38 STUDIES ON THE CENTRAL NERVOUS SYSTEM 



experience teaches that this is not so easy. Not unfre- 

 quently the spine is opened and no tumour is found. A 

 neurologist, who never had such a disagreable experience, 

 has not yet had experience enough. One of the chief causes 

 of this is that the hmits of these dermatoms are not as 

 precisely known as they should be. The dermatoms in 

 our diagrams are a little schematic and all these successive 

 dermatoms partly overlap one another. 



Such an overlap has been carefully determined by Sher- 

 rington, Winkler, van Rynberk and others in animals. 

 They have cut the posterior roots and examined the analgetic 

 areas in the skin, caused by this operation. In most places 

 of the body three posterior roots must be dissected, before 

 an analgetic area appears. Clinical experience indicates 

 that in man there is also an overlap between successive 

 dermatoms. The dermatoms in the figures of Bolk are, it 

 is true, a little too small. Still there remain many differences 

 between the results of the experimental work on the one hand 

 and of the clinical and anatomical work on the other hand. 

 Many of these differences can be explained owing to the 

 different form of the body. In a former article I showed 

 that the form and the position of the dermatoms in man and 

 in animals are influenced by the form of the body. The 

 form of the neck, for example, and the relation between the 

 size of the legs and the trunk is quite different in animals 

 from man. Hence it is not correct to use the dermatoms of 

 Sherrington and others, determined for animals, in our 

 clinical work. 



The diagnosing of the exact level of tumours on the spinal 

 cord has been lately greatly helped by the discovery of 

 Sicard and Forestier. After puncture of the cisterna 

 magna — first done by J. B. Ayer — lipiodol is injected and a 

 X-ray examination of the spine is made. X-rays cannot 

 penetrate lipiodol. When an extramedullar tumour is 

 present, the lipiodol stops, gives a shadow on this part of the 

 spine and thus we are able to control our clinical conclusions. 



