204 ANATOMY OF VEBTEBEATES. 



tlic fossulate papilla? and the mucous tract behind : the ninth 

 or hypoglossal is expended upon the muscular tissue. 



215. Organ of Smell. Most Mammals are remarkable for 

 the degree in which the sense of smell is serviceable. The class 

 is characterised by the extent of the pituitary surface and the 

 size and number of the olfactory nerves; nevertheless, both ex- 

 tremes are therein exemplified, although the family (JDelpMnidoz) 

 in which the organ is wanting is exceptional and maximised 

 development the rule. 



The progress is not, as with the organ of taste, pari passu 

 with the rise in the class : both Man and monkeys are below 

 most quadrupeds in olfactory endowments. In hoofed ones smell 

 is important in the the discrimination of wholesome from noxious 

 food : taste would be a tedious test, the sapid matter needing to 

 be moved about or masticated, mixed with fluid, and more or less 

 dissolved, before the tongue can exert its gustative power ; but 

 ( smell is done at once.' 1 Most flesh-feeders scent afar their 

 food. 



In Mammals, as in all air-breathers, the odorous atoms strike 

 upon the olfactory membrane at the entry of the breathing 

 passages, where the atmosphere is filtered, as it were, through 

 the organ of smell before reaching the windpipe ; and most 

 effectively and instructively in the pinnigrade Carnivora. 



The olfactory organ in Mammals receives its special endowment 

 from nerves which rise in numbers from their proper encephalic 

 centre, fig. 46, 47, R. They pass out by as many holes in the 

 plate of the prefrontal, which is thence called the ( cribriform,' or, 

 from the Greek-root, ' ethmoid:' but the sieve-like structure is a 

 strictly mammalian peculiarity consequent on the multiplicity of 

 olfactory nerves, and is only affected by a single exception in 

 this class, the Ornithorhynchus adhering to the wider Vertebrate 

 rule. 



The nerves carry out with them, each an investment of the 

 brain-membranes ; the dura mater losing itself in the periosteum, 

 the pia mater in neurilemma, the arachnoid being reflected back. 

 The nerves are grouped in all Mammals into a set for the 

 septum, and a second for the upper or ethmo-turbmals, a third or 

 middle short set being, in some, distinguishable for the labyrinth 

 or roof of the nasal chamber. The branches of the second set, 

 after expanding on the ethmo-turbinal, usually converge to 

 become connected with the lateral nasal branch of the ( fifth.' 

 Their mode of distribution is best seen on the ethmo-turbinal : 



1 xx. vol. iii. p. 86. 



