.. ------Posterior carmen tracts The impulse is carried from the periphery to the ganglion of the corresponding nerve, and thence to the end-nucleus within the brain-stem. To the peripheral neurone I is added the central neurone II. The latter arises within the sensory end-nucleus, its axone, the efferent nerve-fibre, passes upward with the medial fillet and ends within the thalamus. From here the neurone III extends to the cerebral cortex.
c. The path for the visceral impulses, from the lungs, heart, oesophagus, stomach, etc., lies in the vagus and the sympathetic.
d. The path for the equilibrium impulses lies in the vestibular nerve, supple mented by spinal fibres. The path leads to the cerebellum, thence by way of the superior cerebellar peduncle to the nucleus ruber and the thalamus, and then to the cerebral cortex.
e. The path for the taste impulses lies in the glossopharyngeus, the intermedius and the third division of the trigeminus. Neurone I leads from the periphery (the tongue) to the end-nucleus (nucleus of the tractus solitarius); neurone II from the end-nucleus to the thalamus ; neurone III from the thalamus to the cortical gus tatory centre.
Concerning the paths, which serve to conduct the gustatory impulses, the following may be noted. It is generally accepted, that the taste impulses from the anterior two thirds of the tongue are conveyed centrally by the lingual branch of the trigeminus; from the posterior third of the tongue, by the glossopharyngeus. While the course of the taste-fibres by means of the glossopharyngeal nerve is readily understood, opinions con cerning the path followed by the taste-fibres from the anterior two-thirds of the tongue vary. Thus, it is assumed by some, that these fibres run backward in the chorda tym pani to the ganglion geniculi • and thence proceed, either through the great superficial petrosal nerve to the spheno-palatine ganglion and on centrally by the maxillary division of the trigeminus, or through the small superficial petrosal nerve to the otic ganglion and centrally by the mandibular nerve. According to others, the chorda fibres reach the
glossopharyngeus by way of the small superficial petrosal and the tympanic nerve. It is assumed, further, that not only the chorda fibres, but also the gustatory fibres of the glossopharyngeus, by way of the small superficial petrosal nerve, reach the trigeminus and in it pass centrally. Finally, according to the view which seems, perhaps, the most reasonable, the chorda fibres pass, by way of the chorda tympani, to the geniculate gan glion and thence, by way of the nervus intermedius, to the medulla oblongata, where they form a descending root, which ends in the nucleus tractus solitarii, that is, the sensory end-nucleus of the glossopharyngeus. This view, further, is supported by the experi mental investigations, which have shown, that removal of the Gasserian ganglion or intra cranial section of the maxillary and the mandibular divisions of the trigeminus are not followed by loss of taste in the anterior two-thirds of the tongue.
f. The path for olfactory impulses leads from the olfactory mucous membrane by way of the fila olfactoria to the bulbus olfactorius, thence to the primary centres and from the latter to the secondary or cortical olfactory centre within the gyrus hippocampi.
g. The path for auditory impulses lies within the nervus cochleae. Neurone I conveys the stimulus from the hair-cells of Corti's organ to the end-nucleus. Neurone II passes from the end-nucleus to the corpus geniculatum mediate and to the inferior colliculus, the fibres forming the lateral fillet. Neurone III unites the corpus geniculatum mediale with the cortical auditory centre within the gyrus temporalis superior.
h. The path of the visual impulses lies within the nervus opticus. Neurone I extends from the retina to the corpus geniculatum laterale, to the pulvinar and to the superior colliculus. Neurone II connects the corpus geniculatum laterale and the pulvi nar with the secondary or visual centre within the cortex of the cuneus.