ARCTIC METABOLIC ECONOMY 349 



insulation available to the hairless surfaces of swine (L. Irving, 1956), 

 for example, or of aquatic animals lies in the vascular system of in- 

 sulation (L. Irving and Hart, 1956), for the insulation of fat and 

 tissue, while of some significance for heat conservation, is invariable 

 and is of no value in regulating dissipation of heat. 



In the physiological insulation of the body, the vascular system can 

 have its greatest effectiveness at the freezing temperature, for the 

 tissue cannot become much cooler without freezing, and it provides 

 one of the means for regulated variation of the superficial insulation 

 on the extremities, unprotected by thick fur and feathers. 



Adaptation of Tissue Function to Low Temperatures 



In order that the extremities may be utilized in the economy of 

 animal heat their tissues must keep in normal operation such functions 

 as sensitivity at temperatures too cold for the tissues of the homoio- 

 thermous body to survive. An example of the adjustment of warm- 

 blooded tissues to functioning while cold was provided in the demon- 

 stration that the metatarsal portion of the peroneal nerve of herring 

 gulls {Lar-us argentatus) continued to show excitation and conduction 

 at 8° C. The central part of the nerve from within the warm body 

 failed at a higher temperature. These gulls had been kept out of doors 

 in winter at Boston. When they were kept in a warm room and with 

 warm water to swim in, the distal parts of the peroneal nerves failed 

 at about the same temperature as the central parts. Evidently the 

 lowest temperature at which these peripheral nerves will function 

 is modifiable by environmental temperature. It is, therefore, a clear- 

 cut adaptation which can be anatomically located in the terminal fiber 

 from centrally located nerve cells and can be defined in terms of 

 electrical measurements (Chatfield, Lyman, and L. Irving, 1953). 



The homologous nerves of domestic hens kept with the gulls showed 

 no modifiability of their peripheral nerves by cold, nor were the legs 

 and feet of the hens cold, as were those of the gulls. It is interesting to 

 consider that these domesticated fowl, derived from tropical ancestry 

 appear to retain, along with their high critical temperature (table 

 18), legs and feet scarely adapted to cold climates. 



The properties of some substances of warm-blooded tissues, such as 

 the high-melting-point fats common in the mammalian body, are 

 critically affected by cold. Fats from within the body of caribou melt 

 near 50° C. At body temperature, which is about 10° cooler, the fats 

 must be sufficiently fluid for normal functions. But at 0° C. these 

 fats are brittle hard. Since the marrow fat from the distal parts 

 of the caribou's leg bones is found to melt at 10° C. it seems that 

 this distinction of peripheral from body fats would allow for the 



