CHAPTER VII 



PHYSIOLOGICAL REGENERATION. REGENERATION AND 

 GROWTH. DOUBLE STRUCTURES 



DURING the normal life of an individual many of the tissues of the 

 body are being continuously renewed, or replaced at definite periods. 

 The replacement of a part may go on by a process of continuous 

 growth, such as takes place in the skin and nails of man, or the re- 

 placement may be abrupt, as when the feathers of a bird are moulted. 

 It is the latter kind of process that is generally spoken of as physio- 

 logical regeneration. In the same animal, however, certain organs 

 may be continually worn away, and as slowly replaced, and other 

 organs replaced only at regular intervals. 



Bizozzero has made the following classification of the tissues of 

 man, on the basis of their power of physiological regeneration. 

 (i) Tissues made up of cells that multiply throughout life, as the 

 parenchyma cells of those glands that form secretions of a definite 

 morphological nature ; the tissues of the testes, marrow ; lymph 

 glands, ovaries ; the epithelium of certain tubular glands of the 

 digestive tract and of the uterus ; and the wax glands. (2) Tis- 

 sues that increase in the number of their cells till birth, and only 

 for a short time afterward, as the parenchyma of glands with fluid 

 secretions, the tissues of the liver, kidney, pancreas, thyroid, con- 

 nective tissue, and cartilage. (3) Tissues in which multiplication of 

 cells takes place only at an early embryonic stage, as striated muscles 

 and nerve tissues. In these there is no physiological regeneration. 



There are many familiar cases of periodic loss of parts of the body. 

 The hair of some mammals is shed in winter and in summer. Birds 

 renew their feathers, as a rule, once a year. Snakes shed their skin 

 from time to time. The antlers of deer are thrown off each year, 

 and new ones formed accompanied by an increase in size and branch- 

 ing of the antlers. In other cases similar changes may be associated 

 with certain stages in the life of the animal. The milk-teeth of the 

 mammals are lost at definite periods, and new teeth acquired. 1 The 

 larval exoskeleton of insects is thrown off at intervals, and after 



1 In rodents, however, the incisors continue to grow throughout the life of the animal. 



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