DISCUSSION 3 59 



atmospheres absolute pressure, or a depth of about 2000 ft., and it is impossible that 

 a whale could ever reach such a depth. 



The chief effects of the dive which must be considered, therefore, are those which 

 arise from its duration rather than from its depth. The main result of a long period of 

 submergence will be that the tissues will acquire a considerable oxygen debt. The chief 

 difficulty, in fact, with which the whale will be confronted during the dive will be that 

 of obtaining, or of taking down, sufficient oxygen for a submergence of twenty minutes 

 to half an hour. At rest a man uses 220 c.c. of oxygen per minute and under conditions 

 of moderate exercise about 500 c.c. Assuming that the total amount of air in the lungs 

 at any moment is 7 litres (1-5 litres of oxygen) there would be enough oxygen in the 

 lungs at any moment to last for three minutes of moderate exercise or 25 seconds of 

 strenuous exercise. Thus if the whale's metabolism is in any way comparable with 

 that of a man he will need to take down, when diving for half an hour, a store of oxygen 

 at least twenty times greater than the amount the lungs could contain at the moment of 

 submergence. It is known, however, that provided the brain and nervous system are 

 kept supplied with oxygen, a man may incur a debt in his tissues equal to ten times 

 the amount of oxygen in the lungs — that is 15 litres, or enough for 30 minutes' moderate 

 exercise and about 4 minutes' strenuous exercise. 



All attempts to ascribe a function to the vascular networks described in this paper 

 must necessarily be purely speculative. It is fairly evident, however, that all theories 

 which attribute to the retia the function of counteracting the effects of hydrostatic 

 pressure on the distribution of blood in the body and on the circulation generally are 

 without value. Hydrostatic pressure probably makes no difference to the circulation 

 at all, since it is equally distributed within and without the body. The networks are far 

 more likely to be concerned in some way with gaseous exchange, especially as they are 

 present in seals and porpoises, which do not dive to great depths but can remain 

 submerged for long periods. In this connection the situation of the networks near the 

 respiratory centre and on the course of the main blood vessels and also around the 

 brain and nerve chord is extremely suggestive. It has already been mentioned that the 

 masses of the rete are abundantly charged with fat. Oxygen is very soluble in fats. 

 In a table of solubilities of oxygen in a selection of animal and vegetable oils Lewko- 

 witsch (1904, chap, vii) gives the oxygen absorption of whale oil, as shown by Livache's 

 method with lead, to be considerably higher than that of any other oil. The figures are 

 as follows : 



After 3 days' exposure at N.T.P. 100 parts of cod liver oil absorb 6-382 2 



whale oil „ 8-266 



sperm oil „ 1-629 

 Japan fish oil „ 8-194 

 The oxygen absorption of whale oil is thus nearly half that of ordinary blood. These 

 figures are only given to show that the fatty tissues of the whale's body, and perhaps 

 the fatty masses of the retia mirabilia in particular, are capable of absorbing com- 

 paratively large quantities of oxygen, but it is not suggested that they have any other 



