HEART, CIRCULATION, AND BLOOD 167 



as well as the posterior vena cava in the abdominal cavity, and may thus 

 be said to form an auxiliary connection between the two veins, enabling 

 the blood from the brain to return to the heart through either of them. 

 Spinal veins have also been described in other aquatic animals, i.e. 

 Sirenians (sea-cows, etc.) and Pinniped Carnivores (seals, etc.) and, for 

 the sake of completeness we might add that they have also been found in 

 some terrestrial mammals such as sloths, bats and cats. 



We have just discussed four special characteristics of the Cetacean 

 vascular system : the arterial retia in the thorax and neck ; the venous retia 

 in the abdominal cavity; the distensions of the hepatic veins and of the 

 inferior vena cava; and the presence of large veins in the vertebral canal. 

 All these characteristics are apparently found in all other aquatic mammals 

 also, though those which do not constantly live in the water - e.g. seals 

 and sea-lions - lack some of them. Clearly, therefore, the retia are an 

 adaptation to an aquatic mode of existence and especially to diving, though 

 it is not easy to say pi'ecisely in what way. We need not, therefore, be 

 surprised to learn that many experts have delved into this problem, and 

 that the literature on it is extensive. Still, even the most recent investiga- 

 tions by Harrison and Tomlinson (1956) have failed to provide a complete 

 answer. 



We shall not go into the details of all the hypotheses put forward in the 

 course of the last hundred years, but concentrate on the known facts, i.e. 

 that the retia are capable of absorbing and releasing vast quantities of 

 blood. This is particularly true of the arterial retia, and though the 

 structure of the venous retia has not yet been studied sufficiently, we can 

 nev^ertheless state that they, too, can store blood for some time, as can the 

 special hepatic veins and the inferior vena cava. The spinal veins, more- 

 over, may, as we have seen, enable the blood from the brain to return 

 both to the thorax and to the abdomen so that, if the flow to the thorax 

 is impeded for some reason, congestion in the delicate central nervous 

 system is avoided. Furthermore, blood from the abdomen can, under 

 certain conditions, be diverted to the thorax through the spinal canal 

 instead of the inferior vena cava which normally carries the blood. 



All these modifications are obviously associated with possible pressure 

 differences between the thorax and the abdomen, and possibly between 

 both and the brain. Unfortunately we know practically nothing about the 

 nature of svich pressure diflferences, nor have we evolved adequate 

 experimental techniques for determining them. All our arguments must 

 therefore be based on anatomical inferences and on what little we know 

 about pressure differences in terrestrial mammals. Hence, the reader is 

 advised to treat what follows with caution. 



Let us imagine that, at a given moment, the pressure in the thorax is 



