48o MAMMALIA— PHYSIOLOGY 



several months). Primitive sheath cells on the early unmyelinated 

 nerves multiply and migrate to the advancing "myelin-emergent" 

 sprouts and myelin segments are added distally in an orderly man- 

 ner. Each segment is formed through the combined action of a 

 sheath cell and a myelin-emergent fiber. Myelin of a segment first 

 appears near a sheath cell nucleus and extends progressively in both 

 directions. Unmyelinated gaps between adjacent myelin segments 

 may be filled in later by the intercalation of new segments. End-to- 

 end anastomosis of segments sometimes occurs with complete ob- 

 literation of the intervening node of Ranvier. A portion of one 

 segment may be appropriated by another segment, accompanied 

 by the establishment of a new node. (Courtesy of C. C. Speidel.) 



Hibernation and Aestivation in Animals. — Certain amphibia, 

 reptiles and mammals are able to go into a sleep when the tempera- 

 ture falls. Hibernation is marked by a reduction of all body proc- 

 esses to the minimum. For example, the hibernating dormouse 

 periodically takes a dozen respirations, then ceases breathing for 

 several minutes. Normally, when awake, it respires at the rate of 

 80 times a minute. A summer resting period, known as aestivation^ 

 is characteristic of the mudfish {Protopterus) and of certain amphibia, 

 reptilia, and a few mammals. 



Organs That Man Can Lose. — A leg, an arm, an eye, his tonsils, 

 gonads, spleen, appendix, gall-bladder, part of his lungs, part of 

 his brain, and as much as twelve feet of his intestine may be re- 

 moved without serious results*. Entire lobes of the lungs have been 

 removed, with beneficial result?. 



Statistics of Vitality. — Muscles of the human heart are alive 2 

 hours after normal heart beat and respiration have ceased. The 

 body muscles are alive 5-6 hours thereafter. Muscles of a rabbit 

 will live Zyi hours after the death of the animal. Those of a sheep 

 survive io>^, those of a dog 11^ and those of a cat I2>^, those of a 

 frog 24-30 hours. 



Susceptibility of Mammals to Poison. — The porcupine or 

 " hedgehog " takes with apparent enjoyment a dose of cantharides 

 that will kill several persons under excruciating pains. In rabbits 

 and dogs, morphine causes vomiting and then light sleep, but more 

 depression in the rabbit. The rabbit can take more morphine than 

 can a man fifty times the animal's weight. Morphine first causes 

 wild excitation in the cat and other Felidae and then depression of 



