METABOLISM 239 



these various processes of heat-production and of heat-loss. No such 

 action of the heat- and cold-spots has been actually demonstrated, 

 however, so that this interesting matter must remain only a fair pre- 

 sumption until its truth has been actually proved. What evidence there 

 is is in its favor. 



About the heat-center or -centers there is more definite information. 

 It is apparently located in the caudate nucleus of the corpus striatum. 

 Puncture of this little area of the interior of the brain even with a fine 

 needle causes in animals a marked rise of body- temperature. Ott 

 found another and more likely thermogenic center in the tuber cinereum 

 of the optic thalmus, and this has been corroborated. This latter region 

 of the brain is closely connected with the vaso-motor apparatus, for 

 puncture of its anterior part causes a marked fall of blood-pressure by 

 vaso-dilatation. The increase of the heat-production which then follows 

 is probably brought about by radiation of the nerve-influence into the 

 adjacent motor paths. This would cause an innervation of the muscles 

 and a rise in their trophic heat-production. Evidence of a probable 

 thermolytic center is more vague. 



Whatever the exact neural mechanism of thermotaxis, it does not 

 reach its full development in the human animal until about the tenth 

 year. We see evidence of this very often in the rapid rise of temperature 

 in young children from causes so slight that they would not at all influence 

 the adult temperature. 



Other Forms of Energy-expense in which katabolism manifests itself 

 are those other than growth and repair, secretion, and heat, which have 

 now been discussed. They are mainly muscular and nervous force. 

 Muscular power is discussed in a chapter by itself and nervous force 

 with the functions of the nervous system. These need only mention in 

 this place for the sake of systematic completeness. 



There remains under the head of katobolism to briefly describe the 

 excretory processes as such and the harmful substances to which these 

 important processes give rise. This is comprised under the general 

 term excretion, the latter phase of external nutrition. 



Excretion. The excretion of substances of no further use to the organ- 

 ism is an indispensable part of nutrition. They are mostly of such a na- 

 ture that they would poison the organism and promptly cause its destruc- 

 tion did they remain within it. The five familiar sorts of food-material 

 (protein, fat, carbohydrate, salts, and water) partake more or less in 

 the structure of the tissue-molecules, are sooner or later katabolized, and 

 their elements at least sent out of the body either by the kidneys, the lungs, 

 the rectum (including the liver's contribution), or the skin. Minute 

 quantities, relatively, are also excreted by the reproductive and nasal 

 organs, and as the dermal appendages, hair and nails, but these are 

 negligible otherwise than in this mention. The most important of the 

 actual end-products of katobolism excreted by man are urea, carbon 

 dioxide, and water, these together representing the ultimate waste of the 

 three basal "proximate principles," protein, fats, and carbohydrates 



