Functional factors iii the morphology of thc forebrain of fishes 163 



detailed description of its fibrous connections given by Johnston permits 

 comparison with Amphibia and the determination of its homologies ín 

 still higher brains. It is clear ¡n all of these cases that this área has 

 differentiated in functional relation with the non-olfactory centers of the 

 diencephalon, that is, the thalanius as distinguished from the epithalamus 

 and the hypothalamus. 



Xo región exactly comparable with the área olfacto-somatica of Aci- 

 penser has been described in elasmobranchs. The área olfactoria laterahs, 



tr. ol.som: 



tUtJTolf. 



tr. som. p. 



tr.th.f. 



16 



Fi;^. 10. — Diagram of the forebrain of Acanthias from the left side, illustrating 

 the connections of the área somática. Sepárate systems of fibers enter this área 

 from the corpus geniculatum laterale (c. gen. lat.) and from the dorsal part of the 

 thalanius ('///í7/.j. These comprise the tractus thalamo-frontalis \tr. tli. f.). The 

 descending somatic projection tract (tr. som. p.) leaves the rostral part of this área 

 to enter the ventral part of the thalamus and pedunculus cerebri. Based on figu- 

 res and descriptions of Johnston (191 1). 



however, is under somatic sensory influence through the olfacto-somatic 

 correlation tract. And at the lower border between the área somática . 

 and the área olfactoria lateralis there is a nucleus of specially large neu- 

 rons which is regarded by Johnston (191 1, p. 32) as a differentiated center 

 of efferent discharge for the somatic projection tract. Ihese cells appear 

 to have been differentiated from the olfacto-somatic projection cells of 

 Acipenser (fig. 4) whose axons form the descending fibers of the tractus 

 strio-thalamicus of that species, which is really an olfacto-somatic projec- 

 tion tract. In Acanthias these neurons are detached from the olfactory 

 área, though still in physiological relation with it through the olfacto- 

 somatic correlation tract. 



