216 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



as the superior nucleus of the lemniscus. From this second or third 

 termination another set of fibers, the auditory radiation, continues 

 forward through the inferior extremity of the internal capsule to end 

 in the superior temporal gyrus (see Fig. 82, E). According to 

 Flechsig,* who has studied the course of these fibers in the 

 embryo by the myelinization method, the main, group passes 

 from the median geniculates to the transverse gyri of the tem- 

 poral lobe within the lateral fissure of the cerebrum (fissure 

 of Sylvius). The median geniculates, in man at least, have, 

 therefore, the function of a subordinate auditory center, as the 

 lateral geniculates have the function of a subordinate visual 

 center. The median geniculates are connected with the inferior 

 colliculus, and also, it will be remembered, with each other, by 

 commissural fibers (Gudden's commissure) that pass along the 

 optic tracts and the inferior margin of the chiasma. The 

 auditory path, therefore, involves the following structures: 

 The spiral ganglion, the cochlear nerve, accessory nucleus and 

 tuberculum acusticum, corpus trapezoideum, medullary striae, 

 superior olivary, lateral lemniscus, inferior colliculus, median 

 geniculate, Gudden's commissure, auditory radiation, and 

 temporal cortex. 



The Motor Responses from the Auditory Cortex. According 

 to Ferrier, stimulation of the cortex of the temporal lobe (inferior 

 convolution) causes definite movements, such as pricking of 

 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 following 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 (whale, 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, f 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 



* Flechsig, " Localisation der geistigen Vorgange," Leipzig, 1896. 



t See Barker, " The Nervous System,'' 1899, for references to literature. 



