34 o DISCOVERY REPORTS 



spinous veins from the neural venous sinuses pass to the posterior thoracic between all 

 the dorsal vertebrae up to the 7th cervical. Anterior to that, up to the 1st cervical, the 

 intraspinous veins, of which there are six in the neck region, appear to take origin from 

 the network of venous capillaries that fills the cervical portion of the neural canal. 

 From the 8th dorsal vertebra to the 7th cervical vertebra there are thus eight veins 

 connecting the neural venous sinuses and the posterior thoracic vein. They become 

 increasingly stout forwards and the first four or five of the series are very wide indeed 

 and have thin walls. Those between the 7th cervical and 1st dorsal and the 1st dorsal 

 and 2nd dorsal join and enter the posterior thoracic by a common trunk between the 

 1st and 2nd rib. The intraspinous veins in the neck have the arrangement shown in 

 Fig. 4 B. The vein emerging from between the 1st and 2nd cervical vertebrae passes 

 through the large foramen in the transverse process of the 2nd cervical vertebra and is 

 then joined by its successor to form a single vein running under the basal processes of 

 the vertebrae in front of the 7th cervical. A stem from the 3rd and 4th, 4th and 5th 

 and 5th and 6th meets it, and after receiving another from the 6th and 7th, it enters 

 the posterior thoracic vein ventral to the head of the first rib in common with the first 

 two thoracic intraspinous vessels (Fig. 4 B). Behind the 8th dorsal vertebra the thoracic 

 intraspinous veins drain into a small independent trunk on each side of the vertebral 

 column. These will be described when the intercostal blood supply is dealt with. In the 

 abdomen the intraspinous veins connect the neural venous sinuses with the postcava and 

 still further caudally with the caudal veins. These posterior connections will be referred 

 to again, but it is desired to mention them here in order to point out that the neural 

 venous sinuses must serve the purpose of reservoirs. They do not form part of the blood 

 drainage back to the heart from any set of organs or from any region of the body. 

 Anteriorly they are cut off from direct communication with the brain by vascular 

 plexiform masses filling the neural canal. They are in direct and wide communication 

 with the great veins of the body, and the blood would appear to flow in either direction 

 in the connecting intraspinous vessels — that is upward in the abdominal region and 

 downward in the thoracic region, or vice versa, without impeding the flow towards 

 the heart. Thus in the Fin Whale they provide direct connection between the precaval 

 and postcaval venous systems alternative to that provided by the heart. 



Veins exactly complementary to the arteries ascending to the dorsal musculature join 

 the intraspinous veins of the posterior thoracic system shortly after these leave the 

 neural canal. Their course follows that of the arteries and they, likewise, increase in 

 calibre forwards. Behind the 6th rib they drain into the segmental veins shortly to be 

 described (Fig. 4 B). 



The vascular masses of the rete mirabile are found surrounding the posterior thoracic 

 artery as far back as the 4th dorsal vertebra and the roots of all the six arteries which 

 ascend to the dorsal musculature from it. The masses of the rete lie between the trans- 

 verse processes of the vertebrae and the heads of the ribs concerned. In the same way the 

 rete surrounds the posterior thoracic vein and all the thoracic intraspinous veins except 

 the last two. It also surrounds the intraspinous veins in the cervical region. It extends 



