698 STRUCTURE OF THE CEREBRUM [CH. XLY1II. 



little difference seen here between man, monkey, and dog. The 

 second layer, on the other hand, is concerned with the psychic or 

 associational functions, and attains its maximum depth in man. 



The archipallium (p. 646) is the portion of the cortex which 

 makes it appearance earliest in vertebrates, and is associated with 

 the rhinencephalon or olfactory lobe of the pala-encephalon. In 

 mammals it is reduced to small proportions in comparison with the 

 rest of the cortex, which is termed, on account of its later appearance 

 in historical development, the neopallium. In man, the archipallium 

 is doubled in to form the hippocampus major, which projects into the 

 lateral ventricle; this is continuous externally around the dentate 

 sulcus with the gyrus hippocampi. This part of the cortex is easily 

 distinguishable from the neopallium, being much simpler in structure 

 the pyramids, for instance, are reduced to a single layer, and the 

 smaller cells nearer the surface are grouped in a characteristic nest- 

 like way. 



The White Matter of the Cerebrum. 



The white matter of the cerebrum, like white matter elsewhere, is 

 made up of medullated nerve fibres. According to the direction of 

 the fibres, they may be divided into three principal groups : 



1. Association fibres. These pass from convolution to convolution, 

 and the principal bundles of these are shown semi-diagrammatically 

 in fig. 436. 



2. Commissural fibres. These pass by the commissures of the 

 brain, of which the most important is the corpus callorum, so as to 

 link the convolutions of one hemisphere with the corresponding 

 convolutions in the opposite hemisphere, where they terminate in 

 arborisations (synapses) around the cells of the grey cortex. 



3. Projection fibres. These are the fibres which run more or 

 less vertically and link the cerebrum to the lower portions of the 

 central^ nervous system. They may be subdivided into the efferent 

 projection fibres, which convey impulses downwards, and the afferent 

 projection fibres, which convey impulses upwards. They are shown 

 semi-diagrammatically in fig. 437. 



The term projection fibre almost explains itself. By means of 

 the efferent projection system the cerebrum is able to project its 

 impulses to the cord, and thence to the muscles at the periphery. 

 By means of the afferent projection system the surface of the body, 

 provided as it is with sense organs, is able to project its impulses to 

 the seat of sensation. 



The efferent projection system. The most important portion of 

 this is the cerebro-spinal motor tract. The cells of the cortex which 

 give rise to these fibres are par excellence the Betz cells, which are 

 found in the fourth layer in the region of the brain, known as the 



