516 URO-GENITAL SYSTEM. 



bulbus arteriosus). The curvature of the heart gradually 

 increases in such a manner that the ventricle, which at first 

 was placed above, turns downwards and forwards, and the 

 auricle upwards and backwards. Simultaneously with the 

 establishment of the placental circulation, a median partition, 

 or septum, originates at the apex of the ventricle, whose 

 extension finally divides the single ventricular cavity into two 

 cavities, called the right and left ventricle. A partition is 

 also formed in the large sinus of the aorta (bulbus arteriosus), 

 the latter having assumed a spiral form ; this partition divides 

 the sinus into two ducts which are twisted on their axes ; 

 one of these ducts communicates with the right ventricle 

 (the origin of the pulmonary artery) and the other with the 

 left ventricle, which latter becomes the origin of the aorta. 



The auricular cavity is also gradually divided into a right 

 find a left auricle, by a septum which originates at the auriculo- 

 -ventricular region. Yet during the remainder of the foetal 

 existence, this incomplete partition or septum contains an 

 opening (foramen Botalis sen ovale) which allows of a com- 

 munication between the two auricles. The relations of this 

 inter-auricular opening, with the mouths of the venaB cava3 

 in the right auricle, form a characteristic feature of the placen- 

 tary circulation. The mouth, or opening, of the inferior vena 

 cava is provided with the Eustachian valve ; this valve is 

 largely developed at this period, and is so arranged that the 

 blood which comes from the inferior vena cava can only go 

 through the postero-inferior portion of the right auricle, and 

 is directed towards the inter-auricular opening; by this 

 means the blood is diverted through the foramen ovale into 

 the left auricle, and thence into the left ventricle, etc. (see 

 farther on). On the other hand, the blood which comes 

 from the superior vena cava, there being no such valve here, 

 passes from the right auricle (which it fills, just as in adult 

 age) by the auriculo-ventricular orifice directly into the right 

 ventricle, etc. (see also farther on). 



c. Arteries. We have spoken of two branches that 

 originate from the anterior extremity of the cardiac tube ; 

 these soon turn towards the back and form what is called the 

 first pair of aortic arches (see p. 511). Soon afterwards two 

 or three other aortic arches are successively developed, and 

 are behind this first aortic arch ; these also unite in the median 

 trunk of the descending portion of the aorta (Fig. 150); yet 

 the continuance of these arches is only very transitory, and 

 most of them are soon obliterated, some of their branches 



