DISCUSSION 357 



circulation during periods when ordinary respiration is suspended, and that the blood 

 so distributed acts in lieu of freshly oxygenated blood from the lungs. The blood from 

 the retia, however, does not, as has been seen, empty into the general arterial circula- 

 tion, but immediately into the venous system, whence it returns to the heart by way 

 of the posterior thoracic vein or neural sinuses from the thoracic rete, and by way of 

 the jugular vein from the basicranial rete. Further, the blood in the thoracic rete at 

 any rate is more venous than arterial. 



Turner (1872, p. 233) suggested that the rete serves for the retention of blood 

 on its course to certain delicate organs, such as the brain and spinal cord, thus 

 equalizing and distributing the blood stream to these organs and preventing their 

 engorgement. Such an apparatus is seen in the intracranial arteries of ruminants and 

 in the arteries of the human pia mater. However, the arteries which lead through the 

 rete do not serve the brain or spinal chord or, indeed, any delicate organs, but continue 

 somewhat diminished in size to the dorsal musculature, or to the sides of the face, and 

 the flow of blood is not impeded or distributed by the rete. It is certain, nevertheless, 

 that the rete has some fairly intimate relation with the nervous system, since it enwraps 

 the spinal chord at its anterior extremity and spreads out over the hinder surface of 

 the brain. 



Murie (1873, PP- 290-1) believed the retia to have a physiological function connected 

 with nutrition. " Their office is equivalent to modified blood glands in some way related 

 to pabulum or nutrient fluid. The Retia Mirabilia in Cetacea and other mammals are 

 not confined to the cerebro-spinal tract and neighbourhood of the respiratory apparatus 

 but principally follow lines where lymphatics and absorbents occur in the greatest 

 profusion. Moreover in Cetacea, Sirenia 1 , and Phocidae, where the Retia are most 

 manifest, the lymphatics are unusually abundant and of large size. I apprehend that 

 countless divisions, by coming into close contact with the lymphatic system, conduce 

 to an interchange or exudation of their contents." Murie's theory is based on the 

 similarity of position of the lymphatics and the tracts of vascular tissue similar to that 

 already described in the inguinal region. The theory may well apply to these plexuses, 

 but cannot hold for the thoracic and basicranial retia, where no lymphatic glands 

 were found in relation with the vascular masses. 



The retia mirabilia, or structures like them, are found in all mammals which are 

 capable of diving and of remaining under the surface for long periods — that is in seals, 

 porpoises, dolphins and whales. According to Owen the littoral and herbivorous 

 Sirenia, which have not the habit of remaining so long submerged, do not possess these 

 vascular networks. It seems reasonable, then, to connect the presence of these blood 

 reservoirs with the capacity for making long and frequent dives. 



It is well known that whales dive to considerable depths, but little or nothing is 



known of the actual depths they may reach. It is usual for a Blue or Fin Whale, when 



pursued, to make repeated dives of anything up to twenty minutes' duration. These 



long dives are known as "sounds". When the whale is progressing normally, surface 



1 According to Owen (1868, p. 547) the Sirenia do not possess thoracic or myelonal plexuses. 



