138 THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



are extraordinarily sensitive to some pathological changes, they are equally 

 resistant to others. It is unsafe at present to hazard even a tentative explana- 

 tion. Why this should he remains one of the great problems of pathology. 



The small amount of work on the pathology of mitochondria in the nervous 

 system is not due to a lack of interest, but rather to a misunderstanding and exag- 

 geration of the difficulties involved. Experience has shown that the older osmic- 

 acid-containing fixatives are almost useless for the study of the central nervous 

 system on accovuit of the large numlier of medullated and non-medullated fibers 

 offering insurmountable barriers to their penetration. It is not generally appre- 

 ciated that this difficulty may be in large measure overcome by the application of 

 Regaud's mixture, which consists of formalin and potassium bichromate. Ex- 

 cellent results may also be obtained by mordanting tissues in bichromate which 

 have been previously injected in the ordinary way with formalin. 



It is important to note also that the mitochondria in the nervous system do 

 not imdergo autolytic changes immediately after death as they do in gland-cells. 

 It is by no means necessary that the tissue be fixed while the body is still warm; 

 6 or 8 hours after death is often soon enough. 



While we are still ignorant of the normal appearance of mitochondria in the 

 human brain, isolated observations of their relations in pathological conditions 

 are of but little value. Great difficulties attend the ])roduction of lesions in the 

 brains of experimental animals, without surgical intervention, because the changes 

 in the other organs often prove fatal before the nervous s^ystem has been affected; 

 but in a field so promising these difficulties will probably be speedily overcome. 

 Mitochondria, being phospholijnns, differ sharply from the Nissl substance, which 

 is nucleoprotein in nature. Accordingly, the}" will serve as clues to a different 

 type of activity and will yield valuable information upon the question of nerve- 

 cell physiology. From their chemical constitution also it is probable that on dis- 

 integration they may elaborate cholin, and inasmuch as organic diseases of the 

 nervous system can be separated from functional neuroses by the formation of 

 chohn in the one and not in the other (Halliburton, 1907, p. 74), it is ciuite likely 

 that a study of mitochondria may afford a cytological basis of distinction between 

 these two great groups of nervous diseases. 



REGENERATION. 



Romeis (1913«, p. 10) has made a study of regeneration in the tails of Triton. 

 In the connective-tissue cells the mitochondria become greatly increased in num- 

 ber and he finds all the stages in the formation of fibrils reported by Meves. The 

 same is true in muscle-cells, and the mitochondria also increase in niunber in the 

 glands of the skin and in bone. He believes that with regeneration the cells 

 become more embryonic with resultant increase in mitochondria and that the various 

 structural differentiations are formed by the mitochondria and the cytoplasm acting 

 together. 



Torraca (1914o, p. 539; 19146, p. 459; 1916, p. 326) has made a number of 

 contributions to the subject, working on cartilage, striated muscle, and glands. 



