WARMS L O ODED ANIMALS. 7 1 3 
feels cold, is not necessarily brought about by a conscious effort of the 
will ; it is to a great extent reflex, and shows itself in the more energetic 
performance of work, or, if no work be done, the reflex may become so 
imperative as to give rise to involuntary movement, shivering, which is 
only of value to the organism as a source of greatly increased heat 
production. There is little doubt but that a normal man, w T ho feels cold 
and is free to follow the dictates of his sensations, will be more active, 
and will produce more carbon dioxide and absorb more oxygen than he 
would in warm surroundings. The man who suppresses increased 
muscular action when he feels cold, is abnormal. It follows, therefore, 
that man is no exception to the general rule that warm-blooded animals 
in cold surroundings increase, in warm surroundings diminish, their 
respiratory exchange and production of heat. 
It has already been shown that a rise or fall in external temperature 
determines in the same direction a variation of the respiratory exchange 
of cold-blooded animals. What, then, is the cause of the totally opposite 
result observed in warm-blooded animals ? To this question only an 
incomplete answer can be given. The difference is due to the nervous and 
muscular mechanisms which maintain the fairly constant temperature 
observed in the warm-blooded animals. For if, as Sanders-Ezn 1 and 
Pfluger 2 have shown, the exposure to cold be excessive, and the animal's 
temperature falls to 26°, then also there is a fall in the intake of oxygen 
and the output of carbon dioxide ; on the other hand, if by means of 
warm baths the internal temperature of the animal is raised above the 
normal, then there is an increase above the average respiratory exchange. 
In fact, a warm-blooded animal responds to a rise or fall in the tempera- 
ture of its surroundings with a decrease or increase of its metabolism, 
only as long as its internal temperature remains near the normal point. 
Moreover, Pfluger has proved the connection between the normal 
response to a change of external temperature and the nervo-muscular 
system, for he shows that a mammal paralysed with curari 3 or with its 
spinal cord cut in the lower cervical region, absorbs more oxygen and 
discharges more carbon dioxide in warm than in cold surroundings ; it 
resembles in this respect a cold-blooded animal. A similar cold-blooded 
condition can be produced in mammals, as Eumpf, 4 Eichet, 5 and 
Pembrey 6 have observed, by exposing the anaesthetised animal to 
changes of temperature. 
The objection that these experiments are associated with markedly 
abnormal conditions, and therefore cannot indicate the true condition of 
normal animals, is met by the fact that it is possible to trace the 
gradual development of the means whereby an animal increases or 
decreases its metabolism and maintains a fairly constant heat of its 
body, notwithstanding wide variations in the temperature of its 
surroundings. This has been shown by Pembrey 7 in a series of 
comparative experiments upon full-grown and newly-born animals. In 
the full-grown mouse the response to a change of external temperature 
1 Ber. d. k. sticks. Gcscllsch. d. Wissensch. Math.-phys. KL, Leipzig, 1867, S. 58. 
- Arch./, d. ges. Physiol., Bonn, 1878, Bel. xviii. S. 247. 
3 See also Zuntz, Arch. f. d. ges. Physiol.., Bonn, 1876, Bd. xii. S. 522. 
4 Ibid., 1884, Bd. xxxiii. S. 538. 
5 Uompt. rend. Acad. d. sc, Paris, 1889, tome cix. p. 190. 
6 "Proc. Physiol. Soc," Journ. Physiol., Cambridge and London, 1894-95, vol. xvii. 
7 Journ. Physiol., Cambridge and London, 1894, vol. xv. p. 401; 1895, vol. xviii. 
p. 363. 
