HEART, CIRCULATION, AND BLOOD 



l6l 



Figure gs. View into the thorax of a Little Piked Whale and a Common Dolphin to show the 

 position of the vascular networks, the ribs, and some afferent vessels. {Bouvier, i88g.) 



of ever-bigger veins which carry the deoxygenated blood back to the heart. 

 This, at least, is the normal situation, but sometimes arteries and veins 

 branch out into a network of small vessels which have neither the very 

 thin walls nor the other characteristics of capillaries. There are very small 

 arteries or veins, with arterial networks interposed between two arteries 

 and venous networks between two veins. Although such networks occur 

 in various terrestrial mammals as well (including cows) they were once 

 thought so unusual that they were given their present name of retia 

 mirabilia, meaning 'wonder-networks'. 



If we can get hold of a very recently killed porpoise and remove its 

 heart and lungs, we shall lay bare a thick, spongy, vascular mass on either 

 side of the vertebral column and between the ribs. We can trace this mass 

 going right up into the cervical and down into the lumbar region, though 

 it clearly thins out in the posterior part of the body (Fig. 92). Even so it 

 continues right into the chevron canals of the caudal vertebrae and ends 

 just in front of the flukes. On closer examination the networks can be seen 

 to consist of twisted blood vessels, and this becomes quite obvious at their 

 intercostal termini, where the convolutions can be seen with the naked eye. 

 Though Tyson (1680) and Monro (1787) were greatly struck by these 

 networks while dissecting porpoises, the first accurate and detailed 

 description was given by the French anatomist Breschet, who published 



