THE CONNEXIONS OF THE OLFACTOKY NEEVES. 



625 



If the brain of almost any other mammal is examined (take the rabbit's as 

 an example), the area piriformis will be found to constitute relatively an enormously 

 larger proportion of the cerebral hemisphere than it does in the human brain ; 

 and it is separated from the part of the hemisphere (neopallium) that lies above 

 it by a longitudinal furrow called the fissura rhinalis. The enormous expansion 

 of the neopallium in the human brain accentuates the flexure of the piriform 

 area at the point x (Fig. 553), and at the point y the exuberant growth of 

 neopallium relegates the swollen posterior part of the piriform area on to the 

 medial surface (Fig. 554), where the posterior part of the rhinal fissure persists to 

 separate it from the neopallium. 



The surface of the piriform area often presents numerous small wart -like 



Olfactory peduncle 



Olfactory bulb -. 



/\ 



Olfactory tract ' ' VjJ, .._ 



/ x \-*&^ f- ^ Rhinal fissure 



Olfactory tubercle ' v 



Nucleus amygdalae Piriform area 



Olfactory bulb^ 



Olfactory tract 



Piriform area 

 (anterior part) 



x - 

 Rhinal fissure - 



y 



Neopallium 



Piriform area 



(posterior 



part) 



Piriform area 

 (anterior part) 



Olfactory tubercle 



Optic chiasma 



Nucleus 

 amygdalae 



y- 



Rhinal fissure -- 



Neopallium ^V - 



FIG. 553. 



Olfactory l bulb 



- Olfactory-tubercle 



-Optic chiasma 



Nucleus 

 amygdalae 



Piriform area 

 (posterior part) 



A, The lateral aspect of the left cerebral hemisphere of a rabbit. B, The inferior aspect of the right half of 

 a rabbit's brain. C, The corresponding view of a human foetal brain at the fifth month. 



Olfactory areas, green ; neopallium, blue. 



excrescences ; and it is whitened by a thin layer of fibres (substantia reticularis 

 alba) prolonged backwards from the stria olfactoria lateralis. By these fibres 

 olfactory impulses are poured directly from . the mitral cells of the bulb into the 

 piriform area. If we call the olfactory nerves the primary olfactory neurones, the 

 fibres which pass from the bulb to the piriform area would then be secondary 

 olfactory neurones. 



Formatio Hippocampalis. From all parts of the area piriformis, as well as 

 the trigonum and tuberculum olfactorium, fibres arise (tertiary olfactory neurones), 

 and proceed on to the medial aspect of the hemisphere, where they terminate in 

 the edge of the pallium, alongside the lamina chorioidea. In the human brain the 

 vast majority of these tertiary neurones proceed from the posterior extremity of 

 the piriform area, but a certain number arise in the neighbourhood of the 

 substantia perforata anterior and proceed at once on to the medial surface of the 

 hemisphere. The large number of small nerve-cells that collect in the medial 

 edge of the pallium become specially modified in structure to form a receptive 

 organ for impressions of smell, known as the fascia dentata; and the axons of 

 these cells pass into the part of the pallium which immediately surrounds the 

 peripheral edge of the fascia dentata and is known as the hippocampus (Fig. 556). 



41 



