CONTRACTILE TISSUES 457 



have very effective means of increasing loss of heat from the surface, including 

 that of the mouth and respiratory passages. This may be done by dilatation of 

 the blood vessels of the skin, mucous membrane, etc., but more effectively by 

 evaporation of water, as pointed out on page 227 above. Hence we see the value 

 of the sweat glands. Increased evaporation of water is also caused by increased 

 rate of breathing, which has a direct cooling action on the mucous membrane 

 of the respiratory passages. Conversely, loss of heat may be decreased by vascular 

 constriction in the skin.' 



It was pointed out by Fredericq (1882) that the cold to be struggled against 

 comes from the outside and acts on sensory nerves in the skin ; whereas increased 

 heat almost invariably arises in the organism itself and acts by raising the 

 temperature of the blood, in that the centres of the sweat nerves and the vaso- 

 dilator nerves are supposed to be excited by a rise of temperature. Naturally, 

 the effect of external cold is also to cool, in a limited degree, the blood leaving the 

 skin, but, since one of the means adopted to counteract cold is constriction of the 

 blood vessels in the skin, thei'e must be comparatively little cooling of the blood. 

 On the other hand, the external temperature very rarely rises above that of the 

 warm-blooded animal, so that stimulation of cutaneous nerves will be secondary 

 in this case, although not entirely excluded. Put in other words, the struggle 

 against cold is preventive and obstructs loss of heat ; that against heat is rather 

 curative and increases the loss of excess heat produced, rarely being able to diminish 

 production to an effective extent. As regards this last point, it may be said to 

 depend on the condition of the animal. If the surrounding temperature is /airly 

 warm, the animal will be quiet and thus unable to diminish muscular contraction 

 to a further extent when the temperature rises ; whereas, if the temperature is low, 

 the animal is active and able to become quiet when the temperature rises. 



Observations on the output of carbon dioxide form a very convenient means 

 of estimating heat production and have been made much use of for this purpose. 

 If we take an animal and measure its respiratory exchange when the temperature 

 is at 15 in the room, the animal is active and we obtain a certain value; raise 

 the surrounding temperature to about 30", the animal lies quiet and probably goes 

 to sleep ; there is diminished production of heat shown by decrease of oxygen 

 absorption and carbon dioxide output ; again, lower the surrounding temperature 

 to about 0, great muscular activity, with shivering, sets in and considerable 

 increase of carbon dioxide output takes place. 



These experiments can be done conveniently on a mouse with the apparatus described by 

 Haldane (1892) and modified by Pembrey (1894) for use with small animals. 



Since the means adopted for regulation of temperature involve the bringing 

 into play of so many and various kinds of efferent nerves, muscular, secretory, 

 vasomotor, and so on, it is plain that a co-ordinating centre is a necessity. 

 Experiments by Aronsohn and Sachs (1885) showed that puncture of the median 

 side of the corpus striatum in the rabbit caused considerable rise in temperature. 

 Further important experiments were made by Barbour (1912), who showed that 

 application of heat or cold to the anterior end of the corpus striatum, in the region 

 of the caudate nucleus, by means of a cylindrical metal tube through which water 

 was circulated, caused definite changes in the body temperature. Any temperature 

 below 33, in the conditions of the experiments, acts as a cold stimulus and 

 produces a rise in rectal temperature, together with shivering and vaso-constriction 

 in the skin. Cold acts, then, as an exciting agent, like puncture, electrical 

 stimulation, or the toxic substances of fever. Heat, on the other hand, that is, a 

 stimulation temperature of 42, in the conditions of the experiments, produces a 

 f all in rectal temperature, muscular relaxation and dilatation of skin vessels 

 (Fig. 140). It is to be presumed, also, that nerve impulses from the endings in 

 the skin, sensitive to cold, are also in relation with the centre. No doubt, under 

 normal conditions, the centre would be still more sensitive than in Barbour's 

 experiments and would react to much smaller temperature changes in the blood. 

 Barbour and Prince (1914) have shown that local heating of the centre causes 

 diminution in the evolution of carbon dioxide, in the intake of oxygen and in 

 the respiratory volume. Cooling the centre has opposite effects. The production 



