466 ANATOMY OF VERTEBRATES. 



double, there being one for each side of the body, and both right 

 and left ( vcnic cardinales ' extend forward, in close contact, along 

 the haemal canal in the tail, then through the abdomen, and in both 

 regions immediately beneath the aorta and vertebral bodies, to near 

 the first vertebra, where each trunk diverges and descends to join 

 its corresponding ( vena jugularis,' fig. 308, v, forming the short 

 ' precaval ' vein, 1 ib. v, which empties itself in the great auricular 

 sinus between the aponeurotic layers of the pericardial and abdo- 

 minal septum. In the Lamprey the vena cardinalis is single along 

 the tail, but it bifurcates on entering the abdomen into two veins, 

 each of which is six times as large as the aorta. The left cardinal 

 vein is larger than the right in the Myxinoids : but the symme- 

 trical disposition of the vertebral venous system is more disturbed 

 in many Osseous Fishes, at the expense of the right side ; the right 

 cardinal vein, after some transverse connecting channels with the 

 left, finally terminating or losing itself therein anteriorly : part of 

 the right jugular vein, also, in this case enters the left or common 

 cardinal vein. 2 In the Tunny the two c vena? jugulares ' unite and 

 form a common trunk, which enters the auricular sinus indepen- 

 dently. 3 The Shad, the Pike, and the Lucioperca are examples 

 where the jugular veins are symmetrical, and terminate distinctly 

 in the precaval veins. With regard to the vertebro-venal system 

 of the trunk, not all the segmental branches terminate in the 

 ( vena cardinalis ; ' the neural twigs form with the myelonal veins 

 a trunk which runs parallel with the cardinal veins, but above 

 the vertebral bodies in the neural canal. This trunk, the ( vena 

 neuralis,' communicates by short lateral and vertical canals with 

 the venre cardinales, and in the region of the abdomen these short 

 aiiastomising veins perforate the substance of the kidneys, and 

 receive the l renal veins ' before terminating in the abdominal 



o 



cardinal veins. The neural vein gradually exhausts itself by 

 these descending branches, and does not extend to or terminate 

 anteriorly in the precaval trunk. Jacobson, observing that the 

 abdominal anastomotic branches of the neural vein, in transferring 

 its contents to the cardinal veins, perforated the kidneys, thought 



1 Ductus Cuvieri, Rathke; quervenenstcimme, Miiller. The precaval veins are the 

 homologues of the two ' superior cavse ' in Reptiles and Birds, which receive the so- 

 called 'azygos' veins or reduced homologues of the ' vense cardinales' of Fishes: in 

 the higher Mammals and in Man they are concentrated into a single ' superior vena 

 cava,' receiving the 'venae cardinales ' by a common trunk, thence called 'azygos ' in 

 Anthropotomy. The anatomical student is usually introduced to the cardinal veins, 

 as represented by their single homologue in the human subject, where their normal 

 symmetrical character becomes masked by an extreme modification, and where the 

 name ' azygos ' is applicable only to so exceptional a condition. 



2 xxi. p. 38. a Ib. p. 37. 



