THE CONNEXIONS OF THE OLFACTOKY NEKVES. 



623 



of white nerve-fibres passing to and fro between the olfactory bulb and the 

 hemisphere ; hence it is called the tractus olfactorius. 



The cerebral hemisphere first appears in the form of a slight bulging upon each 

 side of the fore-brain, but it soon assumes large dimensions. At first it grows 

 forwards and upwards (Fig. 550), and a distinct cleft, the floor of which is the 

 roof-plate and lamina terminalis, appears between the two hemispheres : this is known 

 as the fissura longitudinalis cerebri. The separation of the two cerebral vesicles 

 by the longitudinal fissure begins at the end of the first month. This fissure 

 becomes occupied by mesodermic tissue, which later on forms the falx cerebri. 

 The cerebral hemisphere, in its further growth, is carried progressively backwards 

 over the posterior parts of the developing brain. At the end of the third month it 

 has covered the thalamus. A month later it reaches the corpora quadrigemina, 

 and by the seventh month it has not only covered these, but also the entire upper 

 surface of the cerebellum. 



In the earlier stages of its development the cerebral hemisphere is a thin-walled 

 vesicle with a relatively large cavity, which represents the primitive condition of 

 the lateral ventricle. At first the vesicle is bean-shaped and the cavity is curved. 

 As development proceeds the posterior portion of the hemisphere grows backwards 

 over the cerebellum in the shape of a hollow protrusion, and a distinct occipital 

 lobe enclosing the posterior horn of the lateral ventricle is the result. This 

 developmental stage begins about the fourth month. 



THE CONNEXIONS OF THE OLFACTORY NERVES. 



The olfactory nerves are the axons of the spindle-shaped bipolar cells situated 

 in the olfactory mucous membrane (Fig. 551). These 

 axons collect in the submucous layer to form small 

 bundles, which enter the cranial cavity through the 

 foramina in the lamina cribrosa .of the ethmoid 

 bone. They at once enter the inferior surface of 

 the bulbus olfactorius, and each fibre breaks up into 

 a tuft of terminal filaments. Towards these tufts 

 dendrites proceed from large mitral cells placed in a 

 deeper plane within the bulb, and each dendrite also 

 breaks up into numerous terminal branches inter- 

 twined with those of the olfactory nerves. In this 

 way are formed a large number of globular bodies, 

 each consisting of the arborescent terminations of 

 a mitral dendrite and of certain olfactory nerve- 

 fibres. These are the olfactory glomeruli of the bulb. 

 Each mitral cell gives otf several dendrites and one 

 axon. Only one dendrite enters into the formation 

 of a glomerulus, but several nerve- fibres may be 

 connected with such a body. It thus happens that, 

 through its dendrite, a mitral cell may stand in 

 connexion with several olfactory nerve-fibres. The 

 axon of the mitral cell passes upwards to the white 

 matter of the bulb, enters this, and, bending back- 

 wards, is conducted through the tract towards the 

 cerebral cortex. 



The olfactory bulb is a small, flattened, elliptical mass of gray substance placed 

 upon the upper surface of the lamina cribrosa of the ethmoid. Its posterior 

 xtremity is attached to the rest of the cerebral hemisphere by the long tractus 

 olfactorius (Fig. 476), a prismatic band of white substance placed in a furrow 

 (sulcus olfactorius) on the under surface of the frontal region of the cerebral 

 hemisphere. A short distance in front of the optic chiasma each olfactory tract 

 becomes inserted into the hemisphere (Fig. 552). The swollen pyramidal-shaped 



OLFACTORY MUCOUS 



JJlimill!! MEMBRANE 



