LEFT AURICLE. 509 



S€ems continued ; but immediately within the orifice of the 

 artery, it is formed into three semicircular folds, each of which 

 adheres to the surface of the artery by its circumference, while 

 the edge constituting its diameter is loose. In the middle of 

 this loose edge is a small firm tubercle, called Corpnsculum 

 Arantii,* which adds to the strength of the valve. Each of 

 these valves, by its connexion with the artery, forms a sac or 

 pocket, the orifice of which opens forward towards the course 

 of the artery, and the bottom of it presents towards the ventricle. 

 Blood will, therefore, pass from the ventricle in the artery, and 

 along it without filling these sacs ; and, on the contrary, in this 

 course, will compress them and keep them empty. If it moves 

 in the artery towards the heart, it will necessarily fill these 

 sacs, and press the semicircular portions, from the sides of the 

 artery, against each other ; by this means a partition or septum, 

 consisting of three portions, will be formed between the 

 artery and the heart, which will always exist when the 

 artery compresses, (or acts upon,) its contents. It is demon- 

 strable, by injecting wax into the artery, in a retrogade 

 direction, that these valves do not form a flat septum, but one 

 which is convex towards the heart, and concave towards the 

 artery ; and that this convexity is composed of three distinct 

 parts, each of which is convex. At the place where these 

 valves are fixed, the artery bulges out when extended by a 

 retrogade injection. The enlargements thus produced are 

 called the Siyiuses of Valsalva, after the anatomist who first 

 described them. The valves are called Semilunar — and, 

 although they are formed by a very thin membrane, they are 

 very strong. 



The Left Auricle is situated on the left side of the basis of 

 the heart. It originates from the junction of the four pulmo- 

 nary veins ; two of which come from each side of the thorax, 

 and appear to form a large part of it. It is nearly of a cubic 

 form: but has also an angular portion, which constitutes the 

 proper auricle, that proceeds from the upper and left part of 



* After Arantius, a professor at Bologna, who first described il. 



43* 



