46 

 brain will be treated in the theoretical considerations of 

 Subsection 7. 



b. The Subs tan tiu Gelat inosa. — The gelatinous substance 

 of Rolando is continued frorri the spinal cord into the oblonga- 

 ta. It takes on such an intimate relation to the terminating 

 nerve-fibres here that Barker ('99) has proposed to call it the 

 nuclei tractus scinali^ nervi trigemini . The Folandic sub- 

 Stance occupies a position hi^h up in the dorsal part of the 

 general cutaneous nucleus, just beneath the cap of the rr.olec- 

 ular layer, and itself forming an investment for the bundles 

 of fibres and larger neurones of the central mass. The complex- 

 ity of its structure, and the difficulty of staining it with 

 the usual reagents are facts well known to microscopists. The 

 nerve-cells of Rolando's substance in the spinal cord have 

 been studied by vonLenhossek ('94) from man, the pig, and the 

 mouse; while Ramon y Cajal ( '96) has given us an elaborate 

 description of the oblongata of the mouse. It gives me great 

 pleasure to be able to extend our conceptions of this peculiar 

 formation to the field of the brain of selachians. 



In Mustelus, the structure of the substantia gelatinosa 

 foreshadows to a remarkable degree the organisation which has 

 been described for the higher vertebrates. The neurones are 

 associated in groups; a representative collection is drawn in 

 Pig. 5. The axones and dendrites branch profusely and interlace 

 so closely that the simulacrum of a fine network is given. In 



