THE NERVOUS SYSTEM 



1399 



Olfactory Lobe. — The olfactory lobe appears as a hollow protrusion of the 

 anterior cerebral vesicle on its ventral aspect, and near its anterior part. 

 The cavity of this protrusion, which is continuous with the lateral ventricle, 

 soon undergoes obhteration, and the protrusion becomes solid. Its terminal 

 extremity undergoes enlargement, and the entire protrusion becomes differ- 

 entiated into the following parts: (i) The olfactory bulb; (2) the olfactory 

 tract; (3) the inner or mesial, and outer or lateral, olfactory roots; (4) the 

 trigonum olfactorium; (5) the area of Broca; and (6) the locus perforatus 

 anticus. Of these parts, the olfactory bulb is the enlarged terminal extremity 

 of the original protrusion, and, as stated, it rests upon one-half the cribri- 

 form plate of the ethmoid bone, through the foramina of which half it receives 

 the olfactory filaments, which are the axons of the sensory cells of the olfactory 

 epithelium of the upper part of the nasal fossa. 



Corpus Callosum 



Anterior Pillar of Fornix 

 Septum Lucidum 



Pineal Body 

 Splenium 



Genu passing into 

 Rostrum 



.Corpora 

 Quadri 



Foramen 

 of Monro 



-^ Anterior Commissure 



Optic Thai, and Mid. Com. 

 Optic Nerve 

 ■^ Pituitary Body 

 Tuber Cinereum 

 Corpus Albicans 



Cerebellum 



Fourth Ventricle 



Third Nerve 

 Pons Varolii 

 I Cms Cerebri 

 Aqueduct of Sylvius 

 Medulla Oblongata 



Fig. 589. — The Mesial Surface of the Left Cerebral Hemisphere 



(HiRSCHFELD AND LeVEILL6). 



Olfactory Epithelium. — The first indications of the olfactory organ are 

 the two olfactory or nasal areas. They consist of thickened ectoderm, and 

 are placed on the ventral aspect of the anterior cerebral vesicle, on either 

 side of the mesial nasal process of the fronto-nasal process, and on the cephalic 

 side of the orifice of the stomodaeimi. Each olfactory area soon becomes 

 depressed, and forms the olfactory or nasal pit. The two pits, right and left, 

 are situated one on each side of the mesial nasal process of the fronto-nasal 

 process. Each pit has the corresponding globiilar process of the mesicJ 

 nasal process internally, and the latercil nasal process externally, this position 

 corresponding to the situation of the future anterior naris on either side. 

 At this period the olfactory pits communicate with the stomodseum, and the 

 orifice of each pit, slightly constricted, constitutes the corresponding primitive 

 anterior naris. This naris, on either side, becomes separated from the stomo- 

 dseum by the union of the maxillarj- process (previously joined by the lateral 

 nasal process) with the globular process. 



