4 ANATOMY OF VERTEBRATES. 



blooded denizens of arctic and antarctic zones. The nature of 

 the external covering has much influence in this resistance, 

 whether it be the thick layer of subcutaneous fat in the whale- 

 tribe, the fur and hair of the quadruped, or the down and feathers 

 of the bird. Save in the case of mankind and the whalekind, 

 the warm-blooded Vertebrate may be distinguished at a glance 

 from the cold-blooded one by the non-conducting, heat-preserving, 

 nature of its clothing, which is ^ hair,' as a general rule in Mam- 

 mals, and ' feathers ' in Birds. 



There are, however, gradations of the heat-maintaining power 

 in the Hcematotherma. Some Mammals, e. g. the Alpine Marmot, 

 the Hamster, the Squirrel, the Dormice (3I?/oxus), the Porcupine, 

 the Virginian Opossum, at the approach of winter- cold, seek a 

 retreat, fall into a deep sleep, and lose from ] 0° to 20° Fahr. of 

 heat. In the Squirrel, e. g., the heat of the body has been found 

 to sink from 98° to 78°. Respiration is continued, though slowly, 

 in these winter-sleepers. The Hedgehog (^Erinaceus) and the 

 Bat ( Vespertilio, Linn.) fall into a dee2:)er and more lasting 

 torpor ; in which breathing is suspended, and a slow and languid 

 circulation is the sole sign of animation. In the Bat, the heart's 

 pulsations fall from 200 in a minute, as when in active wake- 

 fulness, to 30 in a minute, during torpidity ; the blood being 

 then in a dark venous state, and the temperature of the body 

 down to 40°. In this condition these Insectivora survive the 

 season during which their allotted food is unattainable. In the 

 tropics some allied species, e. g. the Tenrecs ( Centetes) fall into a 

 similar torpidity, without the excitement of a freezing cold, during 

 the season unfavourable to the presence of their food. 



The feeble and inactive young Hcematotherma use up less 

 oxygen than adults ; and, when exposed to cold, lose their heat, 

 and also their sensibility, differing in this latter respect from the 

 hybernators. The least touch to a spine of a torpid Hedgehog 

 rouses it to draw a deep sonorous inspiration : the merest shake 

 induces respiration in the torpid Bat. 



In all these instances of loss of power to preserve the average 

 mammalian temperature, the physiological conditions of the 

 species approximate more or less to those of the cold-blooded 

 animals ; and it is interesting to observe that the Avinter-sleeping 

 and torpid Mammals are those which most resemble reptiles in 

 their cerebral organisation : they are also of small size. Whether 

 the Edentata and Monotremata would become torpid, and so 

 accommodate themselves to other than their native climates is a 

 question well worthy of experimental determination. 



