TISSUES AND ORGANS 



SKIN 



The protective function of the integument is the most obvious 

 function of the covering of the body. As is well known there are 

 a number of layers forming the skin, and these become more and 

 more impervious as one travels from the deeper to the more 

 superficial strata. The most obvious histological sign of this 

 differentiation is the presence of keratinization, but there are 

 other, more subtle changes, throughout the layers. As one passes 

 from within outwards there is found a decrease of mitochondria 

 and Golgi system, both being lost in the most superficial cells. 

 Whilst keratohyalin and keratin increase, melanin becomes 

 scantier, and the cells change in shape, whilst fibrillae and inter- 

 cellular bridges are formed. It appears that the whole series of 

 changes gives rise to the appearance of an almost impervious 

 covering for the body. 



The intra-cellular fibrils of human skin are relativelv easilv 

 rendered visible and are therefore very suitable subjects for 

 studying the vexed question of the nature of these structures. It 

 is quite commonly held that the mitochondria give rise to these 

 fibrils by rearrangement of their positions ; further, their break- 

 down is supposed to result in the formation of keratohyalin. If 

 such be the case, it is easily understood whv mitochondria are 

 absent from the most superficial layers of the integument. The 

 inter-cellular bridges, which obviously serve the purpose of 

 holding the cells together, may also act as carriers of stimuli from 

 one element to another. It is stated that the small thickening 

 found in the middle of each bridge is a mitochondrial development, 

 but a clear demonstration of this belief would be difficult. 



There has been, and still is, much discussion as to the mode of 

 origin of cutaneous pigment, i.e., melanin, and here only the avail- 

 able facts will be recorded. In the most basal cells there is 



