294 ESSENTIALS OF ZOOLOGY 



relatively than that of the lower vertebrate, and during the 

 history of the mammal it has increased in size. The increase 

 has especially affected the free cortex, which has been augmented 

 in area and in thickness. The materials have not been greatly 

 changed, but the number of the units has been multiplied. 



This means that mental capability has advanced upon a 

 basis of expansion of the cerebrum, and this is true not only of 

 the history of the Vertebrates as a whole, but also of the classes ; 

 not only of the Mammalia, but of the orders into which that 

 class has been resolved. 



A section of the hemisphere will show that it is made up 

 superficially of grey matter and internally of white matter. 

 Microscopically, the former consists of nerve cells, the outer 

 of which are pyriform and the inner of the more general 

 multipolar type. In man the pyriform area is the more promi- 

 nent, and in the lower mammals the deeper types of cells are 

 greater in number. These nerve cells divide into branching 

 processes, called dendrites, and give off each an axon. The 

 grey matter, therefore, is made up of the nerve cells and their 

 dendrites and the proximal and distal ends of the axons, all 

 supported by neuroglia, the connective tissue of the central 

 nervous system. The white matter below is made up of a 

 multitude of axons which connect the cerebral nerve cells 

 with other areas on the other side by commissures, with other 

 parts of the same hemisphere by association fibres between 

 them, and with other parts of the nerve system by sensory or 

 afferent and motor or efferent fibres. The dendrites and the 

 terminal axons are not continuous, but they are close to one 

 another, and these interruptions are called synapses. Ex- 

 periment has shown also, and pathology has emphasised the 

 fact, that certain parts of the cortex are concerned with special 

 senses, others with association, and that the higher and the 

 more intelligent the mammal, the more do these areas become 

 specialised. But while this is true, it is also true that the whole 

 cerebrum is more or less involved, and works as a whole. The 

 cerebrum, developed phylogenetically and ontogenetically in 

 association with the olfactory sense, has become the centre 

 of all the senses. 



The thalamencephalon is formed mainly of the immensely 



