440 C. JUDSON HEREICK 



fiber tracts and synaptic connections are found. Most of these 

 neurons are of undifferentiated form, and the various groups of 

 neurons performing similar functions (the so-called cerebral 

 nuclei) are very incompletely separated from each other. The 

 nerve fibers are for the most part diffusely scattered in the white 

 layer with few clearly defined tracts. 



The sense organs and peripheral nerves of cyclostomes are 

 simply organized, but the pattern is in its broad outlines similar 

 to that of the true fishes (Johnston, '05). Within the brain, 

 however, the primary sensory nuclei into which the various 

 sensory components of the cranial nerves (from skin, internal 



Fig. 16 Diagrammatic longitudinal section through the forebrain of the lam- 

 prey, Petromyzon, illustrating the form of the cerebral hemispheres. The hemi- 

 spheric evagination includes a large olfactory bulb and a smaller olfactory lobe, 

 leaving a considerable residue of tissue in the unevaginated primitive endbrain. 



ear and lateral-line organs, taste buds, etc.) discharge their 

 nervous impulses are far less sharply separated than in the true 

 fishes. And in the higher correlation centers there is little 

 precise localization of the functional areas. All forms of periph- 

 eral sensory excitation tend to converge into relatively few 

 final common motor paths which are very simply arranged. 



From this arrangement of the conduction paths and nerve 

 centers it follows that the nervous organization is such as to make 

 possible a relatively small number of reactions to all sorts of 

 sensory stimulation and that these reactions are for the most 

 part simple total movements of the whole body rather than 

 complex adjustments involving precise coordination of many 



