THE THORAX 993 



IS directed dorsahvards, and migrates cephalicwards, whilst the arterial loop 

 is directed ventrahvards, and migrates caudahvards. 



Transformation of the Single Heart into the Doable Heart. — The double 

 function of the permanent heart is concerned in (1) the pulmonary circula- 

 tion, and (2) the systemic circulation. In order to prepare the organ for 

 the performance of this t^vofold function, the primitive single heart tindergoes 

 a series of changes, which result in the dix-ision of its auricular and ventricular 

 portions each into two halves, and of the aortic bulb into the ascending aorta 

 and trunk of the pulmonary artery. The permanent heart, therefore, is a 

 double organ, consisting of a right or pulmonary half, and a left or systemic 

 half. 



The transformation of the single auricle into two halves, right and left, is 

 of a compHcated nature. Prior to the commencement of the transformation 

 the single auricle presents a pocket-like dilatation or diverticulum on either 

 side of the median line. These two diverticula represent the right and left 

 auricular appendices. A groove then mcikes its appearance on the upper 

 and dorsal walls of the single auricle. This groove indicates the position 

 internally of a falciform fold, which grows downwards and forwards within 

 the auricular cavit>'. The lamina is called the septum primiim (interauricular 

 septum) or septum superius, and it eventually joins the upper part of the 

 septum intermedium, to be presently described. It may here be stated, 

 however, that the septum intermedium di\"ides the auricular canal into two 

 parts, right and left, which represent the permanent auriculo-ventricular 

 orifices. In the foregoing manner the primitive single auricle and the single 

 auricular canal are divided each into t\vo — right and left. The two auricles, 

 right and left, are for the most part separated by the septum primum, and 

 the two auriculo-ventricular orifices are separated by the septum intermedium. 

 The septum primum is incomplete, and the two auricles communicate with 

 each other by an opening, called the ostium ititerauriculare primum, which 

 is situated between the septum primum and septum intermedium. This 

 ostium, however, is soon closed by the fusion of the septum primum with the 

 septum intermedium. 



At the dorsal attachment of the septum primum a new opening, called the 

 foramen ovale or ostium interauriculare secundum, makes its appearance, 

 which establishes a communication between the two auricles. 



Another septum, called the septum secundum, is also developed. It grows 

 downwards from the roof of the right auricle, and hes on the right side of, 

 and parallel with, the septum primum. This secondary septum ends in a 

 semilunar or falciform border, which bounds the foramen ovale above, in 

 front, and below, and forms the annulus ovalis or limbus Vieussenii. The 

 portion of the septum primum within the free semilunar border of the septum 

 secundum projects slightly into the cavity of the left auricle, and is so dis- 

 posed as to allow blood to pass freely from the right auricle into the left, 

 but it guards against regurgitation. This portion of the septum primum is 

 consequently called the valve of the foramen ovale. 



About the period of birth the ventral margin of this valve becomes as a 

 rule adherent to the left side of the annulus ovalis, and the foramen ovale is 

 thereby closed. Therccifter the valve (a portion of .the septum primum) 

 forms the floor of the depression known as the fossa ovalis. 



In some cases the union of the valve with the left side of the annulus ovalis 

 is incomplete, and in these circumstances a patent foramen ovale persists, 

 giving rise to the condition known as morbus ccsruleus. 



Transformation of tiie Auricular Canal. — The auricular canal, which is the 

 narrow communication between the auricular and ventricular divisions of 

 the primirive cardiac tube, undergoes invagination into the ventricular 

 chamber. Internally it presents two thickenings of endocardium, called 

 the endocardial cushions, one of which is situated ventrally and the other 

 dorsally. As these cushions grow they come together and fuse, thus giving 

 rise to the septum intermedium. This septum divides the auricular canal 

 into two compartments, which represent the right and left auriculo-ven- 

 tricular orifices. The upper part of the septum intermedium, as stated, 

 joins the lower pait of the septum primum, a temporary opening, known 



63 



