32 STUDIES ON THE CENTRAL NERVOUS SYSTEM 



In the lowest classes of vertebrates, in fishes, the posterior 

 columns are very small, hence the posterior horns lie close to 

 one another. There are some tracts in these posterior 

 columns, but they are only composed of reflex fibres, con- 

 necting the different levels of the spinal cord. These animals 

 have no nuclei of GoU and Burdach and a real lemniscus 

 medialis as in higher vertebrates is not present. The sys- 

 tems for vital sensibility, the spino-thalamic tracts of 

 Edinger are found however. These tracts lead the sensory 

 stimuli in the direction of the oblongata, the midbrain and 

 the optic thalamus. 



The spinal trigeminal root is also easily found in all sec- 

 tions of fishes and is even often large. Judson Herrick has 

 shown, that in Teleosts a frontal trigeminal nucleus is not 

 present. I studied complete serial sections of Rhombus 

 maximus and of Lophius boudegossa but could not see a 

 nucleus which may be homologised with the frontal trige- 

 minal nucleus of higher animals. All the sensory stimuli 

 which touch the head of the fishes go into the spinal root. 

 The part of these, not immediately flowing down in reflex 

 movements, but sent in an oral direction, is conducted by 

 the secundary trigeminal systems, issuing in the cells of the 

 substantia gelatinosa of the trigeminal root. The fishes 

 therefore have only the vital form of sensibility, which we 

 may call the palaeotype. 



As soon, however, in the scale of evolution, as life on land 

 has been possible, new demands are made on sensibility. 

 Thus a new pathway is formed. This is the system called 

 above the gnostic sensibility. In lower animals, for ex- 

 ample in amphibians, reptiles and in birds, this second 

 pathway is still small, but grows in the ascending scale and is 

 largest in man. We called this the younger form of sensi- 

 bility, the neosensibility. I refer to figure 6 in which sections 

 of the first cervical segment of the spinal cord in various 

 animals are photographed. 



The same relation is present in the trigeminal systems. 



