INVOLUTION 487 



opinion that these two neural circulations, the medullary 

 and axis cylinder, are almost entirely responsible for the 

 production of external ageing, and, therefore, that the 

 varying rates of their terminal output or discharge consti- 

 tute the main factor in the evolution of the physical and 

 physiological changes engaged in marking out the stages 

 of external ageing, and the sequence of cutaneous change, 

 during the entire span of life. These two circulations, 

 being from centre to periphery only, cannot return the substances 

 they are respectively circulating to their sources of origin, 

 or to a common radiating centre, as the haemal circulation 

 does, or secondarily find their way into the haemal circula- 

 tion, as the systemic lymphatic circulation does ; they, 

 therefore, being histologically free at their distal ends, are 

 compelled to expend them in the development of dermal 

 and epidermal structures and appendages, to be ultimately 

 shed, or finally liberated, by external exfoliation, after a 

 shorter or longer incorporation in surface, as well as deeper- 

 seated layers of the skin. Hence, they become the 

 principal sources of dermal and epidermal growth, both in 

 a material and dynamic respect, and so they are the almost 

 entire sources of surface, dermal and epidermal, modifica- 

 tion, and the prime factors of external ageing, as well as 

 the structural source of origin of many of the pathological 

 conditions of the skin, which are liable to arise at all 

 periods of life, mainly from faulty excretion or exfoliation, 

 and deficient or redundant circulation. 



The depth and consistence of the dermal and epidermal 

 matrix, being principally regulated by the rate and amount 

 of the neural incorporation and excretion, varies in amount 

 at every stage of life, according to the degree of physio- 

 logical activity and freedom from mechanical obstacles 

 characterising the functional, or vital, working of the 

 individual organism. From this it follows that the depth 

 of the " wrinkles and lines " of ageing is determined by 

 neural circulation, deposition, and excretion in, and of 

 exfoliation of and from, the cutaneous textures, and their 

 character as to intensity and natural sequence impressed 

 on their subject in more or less easily read type. 



Besides the surface markings of age left on the external 

 envelope of the body, the appendages of the skin, more 



