868 THE NERVOUS SYSTEM 



callosum may be considered among the association pathways, since they extend be- 

 tween the grey substance of the hippocampus (dentate gyrus) and the subcallosal 

 gyrus of the same hemisphere. 



(7) Likewise the longitudinal fibres in the stria terminalis of the thalamus 

 (teeniasemicircularis) may be considered among the association pathways, since these 

 connect the amygdaloid nucleus with the anterior perforated substance. 



(8) The numerous fibres passing in both directions between the cerebral cortex 

 and the nuclei of the corpus striatum belong to the association system. These do not 

 form a definite bundle, but, instead, contribute appreciably to the corona radiata. 

 However, a pathway described as the occipito-jrontal fasciculus probably consists 

 largely of the more sagittally running fibres of this nature. The existence of this 

 fasciculus has been noted in degenerations and in cases of arrested development of the 

 corpus callosum. Its fibres are described as contributing greatly to the tapetum, and 

 as coursing immediately next to the ependyma of the lateral ventricle. As a mass, 

 they appear in intimate connection with the caudate nucleus, and are spread towards 

 both the frontal and the occipital lobes (chiefly the latter), in the mesial part of the 

 corona radiata of those lobes. 



(9) Since the olfactory bulb is a part of the hemisphere proper, the olfactory 

 tract may be considered an association pathway connecting the olfactory bulb with 

 the parolfactory area, the subcallosal gyrus, the anterior perforated substance, and 

 the uncus. As already shown, a portion of the fibres of the tract belongs to the 

 commissural system. 



THE FUNCTIONAL AREAS OF THE CEREBRAL CORTEX 



The known areas of specific function of the human cerebral cortex are relatively small (fig. 

 644). They comprise but little more than a third of the area of the entire hemisphere. They 

 are (i) the area of general sensibility or the somsesthetic area, and (2) the areas for the organs 

 of special sense. They represent portions of the cortex in which terminate sensory or ascending 

 projection fibres bearing impulses from the given peripheral structures, and in which arise motor 

 or descending projection fibres bearing impulses in response. 



Knowledge of the location of the areas lias been obtained (1) by the Flechsig method of 

 investigation, and to a considerable extent by Flechsig himself; (2) from clinico-pathological 

 observations, largely studies of the phenomena resulting from brain tumors and traumatic 

 lesions; (3) by experimental excitation of the cortex of monkeys and apes, the resulting phe- 

 nomena being correlated with the anatomical findings and compared with the observations upon 

 the human brain. The remaining larger and less known areas of the cortex are referred to as 

 'association centres' or areas of the 'higher psychic activities.' 



In development, the sensory fibres to the specific areas acquire their medullary sheaths 

 first, before birth, and then the respective motor fibres from each become medullated. It is 

 not till a month after birth that the association centres show medullation and therefore acquire 

 active functional connection with the specific areas. 



In defining an area it is not claimed that all the fibres bearing a given type of impulse terminate 

 in that area, nor that all the motor fibres leading to the given reaction originate in the area 

 It can only be said that of the fibres concerned in a given group of reactions, more terminate and 

 arise in the given area than in any other area of the cortex. The corresponding motor fibres 

 arise both in the region of the termination of the sensory fibres (sensory area) and also in a zone 

 (motor area) either partia'lly surrounding or bordering upon a part of the region of termination. 



The different areas are as follows: 



(1) The somaesthetic (sensory-motor) area, the area of general sensibility, and the area in 

 which arise the larger part of the cerebral motor or pyramidal fibres for the cortical control 

 of the general'muscular system. As is to be expected, it is the largest of the specific areas. It 

 includes the anterior central gyrus, posterior central gyrus, the posterior ends of the superior, 

 middle, and inferior frontal gyri, the paraceiitral lobules, and the upper portion of the adjacent 

 part of the gyrus cinguli. The ascending or sensory fibres are said to terminate most abundantly 

 in the part posterior to the central sulcus (Rolandi), the posterior central gyrus being the special 

 area of cutaneous sensibility, and the adjacent anterior ends of the horizontal parietal gyri have 

 been designated as the area of 'muscular sense.' Both these areas are carried over upon the 

 medial surface to involve the lower part of the paracentral lobule and a part of the gyrus cinguli. 

 The anterior central gyrus gives origin to relatively more motor fibres than the other portions of 

 the somsesthetic area. In distribution, the muscles furthest away from the cortex are innervated 

 from the most superior part of the area, the leg area being in the supero-mesial border of the 

 hemisphere, while that from the head is in the anterior and inferior part of the area (fig. 644). The 

 muscles of mastication and the laryngeal muscles are controlled from the fronto-parietal oper- 

 culum. Broca's convolution of the left side constitutes the especial area of speech, and Mills 

 has extended this area to include the supero-anterior portion of the'insula below. 



(2) The visual area. The especial sensory portion of this area is that immediately border- 

 ing upon either side of the posterior part of the calcarine fissure. '\ he entire area, motor and 

 sensory overlapping each other, includes the whole of the cuneus. The motor visual area proper 

 is described as the more peripheral portion of the entire area. In addition, there is a small area 

 producing eye movements situated in the posterior end of the middle frontal gyrus an anterior 

 extension of the somfesthetic area. 



