500 A MANUAL OF VETERINARY PHYSIOLOGY 



observing the groups of muscles first affected and knowing the 

 region of the cortex to which they are related, it is possible, 

 in man, to localise with considerable exactitude the seat of the 

 trouble. 



The Sensory Areas. — It will be remembered that these include 

 the higher senses of vision, smell, taste, and hearing, and the 

 senses of the lower order, touch, temperature, and muscle-sense. 

 The term ' sensory ' is employed in its physiological sense. 

 The seat of pain, we have seen, does not lie in the cerebral 

 cortex. The lower order of senses are those of which we are 

 generally unconscious. Of muscle-sense we are, perhaps, under 

 normal conditions, wholly unconscious ; towards heat and cold 

 the consciousness can be awakened, but is ordinarily unrecog- 

 nised unless the stimulation is severe. The areas connected 

 with these senses are situated above the crucial sulcus. 

 The Centre for Vision is situated in the occipital lobe, and 

 destruction of the centre, say on the right side, is followed by 

 blindness in the two right halves of the retina in those animals 

 where decussation of the optic nerves is incomplete. In those 

 in which it is complete a right-brain lesion leads to a left-eye 

 blindness. We shall deal again with this centre in considering 

 the subject of vision. The Auditory Centre has been located 

 in the temporal lobes, and the facts connected with it will 

 be considered in the chapter on Hearing. The auditory and 

 visual centres are capable of eliciting a response in the 

 motor areas, for the ears are pricked, and the head and 

 eyes turned towards an object or a sound, so that con- 

 necting paths between the special sense-centres and the regions 

 in the motor area, connected with the muscles in question, 

 are believed to exist. The Olfactory Centre : In animals 

 with the sense of smell acutely developed the olfactory bulb 

 and tract are large. The dog, rabbit, horse — in fact, most if not 

 all, of the domestic animals — have the sense of smell highly de- 

 veloped. It is not necessary here to consider the reason in 

 detail, but it is obvious that the whole question of their lives 

 in a natural condition hinges largely on the question of smell ; 

 food, the presence of an enemy, sexual instinct, are capable of 

 exciting in the cortex psychical reactions, which, in the higher 

 animals, are brought about through other channels. In the 

 primitive brain, swellings first develop in connection with 

 smell, and in the process of evolution the cortical centre for 

 smell was one of the first to be established. In the brain with no 

 cerebrum — as, for instance, in the brain of the shark — the olfac- 

 tory centre, contrary to the general rule in fishes, is immensely 

 developed. Fishes have excellent sight but generally no sense 

 of smell, but the shark lives by the sense of smell, and in the- 



