REGENERATION OF ORGANS AND TISSUES. 369 



the so-called trophic nerves, influence the metabolism or nutrition of the tissues 

 ( 342, c). 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 ( 145), and by the metabolism 

 during the contraction of bloodless muscles. Increased respiration and apnoea are 

 not followed by increased oxidation (Pjiuger) ( 127, 8). 



[Gaskell has raised the question as to the existence of katabolic and anabolic nerves con- 

 trolling respectively the analytic and synthetic metabolism of the tissues.] 



244. REGENERATION. 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 fins, 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. In amphibians and reptiles the regeneration of organs and tissues, as 

 a whole, takes place after the type of the embryonic development, and the same is true as re- 

 gards the histological processes which occur in the regenerated tail and other parts of the body 

 of the earth-worm. 



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, including the plasma, the colourless and coloured corpuscles. ( 7 

 and 41.) 



2. The epidermal appendages ( 283) and the epithelium of the mucous mem- 

 branes are reproduced by a proliferation of the cells of the deeper layers of the 

 epithelium, with simultaneous division of their nuclei. Epithelial cells are repro- 

 duced 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. Eegeneration, therefore, 

 either takes place from below or from the margins of the wound in the epithelial 

 covering ; leucocytes also wander into the part, while the deepest layer of cells 

 forms large multi-nucleated cells, which reproduce by division polygonal flat 

 nucleated cells. [In the process of division of the cells, the nucleus plays an 

 important part, and in so doing it shows the usual karyokinetic figures ( 431).] 

 The nails grow from the root forwards ; those of the fingers in four to five months, 

 and that of the great toe in about twelve months, although growth is slower in the 

 case of fracture of the bones. The matrix is co-extensive with the lunule, and if it 

 be destroyed the nail is not reproduced ( 284). The eyelashes are changed in 

 100 to 150 days, the other hairs of the body somewhat more slowly. If the papilla 

 of the hair follicle be destroyed, the hair is not reproduced. Cutting the hair 

 favours its growth, but hair which has been cut does not grow longer than uncut 

 hair. After hair has grown to a certain length, it falls out. The hair never grows 

 at its apex. The epithelial cells of mucous membranes and secretory glands seem 

 to undergo a regular series of changes and renewal. , The presence of secretory cells 

 in the milk ( 231) and in the sebaceous secretion ( 285) proves this; the sperma- 

 tozoa are replaced by the action of spermatoblasts. In catarrhal conditions of 

 mucous membranes, there is a great increase in the formation and excretion of new 

 epithelium, while many cells are but -indifferently formed and constitute mucous 

 corpuscles. The crystalline lens, which is just modified epithelium, is reorganised 

 like epithelium ; its matrix is the anterior wall of its capsule, with the single 

 layer of cells covering it. If the lens be removed and this layer of cells retained, 



2 A 



