68 PHYSIOLOGY CHAP. 



The internal organs are, generally speaking, less sensitive than 

 the skin ; their afferent nerves are, according to Head, in close 

 relation with the centres of the cutaneous sensory nerves of the 

 same spinal segment. 



The same theory applies, according to Head, to the cutaneous 

 hyperalgesias observed in visceral affections. When abnormal 

 excitations from a diseased internal organ reach the cord by way 

 of the afferent nerves the excitability of the spinal segment 

 becomes exaggerated, so that when another cutaneous excitation 

 of low intensity reaches the same segment it provokes pain, 

 whereas under normal conditions it arouses merely a sensation of 

 contact. 



These views were strengthened by Head's mapping out of the 

 hyperalgesic zones that are observed in different affections of the 

 viscera. Each diseased organ produces hyperalgesic zones which 

 are characteristic in form and localisation. According to Head 

 the zones of herpes zoster coincide with those present in hyper- 

 algesia. 



Head's hyperalgesic zone corresponds not with the areas to 

 which the cutaneous nerves are peripherally distributed, but 

 with those supplied by the dorsal roots. As shown in Figs. 25 

 and 26, these do not overlap, while the cutaneous metameres or 

 dermatomes of the dorsal roots, on the contrary, according to 

 Sherrington, do overlap to a large extent. But Sherrington's 

 later work showed the overlapping to be different for different 

 qualities of sensation ; it is much more extensive for tactile sensa- 

 tion, much narrower for pain sensation. We saw (Vol. III. p. 306) 

 that the work of Winkler and van Kynberk on the central area 

 of dermatomes has thrown new light on this subject. And it 

 is probable that Head's hyperalgesic zones represent segmental 

 zones of Sherrington's pain areas, or of the central areas of the 

 dermatomes of Winkler and van Kynberk. 



Head's clinical investigations have such great practical importance that 

 it is desirable to reproduce the following diagram and table, which sum up 

 his results. 



Figs. 25 and 26 show the segmental cutaneous areas of the trunk, 

 extremities, and head. The form and extent of these were arrived at : 



(a) by mapping out the areas in a number of cases of cutaneous hyper- 

 aesthesia with coincident visceral affections ; 



(6) from the topography of the eruptions in 52 cases of herpes zoster ; 



(c) by mapping out the analgesic areas in organic diseases of the spinal 

 cord and roots. 



The 8 cervical segments are indicated by Cl, C2...C8; the 12 dorsal or 

 thoracic segments by Dl, D2...D12 ; the 5 lumbar segments by Ll, L2...L5 ; 

 and the 4 sacral segments by Sac. 1, Sac. 2... Sac. 4. 



The areas of the head are indicated as follows : N = nasal or rostral area ; 

 FN = fronto-nasal area ; MO = medio-orbital area ; FT = fron to-temporal area ; 

 T = temporal area ; V = vertical area ; P = parietal area ; O = occipital area ; 

 NL = naso-labial area; Max. = maxillary area; Man. = mandibular area; 



