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A MANUAL OF ANATOMY 



at first separated from each other, but subsequently they come together and 

 fuse, giving rise to a single longitudinal tube, composed of splanchnic meso- 

 derm, which represents the primitive heart, this tube being situated mesially 

 along the ventral aspect of the pharyngeal portion of the fore-gut. It thus 

 occupies the region which subsequently becomes the neck. 



The lumen of each of the two primitive tubes is lined with mesothelium, 

 which becomes part of the cardiac endothelium. The mesenchyme, which 

 lies outside the lining mesothelium, gives rise to the following cardiac tissues: 

 (i) the myocardium, or cardiac muscular tissue; (2) the epicardium, or visceral 

 layer of the serous portion of the pericardium; and (3) the connective-tissue 

 element of the endocardium. ♦ 



After the fusion of the two tubes, which represent the cardiac rudiments, 

 the primitive heart has the form of a longitudinal tube. At the caudal end 

 of the tube is the sinus venosus. 



Mesocardial Folds. — At this stage the primitive heart is connected dorsally 

 with the ventral wall of the pharyngeal portion of the fore-gut, and ventrally 

 with the somatopleuric body-wall, by means of splanchnic mesoderm, dis- 

 posed in two layers. These bilaminar mesodermic folds form the dorsal 

 mesocardium and ventral mesocardium, which are of the nature of mesenteries. 

 They soon disappear, the ventral folds entirely and the dorsal folds to a 

 large extent. 



After the disappearance of these folds the cardiac tube is for the most 

 part free, its ends being more or less fixed. 



Fig. 410. — Scheme showing Four Successive Stages in the Develop- 

 ment OF THE Heart (L. Testut's ' Anatomie Humaine '). 



I. Arterial Bulb. 



2. Ventricle. 



4. Veins opening into the Sinus Venosus, 



3. Auricle. 



Further Development of the Heart. — The cardiac tube soon presents two 

 constrictions, which indicate its drl'ision into three parts — namely, auricular 

 (atrial), ventricular, and bulbar. The auricular portion pertains to the 

 venous end of the doubly-bent cardiac tube; and the ventricular and bulbar 

 portions belong to the arterial end, the bulbar portion being differentiated 

 from the primitive heart as the aortic bulb or hulhus arteriosus (bulbus cordis). 

 The ventricular portion is therefore intermediate in position between the 

 auricular and bulbar parts. The auricular division is closely associated 

 with the sinus venosus, which will be described presently, and which, though 

 not a division of the primitive cardiac tube, becomes ultimately merged into 

 its auricular portion. The constriction between the auricular and ventricular 

 portions gives rise internally to a passage called the auricular, or atrial canal ; 

 and the constriction between the ventricular and bulbar portions gives rise 

 internally to a passage known as the /return Halleri or strait of Haller. 



The primitive sinuous cardiac tube Ihus becomes differentiated into three 

 chambers — namely: (i) the primitive auricle, (2) the primitive ventricle, and 

 (3) the aortic bulb, or bulbus arteriosus. 



Whilst these differentiations are in progress, the tube undergoes two bends, 

 and now resembles a capital S, laid somewhat thus — (J . The \enous loop 



