io8 PHYSIOLOGICAL 



prevent the end parts or epiphyses of the long bones from 

 ossifying so rapidly ; the fact that these retain their cartilaginous 

 character for a longer period admits of the shafts of the bones 

 continuing to grow until prevented from so doing by hard, 

 dense, ossified structures at their ends. 



Growth and fattening in actual practice proceed simul- 

 taneously, but at a certain stage developmental growth and 

 increase of protein tissues cease (excepting in cases where 

 further development of muscle is due to increased exercise) 

 and any subsequent addition is due exclusively to increase of 

 fat. Such a condition is sometimes spoken of as the "par" 

 condition 1 . An animal in "par" condition is in a state of 

 nitrogen equilibrium, the amount of waste nitrogenous material 

 eliminated through the kidneys being equal to the amount 

 absorbed in the digestive tract. 



What it is that determines the precise stage at which the par 

 condition is arrived at is still an open question. It can only be 

 said here that it is apparently an hereditary characteristic varying 

 in different individuals and still more in different varieties and 

 species. All animals follow the same general laws of growth. 

 In the earliest stages development is very rapid, next there is 

 an enormous fall in the percentage of growth, then the decline 

 becomes gradual (varied only in entire animals by fluctuation 

 at about the period at which sexual maturity is acquired), till 

 finally developmental growth ceases altogether. The laws of 

 growth are believed by some to be related to cellular changes, 

 the nuclei becoming relatively smaller and gradually losing 

 their rejuvenating capacity. Hence their power of forming new 

 tissue is gradually reduced. For as stated already the deposition 

 of fat in the cells is not a case of true growth, but may partake 

 rather of degenerative change. 



Fat is formed mainly in the connective tissue cells, that is, 

 in those tissues which bind together the various organs of the 

 body and hold them in position, filling up interstices and 

 forming a net-work which tends to cover or pervade the more 

 highly differentiated tissues. The fat is produced by a process 

 of absorption from the blood vessels, the protoplasmic contents 



1 Murray, "Meat Production," Science Progress, April 1918. 



