270 The CerebraJ Cortex in Cam's Fainiliaris 



spends in position with a fissuret, present in other animals, e.g. Felis Leo, and most Ursidae and 

 Pinnipedia, known as the precrucial fissure. 



The remainder of the anterior boundary slopes forwards and outwards across the free surface 

 of the anterior limb of the sigmoid gyrus to reach the upper extremity of the orbital fissure, and 

 the latter completes the limit. 



In the lateral direction the area does not extend beyond the line of the coronal fissure. 



Reference to the figures will show the position of the posterior limit. In particular, I would 

 draw attention to the point that motor cortex does not reach as far back as the ansate fissure, 

 and that it is limited in a remarkable manner by the small and insignificant looking " compensatory 

 ansate " fissuret, marked " homologue of Rolando " in the diagram. This fissuret has been previously 

 alluded to (motor area, Felis) as the probable homologue of the primate sulcus central is. 



POSTCRUCIAL OR SENSORY AREA. 



This field is perhaps more readily defined in ('anis than Felis. Reference to the figures will 

 show how it forms a posterior buffer zone to the motor area. It just appears on the inner surface ; 

 on the dorsal aspect it lies anterior to the ansate fissure, and, laterally, it covers the caudal two- 

 thirds of the gyrus coronalis. 



As in Felis, the distinguishing cell feature is the existence of two layers of large pyra- 

 midal cells, separated by a lamina of stellate elements. In examining the part care must be 

 exercised not to confuse it with motor cortex, for the internal large pyramidal cells have some 

 of the shape and appearance of Betz cells, and one is apt not to recognise that they are of 

 smaller size ; again, as the large cells of the outer layer are not numerous, they may be over- 

 looked. 



This lamination is accompanied by a remarkable arrangement of fibres. Without doubt there 

 is no part of the cortex in which the fibre wealth is greater, and the apparent density is in- 

 tensified by the presence of an extraordinary abundance of fibres of large calibre. This feature 

 was marked in Felis, but it is even more pronounced in Canis. 



Concerning that part of the gyrus suprasylvius lying just behind the junction with the gyrus 

 coronalis, and also that part external to the ectolateral gyrus (marked by coarse dots in the 

 diagram), I am doubtful whether to include them in the postcrucial area or not. In a transverse 

 section stained for nerve fibres the cortex is obviously denser than that of the gyri above and 

 below, and it also contains a few large medullated fibres, but, at the same time, its general wealth is 

 certainly less than that of the field just described. Moreover, while there is no doubt of the existence 

 of two layers of large pyramidal cells, the individual cells are distinctly smaller than they were 

 anteriorly. 



PARIETAL AREA. 



This area has a distribution resembling that in Felis, and its microscopic appearances are 

 equally distinctive. 



In sections stained for nerve fibres, its cortex looks pallid, the radiations are slender, and the 

 only large fibres present are cut transversely and lie in the depths of the cortex adjoining the 

 white substance. 



In reference to cells, the absence of large internal pyramids is a negative point of some 

 importance, while the presence of a substellate layer of rather large, elongated, intensely stained 

 and prominent pyramidal cells, is a positive differential character. 



