18 GUIDES FOR VERTEBRATE DISSECTION 



artery, which should be followed into the leg, the latter being 

 much the larger and forming the chief supply of the limb. 



Add these vessels and their branches to the sketch of the 

 arterial system. 



The liver abuts in front against a thin membranous partition, 

 the false diaphragm or septum transversum. Cut through this 

 and lay open a second part of the coslom, the pericardial cavity, 

 lined with a pigmented epithelium, the pericardium. In this 

 cavity lies the heart, free behind but attached dorsally and in 

 front. 



Make out in the heart a posterior conical portion, the ventricle, 

 and in front a pair of thinner walled auricles. Which of these 

 latter is the larger, the right or the left? Crossing the right auricle 

 and extending forward from the ventricle is an enlargement, the 

 bulbus cordis, from the anterior end of which arises a cylindrical 

 tube, the truncus arteriosus impar, which extends forward to the 

 anterior wall of the pericardial cavity, where it divides into two 

 trunci arteriosi. 



Tip the heart forward, tearing the membrane (dorsal meso- 

 cardium) holding it, and see, on its dorsal surface, a large triangular 

 sac, the sinus venosus, formed by the union of three vessels, the 

 postcava behind and a precava in front on either side. Follow 

 the postcava backwards to the liver region (p. 17) and then 

 follow out the right precava beyond the pericardium to its divi- 

 sion into three trunks, the anterior (external jugular vein) going 

 straight forward, the middle (vena anonyma), and the posterior 

 (subclavian vein). The anonymous vein soon divides into the 

 internal jugular and the scapularis veins, the subclavian into 

 the great cutaneous and the brachial veins. Trace the former 

 into the skin, the latter into the arm. 



Tip the heart forward again and see the two pulmonary veins, 

 one coming from each lung. Into which auricle do they empty? 



Sketch these blood-vessels in their respective sheets, inserting 

 the heart, in outline in both, showing respectively its arterial and 

 venous connections. 



Follow the truncus. Each one soon divides into three curved 

 vessels, the anterior the common carotid artery, the second the 

 radix aortae, and the third the pulmo-cutaneous artery. Note in 

 the carotid, soon after its separation from the rest, an enlargement, 

 the carotid gland, at which the vessel divides into external and 



