THE THORAX 995 



Sinus VenosUs. — The sinus venosus (saccus reuniens) is at first an appendage 

 of the auricular division of the primitive heart, below and behind which it 

 lies. Subsequently, however, it becomes merged into the right auricle, and 

 forms the posterior part of that chamber of the adult heart. Into it the 

 following veins discharge their blood: (i) The t^vo ducts of Cuvier, from the 

 body of the embrj-o; (2) the two xatelline veins from the yolk-sac; cind (3) the 

 two umbihcal or allantoic veins from the chorion. It is elongated transversely, 

 and its lateral portions, where the ducts of Cuvier open into it, are spoken of 

 as the lateral cornua. 



The communication bet\veen the sinus venosus and the auricular chamber 

 soon becomes small, and it shifts towards the right side, to a point between 

 the right lateral comu and the right half of the auricular chamber. Ulti- 

 mately, when the auricular chamber has become divided into two auricles, 

 the sinus venosus opens entirely into the right auricle. The narrow opening 

 is guarded by two endocardial folds, right and left, which axe known as the 

 venous valves. These valves are continuous superiorly with a ridge called 

 the septum spurium, on the roof of the atrium. The left venous valve dis- 

 appears, but the right venous valve persists, and gives rise to (i) the 

 Eustachian valve, and (2) the Thebesian valve. 



The venous valves, right and left, are situated at the opening of the sinus 

 venosus into the right auricular cavity, and they are formed by the margins 

 of the opening which project sUghtly into the right auricular cavity. The 

 upper end of the right venous valve is continuous with a fold, called the 

 septum spurimn, situated on the roof of the right auricular ca\dty on the 

 right side of the septum primum. The right venous waive gives rise to the 

 Eustachian and Thebesian valves. The left venous valve is continuous 

 with the septum secundum, which bounds the foramen ovale above, in front, 

 and below. The semilunar border of the septum secundum, together with 

 the left venous valve, forms the annulus ovaUs. 



The right lateral cornu, which receives the right duct of Cuvier, undergoes 

 enlargement, and, along with the central portion of the sinus venosus, it 

 forms the atrium of the right auricle. The right lateral comu carries with it 

 the right duct of Cuvier, or right superior vena cava. The left laterad comu 

 which receives the left duct of Cuvier, is not merged into the right auricle, 

 but remains connected with it, forming the coronary sinus, which is the 

 persistent and pervious cardiac end of the left duct of Cuvier, or left superior 

 vena cava. Whilst the right auricle receives the sinus venosus, the left 

 auricle receives the trunk formed by the union of the pulmonary veins, but 

 these latter vessels are, at this stage, of little importance, the pulmonary 

 circulation l>»ng merely of a nutritive nature. 



The common orifice of the pulmonary veins becomes enlarged and forms 

 the atrium of the left auricle; and subsequently the veins open into the 

 atrium by four orifices, t%vo right and two left. 



Formation of the Valves ol the Heart. — The muscular tissue of the ven- 

 tricular walls of the heart is at first loosely arranged in the form of trabeculae, 

 which are disposed in a reticular manner. This muscular reticulum, including 

 its meshes, is lined with endocardium. The superficial trabecuLB soon come 

 together, and form a compact muscular wall, but towards the interior the 

 reticular arrangement is retained. Muscular bundles, of different kinds, are 

 therefore formed — e.g.,columncs carnece, tnusculi papillares, and moderator bands. 



In the portions of the ventricular walls around the auriculo-ventricular 

 orifices the muscular tissue is replaced by fibrous tissue. This fibrous tissue 

 gives rise to thin, somewhat triangular lamellae, and these become the segments 

 of the aurieulo-ventricular valves. 



The chordcB tendinis represent musculzir trabeculae, which, at their valvular 

 ends, have been replaced by fibrous tissue, whilst their ventriculiir ends retain 

 their muscular character, and form the tnusculi papillares. 



The aortic and pulmonary semilunar valves are developed in connection 

 with the ventricular end of the aortic bulb, prior to its division into the 

 ascending aorta and trunk of the pulmonary artery. They appear as four 

 fibrous cushions on the inner wall of the ventricular end of the aortic bulb, 

 and are anterior, posterior, and right and left lateral respectively, ^^"hen the 



