714 OP THE FUNCTIONS OF THE NERVOUS SYSTEM. 



on one side of the body. These convulsive contractions are to be found in 

 every case of circulatory or rotatory movement. As to the cause of these 

 contractions, it exists in the irritation produced in certain parts of the ence- 

 phalon. ED.] 



739. Nerves of Special Sense. Having thus taken a general survey of the 

 information supplied by Comparative Anatomy, Experiment, and Pathological 

 phenomena, in regard to the functions of this division of the Encephalon, we 

 shall next inquire into the attributes of the Nerves of which they are obviously 

 the ganglionic centres. Through the First pair, or Olfactory nerves, are trans- 

 mitted the impressions made by odorous emanations upon the surface it supplies ; 

 and it is not susceptible to impressions of any other kind. Anatomical exami- 

 nation of the distribution of this nerve proves that it is not one which directly 

 conveys motor influence to any muscles, since all its branches are distributed 

 to the membrane lining the nasal cavity; and experimental inquiry leads to the 

 same result, for no irritation of the peduncles or branches excites any muscular 

 movement. Further, no irritation of any part of this nerve excites reflex actions 

 through other nerves. Again, it is not a nerve of common sensation; for 

 animals exhibit no sign of pain, when it is subjected to any kind of irritation. 

 Neither the division of the nerve, nor the destruction of the olfactive ganglia, 

 seems to inconvenience them materially. They take their food, move with their 

 accustomed agility, and exhibit the usual appetites of their kind. The " com- 

 mon" sensibility of the parts contained in the olfactive organ is in no degree 

 impaired, as is shown by the effect of irritating vapors ; but the animals are 

 destitute of the sense of smell, as is shown by the way in which these vapors 

 affect them. At first they appear indifferent to their presence, and then suddenly 

 and vehemently avoid them, as soon as the Schneiderian membrane becomes 

 irritated. Moreover, if two dogs, with the eyes bandaged, one having the olfac- 

 tory nerves and ganglia sound, and the other having had them destroyed, are 

 brought into the neighborhood of the dead body of an animal, the former will 

 examine it by its smell; whilst the latter, even if he touches it, pays no attention 

 to it. This experiment Valentin 1 states that he has repeated several times, and 

 always with the same results. Further, common observation shows that sensi- 

 bility to irritants, such as snuff, and acuteness of the power of smell, bear no 

 constant proportion to one another; and there is ample pathological evidence, 

 that the want of this sense is connected with some morbid condition of the 

 olfactory nerves or ganglia. It is well known that Magendie has maintained 

 that the Fifth pair in some way furnishes conditions requisite for the enjoyment 

 of the sense of smell; asserting that, when it is cut, the animal is deprived of 

 this. But his experiments were made with irritating vapors, which excite 

 sternutation or other violent muscular actions, not through the Olfactory nerve, 

 but through the Fifth pair; and the experiments of Valentin, just related, fully 

 prove that the animals are not sensitive to odors, strictly so called, after the 

 Olfactory nerve has been divided. It is highly probable, however, that the 

 acuteness of the true sense of smell may be diminished by section of the Fifth 

 pair; since the Schneiderian membrane is no longer duly moistened by its 

 proper secretion ; and, when dry, it is not so susceptible of the impressions made 

 by those minute particles of odoriferous substances, to which the excitement of 

 the sensation must be referred. 



740. That the Second pair, or Optic nerves, have an analogous character, 

 appears alike from anatomical and experimental evidence. No chemical or me- 

 chanical stimulus of the nerve produces direct muscular motion ; nor does it give 

 rise, so far as can be ascertained, to indications of pain ; whence it may be con- 



> "De Functionibus Nervorum Cerebralium," &c., Bernae, 1839. 



