308 PHYSIOLOGY CHAP. 



which coincides with the maximum of dorsal innervation (diagram 

 A at point #), was termed by Winkler and Van Rynberk the 

 " ulti'iiiU'in moricns " of the dermatome. 



These observations, as a whole, bring out the important 

 physiological fact that the function and distribution of the root 

 filaments is diffuse, and not localised, in the segmental skin-field. 



The same authors also endeavoured to estimate the precise 

 extent to which overlapping of the dermatonies takes place. 



By a series of ingenious measurements and calculations 

 Winkler and Van Rynberk ascertained that the overlapping of the 

 central areas amounts to one-third near the dorsal median line ; 

 to two-ninths near the lateral line ; while in the ventral median 

 line they do not come into contact. If the marginal zone is also 

 taken into consideration, the total overlapping of the derrnatomes 

 appears at no point to be less than half, so that every point on 

 the skin must be simultaneously related to two dermatomes, i.e. 

 it is innervated from two dorsal roots. This observation holds 

 for the trunk : in the region of the limbs the dermatonies are 

 more compressed, and the overlapping is therefore greater. 



These central areas of the dermatomes are important, more 

 particularly when brought into relation with the clinical facts 

 observed by Head (1893). He describes areas of cutaneous 

 hyperalgesia met with in many visceral diseases, particularly 

 those of the intestines. He observed great constancy in their 

 localisation and extension, and that in these particulars they 

 correspond with the eruptive zones of Herpes zoster. Since it 

 is known that this cutaneous eruption is only the external 

 symptom of an acute infectious inflammation of one or more 

 spinal ganglia, it was natural to assume that the herpetic eruption 

 would follow the cutaneous metamerism, the more so as Sher- 

 rington had already noted that the sympathetic innervation of 

 the skin coincides approximately with it. Head considers the 

 cutaneous hyperalgia, which is symptomatic of internal disease, to 

 be the peripheral expression of irritation in a spinal segment. 



The only serious objection to this hypothesis is that none of 

 Head's zones overlap like the dermatonies. It is probable that 

 the zones of Head, like the herpetic eruption, occur only within 

 the central areas of Winkler and Van Rynberk, where overlapping, 

 as we have seen, takes place to a much smaller extent than for 

 the whole dermatome. In this way it is possible to refer an 

 important series of obscure clinical facts to the system of cutaneous 

 segmentation. 



Another phenomenon pointed out by Langelaan (1900) must 

 be mentioned in connection with cutaneous metamerism. He 

 discovered that a whole system of lines and areas exists in the 

 skin of normal persons, which may, in comparison with the rest 

 of the skin, be termed hyperalgesic. For example, 021 pricking 



