486 BIOLOGICAL PHYSICS 



consistence and thickening of the skin, and for those 

 features of cutaneous change evolved by age. Having 

 thus disposed of haemal circulation as an appreciable genetic 

 influence in the process of external ageing, we are left with 

 the neural circulation to account for the physical changes 

 undergone by the skin, and thus to become the chief 

 instrument in producing and meting out the specific signs 

 and memorials of ageing. 



The neural circulation, as we elsewhere have described, 

 is a compound circulation, consisting of, at least, three 

 distinct circulations, all of which have one feature in com- 

 mon, viz. that they end in and on the skin, and terminate 

 by the excretion of their entire circulative materials, accord- 

 ing to their specific or intrinsic characters as to fluidity, 

 plasticity, and physical qualities generally ; thus, the neural 

 lymph circulation terminates immediately in and by the 

 sweat glandulature as a fluid, or gaseous and evaporable, 

 material, or excrementitious substance, while the medullary 

 and the axis cylinder substances of the axons lend them- 

 selves to the histological increase or growth of the dermal 

 and epidermal textures, and thereafter to gradual exfolia- 

 tion, or shedding, as more or less solid exuviae. 



O ' 



The first of these neural circulations, the neural lymph 

 circulation, may, like the haemal and lymphatic circulation, 

 also be eliminated from the list of agencies primarily con- 

 ducive to the process of external ageing, inasmuch as it is 

 concerned in transmitting through the cutaneous structures 

 a fluid and non-plastic element, which in a normal or 

 physiological state leaves no trace behind, but which, in 

 certain pathological conditions, lends itself to the convey- 

 ance, in suspension, of morbid elements which may undergo 

 changes in transit, rendering them, more or less, permanent 

 ingredients of the dermal and sub-dermal structures, as, for 

 instance, in certain forms of gout and rheumatism, and 

 other allied affections. 



This elimination of all the local circulatory elements 

 concerned in the vital work of the skin, with the exception 

 of the two outstanding neural, or neuro-axonal, circulations, 

 whose final outfall work, or excretional disposition, consti- 

 tutes, in great measure, the active functional raison d'etre of 

 the skin with its appendages, compels us to adopt the 



