464 Journal of Agriculture. [8 August, 1907. 



whose muscles are rendered inactive by tlie poison curare will, if kept 

 alive by artificial respiration, gradually sink to within a few degrees of the 

 temperature of the room and become to all intents and purposes a poikilo- 

 thermal animal. 



Next in importance to the muscles may be placed the liver where 

 chemical transformations are continually occurring most or all of which 

 liberate heat. As has been stated the blood leaving the liver is the hottest 

 in the body. Next in importance to the liver, as heat producing tissues 

 we may place the heart, and the smooth muscles particularly those in the 

 digestive tract. 



The Distribution of Heat. — The heat produced in any muscle or 

 organ tends by ordinary physical conduction to spread to adjacent organs, 

 but the chief factor in heat distribution is the blood-stream which is 

 constantly perfusing each tissue and organ and passing thence tO' the heart. 



The Loss of Heat. — The chief loss of heat is from the skin, con- 

 cerning the structure of which a few words may be said. The skin or 

 integument can be divided for descriptive purposes into two parts. The 

 superficial part is epithelial and is composed of closely packed epithelial 

 cells which are constantly being renewed and pushed up from below and 

 as constantly being shed at the surface. This layer has neither blood 

 vessels nor nerves. Special outgrowths of this layer form hair, feather, 

 nails, claws and the bloodless parts of hoof and horn. The second or 

 deeper part of the skin is formed of connective tissue, is rich in blood 

 vessels and nerves and possesses special sensory nerve-endings. This 

 layer contains two sorts of glands with ducts that pierce the epithelial 

 layer and pour their secretion on the surface. The sweat glands secrete 

 a fluid sweat, consisting chiefly of water and containing traces of salts, 

 urea and fatty acids. The sehaceons glands secrete a semi-solid, olea- 

 ginous substance (sebum) which consists of lipoid and a little protein and 

 water. The sebum acts as a natural ointment keeping the skin and hair 

 soft, and preventing them from being injured by rain and moisture. Both 

 these glands are constantly secreting, though their activity is subject to 

 considerable variations in intensity. In the connective-tissue part of the 

 skin, or below it, we find a layer of fat. 



The skin has many functions to perform ; it protects the muscles and 

 other organs beneath it from mechanical injury and from rain, strong 

 sunshine, &c. ; it helps a little in getting rid of waste matter and excess 

 of water. The skin possesses sensory nerve-endings which are responsive 

 to heat and cold, to injury, and to contact with bodies or to movements of 

 hairs. Only under abnormal and rare conditions can it absorb fluid from 

 tvithout. But one of the most important functions is its blanketing action 

 m keeping the heat of the body from escaping too rapidly and bv allow- 

 ing changes to be made in the rate of heat escape. In the first place the 

 layer of fat already mentioned is a very efficient non-conductor of heat. 

 This layer is, in consequence, enormously developed in those animals which 

 live in the ocean, and particularly in cold latitudes, as, for instance, 

 porpoises, seals, whales, &c. It is also well developed in the pig. The 

 epithelial portion is likewise a feeble conductor especially when it is thick, 

 as it is in the elephant, &c. Then, as outgrowths from the skin, we find 

 in the majority of homoiothermal animals, hair, fur, wool or feathers which 

 act, not so much by their intrinsic feebly conducting powers, as from the 

 fact that they contain large volumes of air enclosed in myriads of tiny 

 spaces. Now air, thus divided, is a very poor conductor of heat and to 

 air is due the chief value of these natural coverings, as well as that of 



