THE ARTERIES AND VEINS 527 



The aortic septum is a composite structure formed partly by a septum growing between the 

 fourth and sixth pairs of aortic arches, and partly by swellings growing in the interior of the conus 

 and truncus arteriosus. When fully formed it extends spirally along the truncus and conus, 

 and enters the right half of the common ventricular cavity, where it joins the right side of the 

 free edge of the interventricular septum. The septum is arranged in such a way that the blood 

 from the left ventricle passes no longer through the right ventricle but along its own channel 

 (the aorta) through the conus and truncus to the first four pairs of aortic arches. The blood 

 from the right ventricle passes through the pulmonary division of the conus and truncus arterio- 

 sus, anterior and to the left of the aorta, into the sixth arches. Further differentiation brings 

 about the external separation of the aorta from the pulmonary artery, but their common cover- 

 ing of epicardium persists as such in the adult. The lower end of the aortic septum persists 

 in the adult as the septum memhranaceum ventriculorum and the crista supraventricularis, the 

 relations of which to the septum musculare are well shown in fig. 428. During the formation 

 of the aortic septum four endocardial swellings appear at the distal part of the interior of the 

 conus. These are arranged as smaller and larger opposite pairs; the smaller and larger swelhngs. 

 therefore, alternating around the lumen. The larger pair of swelhngs assists (by partial 

 blending) in the formation of the aortic septum. When the septum is complete, half of each 

 of the larger swelhngs is contained in the aorta and half of each in the pulmonary artery. One 

 of the smaller swelhngs remains in the aorta and one in the pulmonary artery, so that there are 

 now three sweUings in each vessel. Each of the six swellings becomes undermined to form 

 a semilunar valve of the adult. 



The atrio -ventricular valves. — The interior of the ventricular cavity, which is at first 

 smooth, becomes undermined in an irregular way, to form a system of myocardial trabeculse. 

 The hps of the transversely directed atrial canal become thickened into prominent anterior and 

 posterior endocardial cushions; these project into the ventricular cavity and become involved 

 in its myocardial trabecular system. The atrial canal, which has now moved to the right, be- 

 comes divided sagittally, into right and left venous ostia, by the septum primum. The inter- 

 ventricular septum joins the ventricular side of the posterior endocardial cushion. The anterior 

 and posterior endocardial cushions, where they blend with one another and with the septum 

 primum on the medial side of each venous ostium, form an atrio-ventricular valve-cusp on 

 either side, viz., the anterior cusp of the mitral in the left ostium, and the medial cusp of the 

 tricuspid in the right. The posterior cusp of the mitral and the anterior and posterior of the 

 tricuspid are formed later, partly, by lateral tubercles developing in either ostium, and partly 

 by undermining of the ostia from the ventricular side. The atrial musculature extends into the 

 atrio-ventricular valves and, until a late stage, is continuous with the trabecular system of the 

 ventricles. Gradually, however, this connection between atrial and ventricular musculature 

 is lost, leaving only the chordee tendinese connecting the papillary muscles with the valves 

 Muscle is found at the basal region of the valve-cusps in the adult, and occasionally persists in 

 the chordae tendineae. 



The connection between the atrial and ventricular musculature is not confined to that 

 occurring by means of the valves and trabecular system. The original myocardial connection 

 between the atrial and ventricular portions of the heart remains complete until the embryo 

 has reached the length of about 11 mm. From that time on the epicardium begins to blend 

 with the fibrous annuh of the venous ostia. Meanwhile the atrial musculature rapidly loses its 

 connection with that of the ventricles until they are connected in one place only, i. e., the site 

 of the atrio-ventricular bundle. 



The pericardial cavity is the original cephalic end of the intraembryonic coelom. The 

 somatic mesoderm of the pericardial region forms the adult pericardium. The splanchnic 

 mesoderm persists only in the part which furnishes the myo-epicardium. The ventral and dorsal 

 mesocardia, both of which are formed by the splanchnic mesoderm, are, in the main, transitory. 

 The early disappearance of the ventral mesocardium unites the right and left sides of the peri- 

 cardial coelom ventral to the heart. The dorsal mesocardium persists at the arterial and venous 

 ends of the heart only. The loss of the dorsal mesocardium between the latter points gives 

 rise to the sinus transversus pericardii of the adult. 



During development, the heart and pericardium migrate from a point opposite the cephahc 

 end of the pharynx to one opposite the caudal end of the oesophagus; in fact, from the neck well 

 into the thorax. In the adult, instead of being at the cephahc end of the coelom, the heart 

 and pericardium are contained between the right and left layers of the ventral mesentery of the 

 oesophagus; the pericardial pleura of the adult. 



The cranio-caudal migration is evidenced in the adult by the course of the recurrent and of 

 the cardiac nerves, and also by the apparent migration of the vessels derived from some of 

 the dorsal segmental arteries. 



B. THE ARTERIES AND VEINS 



The arteries [arterise], proportionately to their size, have much thicker walls 

 than the veins. After death they retain their natural form, but are contracted 

 and contain usually a small amount of pale clot. In a very general way the 

 thickness of wall is proportional to calibre. Some arteries, however, are con- 

 stantly thicker or thinner than could be predicted from size alone. 



The larger arteries usuallj^ take a direct course and branch dichotomously. In descriptive 

 anatomy if dichotomous branches are of nearly equal size it is common for each to take another 

 name; if one branch preponderates in size, it is apt to retain the name of the parent trunk 

 while the smaller is regarded as a collateral branch [vas coUaterale] . There are numerous 



