934 THE CENTRAL NERVOUS SYSTEM 



further source of fallacy is the fact that other sensations than those 

 of smell are caused by stimulation of the mucous membrane of the 

 nose. Substances like ammonia, for example, affect entirely the 

 endings of the trigeminus, which is the nerve of common sensation 

 for the nostrils. Pathological and clinical evidence would be of great 

 value, but it is as yet scanty, and of itself indecisive. Some cases 

 of epilepsy have been reported in which the attack was heralded by 

 smells for which there was no objective cause. At necropsy the un- 

 cinate gyrus was found diseased. So far as it goes, such evidence 

 supports the view derived from the anatomical connections of the 

 olfactory tracts, that the centre for smell is situated in the uncinate 

 gyrus on the mesial aspect of the temporal lobe, for the olfactory 

 track may be traced into this region. In animals with a very acute 

 sense of smell, this gyrus is magnified into a veritable lobe, called 

 from its shape the pyriform lobe; from its supposed function, the 



Fig. 381. Sensory Areas of Mesial Surface of Human Brain. The front of the brain 



is towards the right. 



rhinencephalon. The centre for taste is supposed to be situated in 

 the same region as the centre for smell (in the hippocampal convolu- 

 tion posterior to the uncinate gyrus). 



Ordinary and Tactile Sensations, including the muscular sense, 

 have been located in the Rolandic area (p. 929) ; and there are good 

 grounds for believing that afferent fibres from the joints, the muscles 

 and their accessory structures and the skin terminate here in arboriza- 

 tions which come into contact either with the motor pyramidal 

 cells, or with intermediate cells which link them to the pyramidal 

 cells. 



Aphasia. Words are, at bottom, arbitrary signs by which certain 

 ideas are expressed. The power of intelligent communication by spoken 

 or written language may be lost: (i) by paralysis of the muscles of 

 articulation or the muscles which guide the pen; (2) by inability to 

 hear or see the spoken or written word i.e., by deafness or blindness; 

 (3) by inability to comprehend the meaning of spoken or written lan- 

 guage, although sensations of hearing and sight may not be abolished 



