SCHULTE, SEI WHALE. 467 



The azygos system is rudimentary. It is described as absent in Phoccena by v. Baer. 1 

 The azygos major in this foetus is absent; no vessel arches over the root of either lung, and the 

 absence of this vein on the right side may well be associated with the presence and large size of 

 the tracheal bronchus. Three lines of drainage for the intercostal veins are however present 

 on each side, in addition to the intraspinous trunks, with which the intercostals communicate 

 through the intervertebral foramina. These are the internal mammary and lateral internal 

 mammary of the subclavian, and the superior intercostal of the posterior thoracic vein. On 

 the right side of the bodies of the thoracic vertebrae, dorsal in position to the ganglionic cord, 

 there is a small zigzag vessel, which rostrad is continued into the superior intercostal vein. 

 Laterad it receives the intercostals of the right side, mesad those of the left, which cross the 

 vertebral centra at their middle to reach it. In this way a portion of the return flow from the 

 left intercostal spaces below the fifth reaches the right side. The upper left spaces drain into the 

 superior intercostal of the left side. In the small longitudinal vessel of the right side must be 

 seen the representative of the hemi-azygos major, but in a reduced condition, for the physio- 

 logically important drainage seems to be into the vertebral canal. 



Through the intraspinous vessels a communication is established between the precaval 

 and postcaval veins, for the latter has large spinal taps, especially in the subhepatic segment. 

 These intraspinous vessels are placed ventral to the nerve roots, against the bodies of the ver- 

 tebrae, in the space between the theca medullse spinalis and the periosteum and posterior common 

 ligament of the vertebrae; beyond these points they are continued as smaller vessels, rostrad 

 communicating through the foramen magnum with the occipital sinuses. Through the inter- 

 vertebral foramina they communicate with the segmental veins. On the centra they are con- 

 nected by cross branches, which receive tributaries from the centres of ossification. In spite 

 of these numerous cross-anastomoses these intraspinous trunks are not plexiform, but are well 

 formed veins of sinuous course, in size exceeding that of the postcava distad of the renal vessels, 

 from which it is evident that physiologically they form an important element of the venous 

 system. From their connections it is evident that they may serve as an equilibrating anastomo- 

 sis between the precaval and postcaval systems. With the former they communicate by means 

 of the large tap between the third and fourth thoracic vertebra into the posterior thoracic vein. 

 In the lumbar region they communicate with the postcava by a large communication with its 

 subhepatic segment, and by a series of smaller anastomoses between the lumbar veins and the 

 supracardinal cross-anastomoses between the postcavse on the dorsum of the aorta. It would 

 seem probable that the great development of this system is associated with the suspension of 

 thoracic respiration for considerable periods, and the less favorable condition of the venous 

 return depending upon the cessation of thoracic aspiration. Assuming this relative impediment, 

 the system may operate to prevent venous congestion of the cerebrum by affording an outlet 

 from the precaval to the postcaval drainage area, in which latter engorgement might be better 

 tolerated by virtue of the great distensibility of the abdominal veins, which is usual in mammals. 

 There is thus a means of maintaining equilibrium between the precaval and postcaval systems, 

 in accordance to the demands of which the current in the vertebral plexus may be reversed and 

 drain at need into either cava. Of such collateral anastomoses between the caval districts there 

 are in general among vertebrates three longitudinal paths, the spinal, the lateral abdominal, and 

 the azygos, of which the first two are characteristic of reptiles, the last of mammals. 



1 Baer, C. E. von. Uber die Gefassysteme des Braunfisches. Nova acta physico-medica, T. 70, 1835. 



