i86 Retrogressive Processes. 



i.' 



phy of worn out, aged cells with replacement by new and func- 

 tionally capable cells, as the protective epithelial cells, glandular 

 cells or the blood corpuscles. Dependent largely upon inherited 

 capacity of length of life peculiar to the individual, sooner or later 

 a retardation in the cytogenic energy makes its appearance as a 

 characteristic of old age; this physiological loss of tissue-forming 

 power leading to shrinkage of the affected parts is termed senile 

 atrophy. This is particularly noticeable in the sexual glands. The 

 ovaries dwindle after the discharge of the ova generated in early 

 life ; no more follicles mature ; the connective tissue stroma and 

 tunica albuginea become dense and the organs assume a cicatricial 

 appearance. The testicular epithelium leaves off its spermatogenic 

 activity, becomes lower and smaller. In the same way in the lym- 

 phatic structures, the spleen, lymph nodes and bone marrow, the 

 ability for cellular proliferation is curtailed and hand in hand there 

 occurs reduction in the formation of blood corpuscles. Even the 

 skeletal system undergoes considerable atrophy in age. the bones be- 

 coming more porous and brittle and lose sensibly in weight ; a pro- 

 gressive thinning of the bone trabeculse is especially notable, probably 

 caused by increase in a special group of cells (giant cells or osteo- 

 clasts ; 6(TTiov, bone, and K\a(T(reiv, to break) which are concerned 

 in the absorption of calcareous matter. The ganglion cells of the 

 brain become reduced in size. At least in part senile atrophy is due 

 to a physical wearing out, as when certain structures become dilated 

 from mere loss of elastic resistive power (elastic fibres which have 

 lost their tone like a worn out rubber band). 



The following pathological conditions productive of atrophy are 

 to be considered. 



I. Lack of use of organs. Repeated physiological stimula- 

 tion is essential for- the preservation of all cells, keeping cellular 

 function active by constant use. Cessation of this tvpe of stimula- 

 tion and the resultant failure of function are productive of atrophy 

 (atrophy from inactivity). For instance, prolonged fixation of a 

 limb by skillful bandaging or the immobility from ankylosis of a 

 joint may be followed by atrophy of the muscles and bones of the 

 part. As the stimuli are conveyed by the nerve tracts it must be 

 evident that disturbances in the conductivity of the nerves or loss 

 of nervous impulse are especially apt to occasion atrophy (neuro- 

 pathic atrophy). This may be observed in palsies, as those of the 

 hind quarters (frequently seen in dogs from injuries to the spinal 

 cord), in which the musculature of the hind legs may be reduced a 



