204 PH.YSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



the ears and turning of the head and eyes to the opposite side. 

 As in the case of the visual area, therefore, we must suppose that 

 distinct motor paths originate in the auditory region, and it is 

 natural to suppose that these paths give a means for cortical reflex 

 movements, folio wing upon auditory stimulation. 



The Olfactory Center. The olfactory sense is quite un- 

 equally developed in different mammals. Broca divided them from 

 this standpoint into two classes: the osmatic and the anosmatic 

 group, the latter including the cetacea (whales, porpoise, dolphin). 

 The osmatic group in turn has been divided into the microsmatic 

 and macrosmatic animals, the latter class including those animals 

 in which the sense of smell is highly developed, such as the dog 

 and rabbit, while the former includes those animals, such as man, 

 in which this sense is relatively rudimentary.* The peripheral end- 

 organ of smell consists of the olfactory epithelium in the upper 

 portion of the nasal chambers. The physiology of this organ will 

 be considered in the section on special senses. The epithelial 

 cells of which it consists are comparable to bipolar ganglion 

 cells. The processes or hairs that project into the nasal chamber 

 are acted upon by the olfactory stimuli, and the impulses thus 

 aroused are conveyed by the basal processes of the cells, the olfac- 

 tory fibers, through the cribriform plate of the ethmoid bone into 

 the olfactory bulb. 



The Olfactory Bulb and its Connections. The olfactory 

 bulbs are outgrowths from and portions of the cerebral hemi- 

 spheres. Each bulb is connected with the cerebral hemispheres 

 by its olfactory tract. The connections established by the fibers 

 of this tract are widespread, complicated, and in part incom- 

 pletely known. All those portions of the brain connected with 

 the sense of smell are sometimes grouped together as the rhinen- 

 cephalon. According to von Kolliker, the parts included under 

 this designation are, .in addition to the olfactory bulb and tract, 

 Ammon's horn, the fascia dentata, the hippocampal lobe, the fornix, 

 the septum lucidum, and the anterior commissure. The schematic 

 connections of the olfactory fibers are as follows (Fig. 90) : After 

 entering the olfactory lobe the fibers terminate in certain globular 

 bodies, the glomeruli olfactorii (5), whose diameter varies from 0.1 to 

 0.3 mm. Here connections are made by contact with the dendrites 

 of nerve cells of the olfactory lobe, the mitral and brush cells ((7). 

 The axons of these cells pass toward the brain in the olfactory tract. 

 Three bundles of these fibers are distinguished: (1) The precommis- 

 sural bundle, the fibers of which terminate in part in nerve cells sit- 

 uated in the tract itself, but, for the most part, enter the anterior 

 commissure and pass to the same or the opposite side, to end in the 

 * See Barker, "The Nervous System," 1899, for references to literature. 



