secundum ; so 

 named because the first ap- 

 pears and partly disappears 

 before the: second is formed. 

 Both the septum primum and the septum secundum grow from the dorsal towards the 



Central wall of the primitive atrium, the septum secundum to the right of the septum primum. 

 When the septum primum reaches the ventral wall of the atrium it fuses with the dorsal 



mds of the endocardial cushions of the atrio-ventricular canal. Until this fusion occurs an 



iperture, the primary foramen ovale, exists between the two atria and a number of perforations 

 formed in the more dorsal portion of the septum ; therefore the passage of blood from the 



nlerior vena cava through the right part of the atrium into the left part is never prevented. 



Right posterior , . __ ._ _- ^_- 



cardinal vein ] / ^ ^fca^^B Ventricle 



Right duct of Cuvier r' 



Left venous valve j 

 Right venous valve 



PIG. 830. MODEL SHOWING THE INTERIOR OF THE RIGHT ATRIUM OF A 

 HUMAN EMBRYO 5 '5 MM. LONG. (Edinburgh University collection. 

 Modelled by C. C. Wang.) 



