656 THE NERVOUS SYSTEM. 



takes origin, and by the approximation of these opercula, as they grow over the surface of the 

 fossa, the insula becomes closed in and the rami of the lateral fissure are formed (Fig. 583, C). 



The lateral fissure is an example of the fourth category of furrows enumerated 

 above. It is largely the result of the operation of the mechanical factors incidental 

 to the bending downwards of the pallium in front of and behind the place where 

 the hemisphere-wall is supported and held in position by the corpus striatum. 

 The cortical area roughly corresponding to the surface of the corpus striatum is the 

 insula ; the temporal region extends downwards behind it, and to a less extent 



the frontal region 

 in front of it (Fig. 

 583, A). Then 

 towards the end of 

 the fifth month of 

 foetal life the ex- 

 uberant growth of 

 the free fronto - 

 ^^^ parietal pallium 



v v^. /""%/ T ^^ I above the insula 



^PHHMBB^^ ^-^:-: ~m*^F ( Fi g- 5S3 > B ) aiid 



c ^^^ B the temporal pal- 



FIG. 583. RIGHT HEMISPHERES OF HUMAN FCETUSES SHOWING THREE STAGES l lum below and 



IN THE DEVELOPMENT OF THE INSULA AND THE INSULAR OPERCULA. behind it leads to 



A, Right cerebral hemisphere from a foetus in the latter part of the fourth month the development of 



of development ; B, Right cerebral hemisphere from a foetus in the fifth lip-like folds of n60- 



month of development ; C, Right cerebral hemisphere from a foetus in the |i 



latter part of the eighth month of development. 

 In C the temporal operculum has been removed, and thus a large part of the CUla which gradu- 



insula is exposed. The outline of the temporal operculum is indicated by a ally approach One 



F.P, Superior operculum. F, Frontal operculum. 0, Orbital opejculum. another (Fig. 583,0) 



and eventually 



cover up the insula. Other factors come into play in determining the form and 

 topographical relations of the fissura lateralis. For example, the posterior part 

 of the fissure is the morphological boundary between the acoustic and tactile terri- 

 tories of the neopallium. 



THE ACOUSTIC AEEA AND FIBEE-TKACTS. 



In the embryo of the fifth month (Fig. 583, B), as well as in every later stage, 

 even up to the adult condition (Fig. 582), an area upon the superior surface of 

 the temporal operculum can be seen to slope medially towards the upper limb of 

 the sulcus circularis, behind the insula. This area constitutes the receptive centre 

 for acoustic impressions the gyrus temporalis transversus or Heschl's convolution 

 although the extent of this acoustico-sensory area does not coincide exactly 

 with that of the transverse temporal gyrus. The area formed by the upper surface 

 of the temporal operculum immediately behind this prominent transverse gyrus 

 is also called by the same name, so that there are anterior and posterior transverse 

 temporal gyri (Fig. 582; the posterior transverse temporal gyrus is not labelled in 

 the figure). 



In studying the brain-stem we have seen that a tract of fibres originating in 

 the cochlear terminal nuclei (in the medulla oblongata) crosses the median plane 

 (corpus trapezoideum) and bends upwards in the lateral lemniscus of the other 

 side (Fig. 580) to end in the medial genie ulate body of the metathalamus. 

 From the medial geniculate body a new tract arises (composed of tertiary 

 acoustic neurones), which passes laterally (Figs. 580 and 584) to end in the 

 transverse temporal gyri. This tract may be called the radiatio thalamotemporalis. 



The area into which this acoustic radiation is inserted occupies not only the 

 region of the anterior transverse temporal gyrus (Fig. 582) hidden within the lateral 

 fissure, but also extends over its inferior lip, on to the exposed surface of the superior 

 temporal gyrus (Fig. 581). Surrounding this area there are two concentric bands, 



