i66 



THE QUARTERLY REVIEW OF BIOLOGY 



that no pharmacological substances 

 exhibit exactly the same array of bio- 

 logical reactions as snake- venoms ; in other 

 words, they are specific or distinctive 

 products. It has often been stated that 

 the poison-glands of snakes are homolo- 

 gous with the parotid glands of mammals, 

 and this particular example was long 

 considered by the author as a crucial 

 case, rendering untenable the supposition 

 that homology and intrinsic functional 

 mechanism always coincide. Owing to 

 their different innervation the venom- 

 glands of snakes are now known not to 

 correspond to the parotid gland of mam- 

 mals. As to their origin, Wiedersheim 

 says that they "become differentiated 

 from" the upper of the two pairs of 

 labial mouth glands common to reptiles. 

 We thus see that in accordance with the 

 individuality of their intrinsic functional 

 mechanism they have no known homo- 

 logues in other animals. 



The cerebral cortex. As another case let 

 us take the grey matter on the surface of 

 the mammalian cerebrum, the so-called 

 cerebral cortex. This is spread over the 

 surface in a layer whose thickness and 

 general cellular composition present an 

 appearance of unusual uniformity. At 

 the same time experiment has displayed 

 a definite association of particular func- 

 tions with particular areas of the cortex, 

 one region controlling voluntary move- 

 ments of different parts of the body, 

 another being concerned with conscious 

 vision, another with hearing, and so on. 

 It is plain that the intrinsic mechanism 

 of these different regions must vary. An 

 area whose duty is to control voluntary 

 movement must differ in ultimate action 

 from one which receives and interprets 

 visual impressions. One of the achieve- 

 ments of recent histological examination, 

 beginning with Cajal and continued by 

 A. W. Campbell and by Brodmann, has 



been to show that on a basis of simple 

 histological structure the cortex may be 

 mapped into a large number of regions, 

 each with its own distinctive characters. 

 Instead of being of uniform texture it is 

 a mosaic of areas of different histological 

 pattern. While this field of tedious and 

 time-consuming histological work is as 

 yet in its infancy and hard and fast- 

 statements can scarcely be made, in two 

 cases at least, viz. that of the motor 

 region and of the visual region, the limits 

 as histologically defined correspond with 

 those experimentally determined. 



The facts with regard to the motor 

 area are of especial interest in this connec- 

 tion. At quite an early period of investi- ; 

 gation of cerebral localization Bevan 

 Lewis and Henry Clarke, on the basis of 

 a histological difference between the pre- 

 Rolandic and the post-Rolandic cortex, 

 concluded that the motor region proper i 

 lies in front of the fissure of Rolando. 

 This deduction did not agree with the ( 

 findings of subsequent experimenters, who, 

 using strong electrical stimulation ob- 

 tained motor effects on excitation of the : 

 cortex both behind and in front of the i 

 Rolandic fissure. The histological com- 

 munication of these authors was in : 

 consequence ignored. Griinbaum and : 

 Sherrington, employing a more refined: 

 experimental technique, eventually proved : 

 that the motor area is strictly limited to : 

 the pre-Rolandic region. In this case: 

 histological differentiation between two , 

 areas had been able to indicate differences I 

 in their function which took more than 

 twenty years fully to confirm. J. S. 

 Bolton by histological methods first 

 determined the exact limits of the visuo- 

 sensory area of the cortex, which limits 

 corresponded with those already estab- 

 lished by the method of combined stimu- 

 lation and ablation. 



We should have no hesitation in naming 



