92 ESSENTIALS OF PHYSIOLOGY. 



fibres have cell stations in the trapezoid and superior olivary nuclei. 

 The lateral fillet terminates in the inferior colliculus and internal 

 geniculate body ; from the latter new fibres arise to be distributed to 

 the superior temporal convolution as the auditory radiation. 



The Area for Smell and Taste. The area for smell and taste is 

 located in the hippocampal convolution and the neighbouring structures. 

 Extirpation of this area has not yielded definite results. Stimulation 

 causes movements of the lip and nostril on the same side, such as 

 would be caused by a disagreeable or irritating odour applied to the 

 nostril. Further information is derived from a comparative study of 

 the degree of development of these parts of the brain in animals which 

 have, and others which have not, a highly developed sense of smell. 



The olfactory nerve fibres are non-medullated processes of cells in 

 the olfactory mucous membrane, and terminate in structures known as 

 glomeruli in the olfactory lobe in relation with the dendrites of certain 

 " mitral " cells, the axons of which convey the impulses transmitted to 

 them by the olfactory fibres to the hippocampal region of the same or 

 the opposite side. 



The nerves of taste are the chorda tympani to the anterior two- 

 thirds and the glossopharyngeal to the posterior third of the tongue. 

 The taste fibres of both nerves terminate in a column of cells in the 

 pons, which also receives afferent fibres from the fifth nerve. The con- 

 ducting tract from this nucleus to the cortical area for taste has not 

 been traced. 



The Association Areas. There are three great association areas : 

 the posterior in the parieto-temporal region, the middle in the island 

 of Reil, and the anterior or pre-frontal. Stimulation of these areas 

 gives rise to no obvious motor response, but disease or imperfect develop- 

 ment in man is accompanied by various forms of mental deficiency. 

 The pre-frontal area is the highest associational centre : it is the last to 

 develope and the first to retrograde. In it the outer cell lamina attains 

 its greatest depth, and atrophy of the cells of this lamina is found in 

 cases of dementia. The posterior association area, lying between the 

 visuo-psychic and the audit o- psychic areas, is concerned with mental 

 images, especially with the processes involved in the perception of 

 spoken and written language. Lesions of this area are accompanied 

 by interference with the appreciation of words, the different forms of 

 sensory aphasia. 



The human brain is characterised by the great development of the 

 association areas. They represent the material basis for the memory 

 of past stimuli, and for the comparison of one set of stimuli with 

 another. In other words, they are the anatomical structures concerned 



