806 MAMMALIA. 



chaotic assemblage of names applied by the earlier anatomists, in their 

 bewilderment, to every definable portion of its substance, we have little 

 doubt that, when the grand laws that have hitherto guided us in in- 

 vestigating the nervous system of the lower animals are had recourse 

 to, the student will soon perceive how little difficulty there is in com- 

 paring even the brain of Man with the encephalon of the humbler 

 Yertebrata examined in preceding pages, and thus tracing the pro- 

 gressive advances from simple to more complex organization. 



(2358.) The great lessons deduciblo 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 pre- 

 siding over the functions attributable to the individual nerves. Secondly, 

 that in the lower animals where these ganglia exist, they are com- 

 paratively small, and more or less completely detached from each other ; 

 but that in the Yertebrata such is the increased development 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. 



(2359.) Taking the above for axioms (and they are incontrovertible), 

 let us proceed to analyse the cerebro-spinal axis of the Mammalia, and 

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

 already examined. 



(2360.) 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 anatomical structure and rela- 

 tions 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 ( 1802) they were found to equal or even to surpass in size 

 the hemispheres themselves. In Reptiles and Birds they became gra- 

 dually concealed by the development of the hemispherical masses ; and 

 in the Mammalia such is their diminutive appearance, when compared 

 with the cerebrum, that they are scarcely recognized as elements of the 

 encephalon at all. 



(2361.) In all the oviparous Yertebrata 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 Mam- 

 mifers these nerves are extremely numerous, in proportion to the extent 

 of the surface to be supplied, and escape from the skull through the 



