592 



COMPAEATIVE ANATOMY. 



§ 439. 



The embryonic venous system of the Mammalia is completely 

 similar to that of the lower Vertebrata. Two jugular veins (Fig. 334) 

 receive the cardinal veins, and the common trunks on either side 

 pass into a venous sinus, which is connected with the auricle, and, 

 later on, forms a part of the right one. Two distinct venous trunks 

 then open into this auricle, each of which is continued into an 

 anterior and larger, and a posterior and smaller, trunk. When the 

 anterior extremities are developed, the subclavian veins (s) fall into 

 the anterior ones (Fig, 337, A), and the two venous trunks tkus 

 formed are distinguished as the superior venae cavje. 



When the system of the inferior venae cavse is developed, the area 

 of the cardinal veins is diminished, for part of the blood whicb was col- 

 lected by the cardinal 

 veins is now carried 

 to the inferior venae 

 cavae. The cardinal 

 veins also undergo de- 

 generation, owing to 

 some of their roots 

 passing into the new 

 longitudinal trunks, 

 which, as in the 

 Reptilia, represent the 

 vertebral veins, and 

 are continued into the 

 end of the cardinal 

 veins, which open into 

 the ductus Cuvieri. 

 Owing to the decrease 

 in the size of their 

 area, these vertebral 

 veins (Fig, 337,^ J5,i)) 

 appear to be branches 

 of the trunks derived 

 from the ductus Cuvieri and the jugular veins, or superior venae cavee. 

 These are found in the Monotremata, Marsupialia, many Eodentia, 

 and Insectivora. In others, part of the area of the left superior cava 

 (B) is handed over to the right one (cs), owing to the development 

 of a transverse anastomosis ; the left superior vena cava is now 

 atrophied (Eodentia, Euminantia, Solidungula), When this arrange- 

 ment is complete, the greater part of the trunk of this vein disap- 

 pears, and the only part that remains is the terminal portion, which 

 primitively formed the left ductus Cuvieri, and which is placed 

 between the left ventricle and auricle (C, cor); the cardiac veins open 

 into it, and it forms the sinus of the coronary vein of the heart. In 

 Man, even, a semilunar fold separates this sinus from the true 



Fig. 337. Diagvam of the primitive paired veins in 

 Mammals. A The vertebral have taken the place of 

 part of the cardinal veins, which are indicated by 

 dotted lines. B The left jugular has its lower portion 

 atrophied, and its area is united with that of the right 

 jugular by a transverse trunk. C The left jugular vein 

 has completely disappeared, with the exception of a 

 rudiment on the heart, j Jugular. s Subclavian. 

 cs Vena cava superior, c Cardinal vein, v Vertebral 

 vein, cor Coronary vein, az Vena azygos. 



