670 WHALES xxvi 



during diving, and special cartilaginous rings and muscles in the 

 bronchioles. The epiglottis is extended as a tube inserted into the 

 posterior narial cavity so that an uninterrupted air passage is provided 

 from the blow-hole to the lungs. The very elastic and extensible lungs 

 can thus be quickly filled with large volumes of air. In spite of the 

 enlarged tracheo-bronchial tree the respiratory surface is small, but 

 there are special arrangements of valves and venous plexuses to ensure 

 economical distribution of the air and blood. Some whales can remain 

 submerged for half an hour and reach depths of 500 metres or more. 

 There is relatively little experimental evidence about the means by 

 which they obtain oxygen and resist compression. The whole arrange- 

 ment ensures the taking down of a maximum of air. There is rapid 

 ventilation while on the surface, followed by slower heart-beat and 

 presumably reduced tissue respiration while below, so that the whale 

 can take down enough oxygen to last throughout its dives. However, 

 the heart rate slows only to one-half in the only cetacean fully 

 investigated (Tursiops) and there is no evidence about retention in 

 venous sinuses or other means of reducing circulation such as are 

 found in seals (p. 692). The respiratory centre in the medulla has a 

 lesser sensitivity to C0 2 than in land animals. 



Besides the air in the lungs there may also be some provision for 

 storage of extra oxygen in the large blood-volume of the retia mira- 

 bilia, networks of blood-vessels, which abound throughout the body, 

 especially in the thorax. However, the function of the retia mirabilia 

 is probablv connected with the accommodation of the animal to varying 

 hydrostatic pressures. They expand and contract to occupy the space 

 in the thorax as the air in the lung is diminished or increased as the 

 animal rises or descends while swimming. There is much myoglobin 

 in the muscles. The brain is supplied with blood entirely from menin- 

 geal arteries, which draw on the thoracic retia. The basilar artery and 

 intracranial carotid close early. 



No doubt the metabolism is also arranged to allow accumulation of 

 a high oxygen debt, but the special metabolic peculiarities that allow 

 for this are not known. It is perhaps not necessary for whales to have 

 a special defence against caisson sickness if they are using oxygen 

 reserves. There is no continuous addition to the nitrogen dissolved in 

 the blood such as would lead to the formation of the bubbles that 

 occurs when miners or divers rise suddenly after breathing air at great 

 depths. The sudden expiration on surfacing produces a cloud of 

 foetid vapour, the blow. This is generally supposed to be due to 

 condensation, but it occurs even if the air is hot. 



