692 MAMMALIA. 



phalon of the humbler Vertebrata examined in preceding pages, 

 and thus tracing the progressive advances from simple to more 

 complex organization. 



(798.) The great lessons deducible from all that we have as yet 

 seen relative to the essential organization of the nervous system 

 are obvious enough. First, that all nerves, whether connected 

 with sensation or the movements of the body, emanate from or 

 are in communication with nervous masses called ganglia, which 

 are in fact so many brains presiding over the functions attributa- 

 ble to the individual nerves. Secondly, that in the lower animals 

 where these ganglia exist, they are comparatively small, and more 

 or less completely detached from each other ; but that in the Ver- 

 tebrata such is the increased developement of the central masses 

 of the nervous system, that they coalesce, as it were, into one great 

 organ called the cerebro-spinal axis ; and thus that the encephalon 

 and medulla spinalis are both made up of symmetrical pairs of 

 ganglia appointed to different functions, but so intimately blended 

 together that they are no longer distinguishable, except from the 

 pairs of nerves with which they are connected. 



(799.) Taking the above for axioms, and they are incontro- 

 vertible, let us proceed to analyze the cerebro-spinal axis of the 

 Mammalia, and to compare it in simple terms with that of Birds, 

 Reptiles, and Fishes already examined. 



(800.) Commencing at the anterior extremity of the series, the 

 first encephalic masses that present themselves are the " olfactory 

 nerves^ as the human anatomist has been pleased to call them, 

 although in every one of the details connected with their anato- 

 mical structure and relations they confessedly differ from every 

 nerve in the body. They are, in truth, not nerves at all, but 

 brains, the ganglia or brains of smell, from which the olfactory 

 nerves properly so called invariably emanate. In Fishes ( 557) 

 they were found to equal, or even to surpass in size, the hemi- 

 spheres themselves. In Reptiles and Birds they became gradually 

 concealed by the developement of the hemispherical masses ; and 

 in the Mammalia such is their diminutive appearance when com- 

 pared with the cerebrum, that they are scarcely recognized as ele- 

 ments of the encephalon at all. 



In all the oviparous Vertebrata the nerves of smell were two 

 simple cords, one derived from each of the olfactory ganglia, from 

 which they proceeded through osseous canals to the nose. But in 

 the Mammifers these nerves are extremely numerous in proportion to 



