214 



THE VASCULAR SYSTEM. 



and so forms a closed lymph-sac. The pericardium is more or less 

 pigmented^ and contains both vessels and nerves ; it may contain 

 fat-cells]. 



B. The Heart. 



[The heart is a hollow muscular organ, composed of : ( i ) two 

 auricles, forming- the wider anterior portion ; (2),the ventricle, placed 

 behind the auricles ; (3) the sinus venosus, situated clorsally ; and (4) 

 the truncus arteriosus, lying ventrally. Examined while still living, 

 the auricles are seen to be much darker in colour than the ventricle, 

 due to the blood being seen through their thinner walls ; between 

 the auricles and the ventricle is a distinct groove, the auriculo- 

 ventricular groove. The whole organ is more or less pigmented, 

 especially the ventricle {E. temjjoraria has much less pigment).] 



a. The Sinus venosus 



[Simis venarum cavarum), (Fig. 

 133 SV.) is a thin-walled sac 

 lying on the dorsal surface 

 of the heart, and very slightly 

 to the right side (Hoffmann). 

 It is somewhat triangular in 

 form, receiving an anterior 

 caval vein at each anterior 

 angle {V.), and the posterior 

 caval vein (/C.) at the poste- 

 rior angle. The ventral wall 

 has a transverse opening {SF'.), 

 by which it communicates with 

 the right auricle ; the opening 



Tlie heart, seen from above, and dissected to show the pOSSCSSCS twO ValvCS, an ante- 

 rior and a posterior, and is 

 placed close to the auricular 

 septmn. 



b. The auricles [are not 

 always separated, as the sep- 

 tum which usually brings about 

 the separation is in some cases 

 incomplete, and in rare in- 

 stances appears to be absent (Hoffmann)] ; according to Eeker, the 

 auricular septum is sometimes placed so far to the left side that it 



r-SV. 



PV 



opening from the Sinus venosus to the right auricle ; 

 after Howes. 



C". 



IC. 



LA. 



P'. 



PV. 



RA. 



S'. 



SV. 



s v. 



Right carotid arch. 



Posterior caval vein. 



Left auricle. 



Right pulmo-outaneous arch. 



Pulmonary vein. 



Right auricle. 



Right systemic arch. 



Sinus venosus. 



Opening of Sinus venosus into the 



auricle. 

 Right anterior caval vein. 



•ight 



