DIFFERENTIATION AND REJUVENATION 147 



muscle fibre, Fig. 53, which you may recall from the 

 second lecture, so that it will suffice if your atten- 

 tion is again directed to the oval 

 nuclei, and to the lines stretching 

 crosswise on the muscle giving it 

 a " striated " appearance. You re- 

 member, doubtless, that such fi- 

 bres are the ones which enable us 

 to make voluntary motions. Orig- 

 inally each fibre was a set of cells, 

 and the cells had some protoplasm, 

 but, gradually, as development pro- 

 gressed, there appeared in them 

 longitudinal fibrils different from 

 the protoplasm, and the fibrils 

 also created ultimately the appear- 

 ance of cross lines on the fibre. 



T 1 ^i -1 i i r i FlG> 53 ' PARTOFA 



It IS the fibrils which perform the HUMAN MUSCLE FIBRE. 



muscular contractions. It is not 

 the original unmodified protoplasm, but the modified 

 or differentiated muscular cell which is capable of vol- 

 untary contraction. 



The next picture, Fig. 54, shows us clearly and 

 strikingly how much the differentiation may vary. 

 We have here another type of differentiation. These 

 are gland cells ; we can see here, as I pointed out to 

 you before, the material in the form of granules, which 

 is to produce the secretion from these gland cells. 

 This is an orbital gland, and here are the cells, which 

 are very much smaller because they have discharged 



