1152 



HUMAN ANATOMY. 



which in many animals is continuous with the fore-part of the lateral ventricle, as well as some 

 of the six layers that may be typically represented, as in the dog's bulb. The ventral aspect of 

 the bulb, receiving the olfactory nerves, retains most completely its nervous character and pre- 

 sents three chief strata (Fig. 995). (i) The stratum of olfactory fibres appears as a narrow 

 zone made up of the irregularly intermingled bundles of a.xones of the olfactory cells situated 

 within the olfactory area of the nasal mucous membrane. This layer is succeeded by a broader 

 tract, (2) the stratum of the mitral cells, so named on account of the numerous nerve-cells of 

 peculiar bishop' s-hat form which occupy its upper border. Along its lower margin extends a 

 narrow zone of large spherical masses, the olfactory glomeruli. These bodies, from .065-.090 

 mm. in diameter, consist of an intricate complex formed by the intertwining of the richly 

 branching axones ascending from the olfactory cells and of the dendrites descending from 

 the mitral cells. The interval between the upper and lower margins of the second stratum is 

 occupied by the molecular layer, composed of small nerve-cells whose dendrites also enter the 

 glomeruli. (3) The stratum of central fibres includes the centrally directed axones of the 

 mitral and other nerve-cells which constitute the second link in the complicated paths by which 

 the olfactory stimuli are carried to the cortical areas. The outer zone of this stratum is known 



Fig. 993. 



Olfactory bulb 



Olfactory tract 



Mesial 

 olfactory stria 



Lateral 

 olfactory stna 



Island of Reil 



Anterior 

 perforated space 



Cut surface of 

 temporal lobe 



Cerebral peduncle 

 crossed by 

 optic tract 



Lateral 

 geniculate body 



Olfactorj' sulcus 

 Parolfactory area 



Tuberculum 

 olfactoriuni 



Trigonum 

 olfactorium 



Optic chiasm, 

 partly cut away 



Mammillary body 

 in interpeduncular 

 space 



Oculomotor nerve 



Cerebral peduncle 



Pulvinar 

 Median geniculate body 



Sylvian aqueduct 



Anterior part of* inferior surface of brain, showing parts of olfactory lobe and structures within interpedun- 

 cular space.; tip of right temporal lobe has been removed. 



as the granular layer and consists of man)- small nerve-cells intermingled with the fibres. The 

 deeper part of the stratum of nerve-fibres encloses some larger nerve-cells of stellate or 

 elongated form. The central part of the bulb, which represents the obliterated ventricular 

 space, is filled by a gelatinous substance resembling modified neuroglia. 



The olfactory tract Ttractus olfactorius) is a narrow band of light color, which 

 extends from the olfactory bulb in front to the olfactory trigone behind (Fig. 993). 

 It measures about 2 cm. in length and 2.5 mm. in width, but is broader at its pos- 

 terior extremity, from which the olfactory strice, as its roots are called, dix-erge. Its 

 ventral surface is flat and its narrow dorsal one ridged, the tract appearing in 

 transverse section more or less triangular in outline. 



The structure of the olfactory tract further emphasizes the rudimentary condition of the 

 part in man. The ventral aspect and the rounded adjoining borders consist of : (i) a stratutn 

 of nerve fibres, longitudinally coursing and therefore transversely cut in cross-sections, wliich 

 covers the sides and dorsal surface of the tract and is reduced to an extremely thin and rudimen- 

 tary sheet. Next follows (2) 2i gelatinous stratum, which represents the obHterated ventricular 

 cavity seen in many lower animals. .Succeeding this and forming the thickest layer of the tract 

 /ies (3) the dorsal stratum of gray matter, which still retains its importance as a tract of cortical 

 gray substance from which fibres pass toother parts of the hemisphere (page 1222). 



