REGENERATION OF ORGANS AND TISSUES. 493 



agency of vaso-motor nerves, whereby it influences the amount of blood 

 supplied, and also affects the blood-pressure. But in addition to this, 

 and quite independently of the blood-vessels, it is probable that certain 

 special nerves the so-called trophic, nerves, influence the metabolism or 

 nutrition of the tissues (see Trophic Nerves). That nerves do 

 influence directly the transformation of matter within the tissues is 

 shown by the secretion of saliva resulting from the stimulation of 

 certain nerves, after cessation of the circulation (p. 287), and by 

 the metabolism during the contraction of bloodless muscles. Increased 

 respiration and apnoea are not followed by increased oxidation (Pfluger) 

 (compare p. 259). 



244, Regeneration of Organs and Tissues. 



The extent to which lost parts are replaced varies greatly in different organs. 

 Amongst the lower animals, the parts of organs are replaced to a far greater 

 extent than amongst warm-blooded animals. When a hydra is divided into two 

 parts, each part forms a new individual nay, if the body of the animal be 

 divided into several parts in a particular way, then each part gives rise to a 

 new individual (Spallanzani). The Planarians also show a great capability 

 of reproducing lost parts (Duges). Spiders and crabs can reproduce lost 

 feelers, limbs, and claws ; snails, part of the head, feelers, and eyes, provided the 

 central nervous system is not injured. Many fishes reproduce h'ns, even the tail- 

 fin. Salamanders and lizards can produce an entire tail, including bones, muscles, 

 and even the posterior part of the spinal cord; while the triton reproduces an 

 amputated limb, the lower jaw, and the eye. This reproduction necessitates that 

 a small stump be left, while total extirpation of the parts prevents reproduction 

 (Philippeaux). 



In amphibians and reptiles, the regeneration of organs and tissues as a whole, 

 takes place after the type of the embryonic development (Fraisse, Gotte), and the 

 same is true as regards the histological processes which occur in the regenerated 

 tail and other parts of the body of the earth-worm (Biilow). 



The extent to which regeneration can take place in mammals and in 

 man is very slight, and even in these cases, it is chiefly confined to 

 young individuals. A true regeneration occurs in 



1. The blood (compare 7 and 41), including the plasma, the 

 colourless and coloured corpuscles. 



2. The epidermal appendages (see Skin, vol. ), and the epithelium of 

 the mucous membranes are reproduced by a proliferation of the cells of 

 the deeper layers of the epithelium, with simultaneous division of their 

 nuclei. Epithelial cells are reproduced as long as the matrix on which 

 they rest and the lowest layer of cells are intact. When these are 

 destroyed cell-regeneration from below ceases, and the cells at the 

 margins are concerned in filling up the deficiency. Regeneration, 

 therefore, either takes place from below or from the margins of the 

 wound in the epithelial covering ; leucocytes also wander into the part, 



