164 Discussion 



nearer its adult composition in foetal life than the skeletal muscle. I 

 think a great deal of this change is in the skeletal muscle and not in 

 connective tissue. 



Fourman : Then is there a difference in the mode of growth of skeletal 

 muscle on the one hand, and liver and heart on the other? Does skeletal 

 growth occur simply by hypertrophy without cell multiplication, and do 

 heart muscle and liver grow by cell multiplication? Are babies' muscle 

 cells smaller than those of adults and their liver cells the same size? 



Kennedy: By and large what you have said is right. There is con- 

 siderable hyperplasia in liver during growth although there is an over-all 

 expansion in size of the cells with age. There is a much bigger change in 

 muscle cell size than in the liver cells. 



Fourman : If the extracellular fluid is considered as a film over the cells, 

 that would account for the fact that the percentage of extracellular fluid 

 does not change with age so much in liver as it does in muscle. 



Kennedy : Within any one tissue it should be quite easy to test that, 

 because cell size data based on nucleic acid determinations are available 

 for many different ages in a number of species, and equally, extracellular 

 fluid determinations are available in the same tissues. 



Wallace : Muscle composition does not change much with age per unit 

 of muscle ; you are talking about more muscle, not per kilogram of muscle. 



Widdowson : I am talking about per unit of muscle. As I have just 

 said, skeletal muscle changes very much in composition during develop- 

 ment. 



Fourman : Dr. Shock, is the water content in the muscle larger in old 

 people than in the young ones, since muscles do atrophy in old age? We 

 have had that answered indirectly in Dr. Olesen's paper, but are there 

 any direct analyses? 



Shock : I cannot answer for the human, but we have some data on the 

 electrolyte and water composition of rat muscle tissue. We found that 

 the total water content per kilogram of muscle tissue does not change 

 significantly with age. There was a definite shift in the water distribu- 

 tion in that the extracellular phase increased as the intracellular phase 

 decreased. The potassium, phosphorus, and nitrogen contents all went 

 down, but the chloride and sodium contents went up. The ratio of 

 potassium to nitrogen and of phosphorus to nitrogen remained constant. 

 Our interpretation of this was in the light of our beliefs about the reduc- 

 tion in active protoplasm in old age. It is as if a certain mass of proto- 

 plasm had disappeared and been replaced by extracellular compounds 

 with the appropriate amount of sodium and chloride to make up the 

 total water composition. 



Fourman: As I said, it is not a replacement, but — to borrow Dr. 

 Davson's expression — a geometrical necessity to keep a film of water 

 around the cells. 



Kennedy : But you would need to know whether the atrophy was due 

 to a loss of whole structural units or to a change in the size of each unit. 



Shock : We do not really know this. We have not done the histology on 

 these muscle tissues, but we have sent some to Dr. Warren Andrew for 

 examination. 



