486 A MANUAL OF VETERINARY PHYSIOLOGY 



relative weight of brain to body is of limited value. The modern 

 method of determining the mind of an animal from the structure 

 of the cortex of the cerebrum will be dealt with later on. 



Structure of the Cortex. — The cerebral cortex is regarded as 

 made up of several layers of cells, the number of which is not 

 agreed upon by histologists, but may be regarded as consisting 

 of four principal layers. These layers vary in different animals, 

 and we are compelled, as in other portions of the physiology of 

 the nervous system, to fall back upon what is known of the 

 matter in the dog, pig, and man, in the absence of direct infor- 

 mation regarding the horse. The layer of cells immediately 

 beneath the pia mater is spoken of as the molecular ; it consists 

 of some very small nerve-cells with their dendrites and axons, 

 also of dendrites projecting from cells deeper seated in other 

 layers, and the terminal processes of axons which end there 

 belonging to fibres coming from other regions. Perhaps the dis- 

 tinct feature of the layer is the structure first spoken of, the 

 small cells with their dendrites and axons. These begin and 

 end in this layer, and appear to be of a linking-up, or, as it will be 

 described later, associative nature. In the second layer are small 

 pyramidal cells, with the apex towards the surface and the axon 

 passing inwards to the white substance. The depth of this layer 

 increases the higher the animal scale is ascended. Next comes 

 a layer of large pyramidal cells, sometimes called ' giant pyramids 

 of Betz/ found mainly in that portion of the brain anterior 

 to the fissure of Rolando,* and in particular in the region of that 

 fissure. These cells are of great physiological importance, as 

 from them the fibres constituting the great motor tracts are 

 derived. The last layer is that of polymorphous cells, which are, 

 generally speaking, small, and many of them fusiform in shape. 

 With these are cells of Golgi of the second type ; but whereas 

 the axons of the former pass inwards to form white fibres in 

 the medullary portion, the axons of the Golgi cells pass outwards 

 and end by arborising in the molecular layer. The function of 

 the cerebral cortex has been examined, not only by the method 

 of direct experimental excitation, but also from its histological 

 side ; this has enabled the cortex to be mapped out into different 

 regions, and in this connection it is desirable to make it clear 

 that the histological method does not wholly consist in com- 

 paring the structure of one fully-developed area with another, 

 but in studying the period when the fibres leading to the area 

 assume their sheath of myelin, both in the embryo and young 



* It is convenient for the present to retain the term ' fissure of Rolando '; 

 it will be pointed out later that there is no fissure of Rolando in the dog, 

 nor is the crucial fissure of this animal, in the vicinity of which the motor 

 area is located, the equivalent of the fissure of Rolando. 



