6io TEXT-BOOK OF PHYSIOLOGY 



Association Centers. The sensor and motor areas to which specific 

 functions have been assigned do not constitute more than one-third of the 

 total cerebral cortex. There yet remain large regions to which it has been 

 impossible to assign specific functions based on physiologic experiments. 

 Three or four such regions separated by the sensor and motor centers are to 

 be recognized on the lateral and mesial aspects of the hemisphere. In Fig. 

 251 the location, extent, and names of these regions are represented. The 

 fibers which are found in these regions belong almost exclusively to the 

 association system, and become medullated at a later period than do the fi- 

 bers of the projection system; moreover, from the method of their medulliza- 

 tion it would appear that many of these fibers grow out directly from the 

 sensor centers into these regions and become related to the nerve-cells of 

 their convolutions, while others grow out from adjacent as well as distant 

 convolutions. From histologic and pathologic evidence these regions were 

 termed by Flechsig association centers or areas, implying the idea that 

 through the intervention of their cell mechanisms the sense areas are in- 

 directly associated anatomically and physiologically, and together constitute 

 a mechanism by which sensations are associated and elaborated into concrete 

 forms of knowledge or related to definite forms of movement. 



It has been assumed by Flechsig that the frontal association center, from 

 its connections with the sensor and motor areas of the Rolandic region, the 

 olfactory, and perhaps other regions, is engaged in associating and registering 

 body sensations and volitional acts, and that the knowledge thus gained has 

 reference largely to the personality of the individual ; that the parieto-occipital 

 association area, from its relation to the visual, auditory, and tactile sense 

 areas, is engaged in associating and registering visual, auditory, and tactile 

 sensations, and that the knowledge thus gained has reference mainly to the 

 external world. These assumptions in a general way are supported by the 

 phenomena of disease. In certain lesions of the frontal lobe the symptoms 

 indicate a loss or change of ideas regarding personality rather than of the 

 objective world, while the reverse is true in disease of the parieto-occipital 

 lobe. 



The Intra-cranial Circulation. The circulation within the cranium 

 presents certain peculiarities which distinguish it from that in other parts of 

 the body. These peculiarities reside in part in the anatomic arrangement of 

 the blood-vessels, in the probable absence of vaso-motor nerves to the blood- 

 vessels, and in greater part in the fact that the brain and its blood-vessels 

 are contained in a case with rigid, unyielding, and closed walls. 



The Blood-supply. As stated in a previous paragraph the arteries 

 supplying the brain with blood are four in number, viz. : the two internal 

 carotids and the two vertebrals. These four arteries anastomose very freely 

 at the base of the brain, the anastomosis constituting the circle of Willis. 

 From this circle there arise the anterior, middle and posterior cerebral arteries 

 which are distributed to the cortex and the underlying white matter. The 

 basal ganglia, the capsule and adjacent white matter are supplied by a num- 

 ber of branches which arise from the circle of Willis or from the three cerebral 

 arteries immediately after their origin. From the distribution of these two 

 sets of vessels they have been named the cortical and the central ganglionic 

 respectively. 



The venous blood is returned by a system of vessels which present charac- 



