1 68 



ORGANIZATION OF THE HEAD 



v. 15 



All parts of the cerbral hemispheres receive fibres from the olfac- 

 tory bulbs and the forebrain serves mainly for analysing the olfactory 

 impulses. However, it is stated that there are fibres reaching forward 

 to one area at the back of the roof of the hemispheres from other 

 centres. Johnston therefore called this region the 'general somatic 

 area' and suggested that it represents the beginnings of that develop- 

 ment so characteristic of mammals by which all the senses are centred 

 on the cerebral hemispheres. Further work is needed to confirm the 



Dec interhemisph 



., Tr medianus 



Ventr o/f 



Tr olFacb. 



Tr olFacb. epistr cruc. 



Fiss Urn tel. 



Ventrical, /at. 



Fig. 107. A cross-section through the forebrain of a shark. 



Dec. interhemisph. decussatio interhemispherica; Fiss.lim.tel. fissura limitans telencephali; 



Prim. hip. primordium hippocampi; 5. septum; Striat. striatum; Tr. medianus. tractus 



medianus; Tr.oljact. tractus olfactorius; Tr.olf act. epistr. cruc. tractus olfacto-epistriaticus 



cruciatus; Ventricul.lat. ventriculus lateralis; Ventr. o.f. ventriculus olfactorius. 



(From Kappers, Huber, and Crosby.) 



existence of this pathway, and even if present its significance must 

 not be exaggerated. There is of course no cortical arrangement of 

 tissue in the hemispheres. The cells form thick masses around the 

 ventricle (Fig. 107). The roof is quite thick and contains decussating 

 fibres in the midline. The sides and floor make up the main bulk of the 

 organ, the lateral wall being known as the striatum, its upper part the 

 epistriatum. The medial wall is known as the septum and its upper 

 portion is often referred to as the primordium hippocampi, having a 

 position similar to that of the hippocampus of mammals. The main 

 efferent pathways are tracts leading to the hypothalamus and to the 

 optic lobes. After removal of the forebrain the sense of smell is lost 

 but the fish shows no obvious disturbance of posture, locomotion, or 

 behaviour. 



The diencephalon is a narrow band of tissue, there are no extensive 

 tracts leading forward through it, and the optic and other pathways 

 do not end here as they do in higher animals. The lower part of the 



