186 CIRCULATORY SYSTEM. 



the descending vena cava, and at its posterior inferior angle by 

 the ascending vena cava. The structure of the auricle, between 

 these two points, seems to be only a continuation of that of the 

 veins. These veins enter with a direction slightly forwards, so 

 that their columns of blood are not directly opposed to each 

 other. In front of this continuation of the two veins, the auri- 

 cle is dilated into a pouch called its sinus; the upper extremity 

 of the latter, just in front of the descending cava, is elongated 

 into a process with indented edges, and has some general re- 

 semblance to the ear of an animal, from which it is probable 

 that the term Auricle has been derived. 



The exterior surface of this cavity is smooth and uniform, but 

 its internal surface is varied at several places. About midway 

 between the orifices of the two cavse is found a transverse pro- 

 minence, the Tuberculum Loweri, which is occasioned by the 

 continuous structure of the veins meeting at an obtuse angle. 

 This cavity is separated from the left auricle only by a thin sep- 

 tum, which is common to the two auricles. On the septum, 

 below its middle, is a superficial circular depression, the Fossa 

 Ovalis; it is more distinct above than below, and varies much 

 in its dimensions. It is surrounded by an elevated margin, com- 

 posed of muscular fibres, and called its Annulus,or the Isthmus 

 of Vieussens. The septum of the auricles is thinner at the fos- 

 sa ovalis than elsewhere, and is frequently perforated by one 

 or more foramina. I have, in several instances, seen a hole 

 there, large enough to transmit the finger. On such occasions, 

 from the valvular arrangement of the opening, it is probable that 

 the blood of the two auricles is still kept distinct. The fossa 

 ovalis always presents this foramen in the foetal state. 



Just below the fossa ovalis is found the Eustachian valve, 

 consisting in a duplication of the lining membrane of the auri- 

 cle. It is crescentic, but varies much in its dimensions and 

 shape. Its left extremity commences at the left, inferior mar- 

 gin of the annulus ovalis; it then extends itself along the front 

 of the orifice of the ascending cava, where the latter is con- 

 nected with the auricle, but never to an extent sufficient to ar- 

 rest the circulation there. Sometimes it is reticulated at its 

 margin, and half an inch wide; on other occasions, it is scarcely 

 developed. Its loose edge looks upwards, and to the right side. 



