THE NERVOUS SYSTEM 



501 



old type of brain with which it is furnished the olfactory nerves 

 spread out into an area of considerable size. In the new 

 type of brain — viz., one with cerebral hemispheres — the olfactory 

 lobes form the principal part of the earlier structures, and in 

 relatively high types of brain, such as are met with in the 

 domestic animals, the olfactory lobes are large and important. 

 In man, who has a poor sense of smell, they have atrophied, 

 and in the whale they have practically disappeared. The 

 cortical centre for smell lies in the region of the hippocampus 

 and temporal lobe, but the central connections of the olfactory 

 apparatus are not fully known. The Centre for Taste is ultimately 

 mixed up with that of smell, but its precise locality is unknown. 

 With the Centre for Speech we have nothing to do. It is interest- 

 ing to know that stimulation of the corresponding area in the 

 ape does not lead to the production of voice. 



When the sense and motor areas are removed from the 

 cortex, there is a considerable amount of substance left not 

 associated with either of these functions. These have been 

 termed by Flechsig association areas; the term silent areas, 

 employed by some physiologists in speaking of them, appears 

 especially suitable to veterinary physiology. These association 

 or silent areas are the region of the higher intellectual faculties ; 

 the organs of thought, the region in which impressions conveyed 

 to the sense-centres are interpreted, for it is through the sense 

 centres that intelligence is developed. An association area in the 

 anterior part of the frontal region of man is an important intel- 

 lectual centre, though of secondary importance to one situated 

 in the parietal region posterior to the fissure of Rolando. 



It will be observed that up to this point the functions of the 

 various parts of the cerebrum have been largely determined by 

 direct experiment or pathological observation. This, however, 

 as we indicated previously, is not the only method of inquiry 

 open. Flechsig's embryological method has furnished important 

 results. It will be remembered that it is based on a knowledge 

 of the period at which the fibres in the various tracts acquire 

 their medullary sheath. In the human embryo the nerve-fibres 

 are sheathed three or four months after the axis-cylinder is formed, 

 and the order in which this occurs appears invariable. The 

 afferent fibres are first myelinated, then the efferent, and lastly 

 the association fibres ; this holds good for the nerves of the 

 whole body. At birth the human infant has the afferent 

 system myelinated, but not all the efferent. This, of course, does 

 not apply to those animals which are born with the power of 

 locomotion, but the matter is deeply interesting ; also the fact 

 that the functional activity of nerve tracts is largely dependent 

 on the amount of myelin present. In man the myelination of 



