126 Mammalia. Nervom System, 



NERYOTJS SYSTEM. Special. 



1. Spinal Cord, or Myelon. 



The proportmi of, to Neural Canal, 



is greatest in the Pinnigrade Sedorialia {e.g. Seal), the Cete, and 

 the Sirenia. The space between the myelon and neural arches i& 

 occupied by blood vessels, which are chiefly arterial plexuses. 



The Mass of, is in direct ratio to that of the body. 



except in certain Insectivora, e.g. HedgeJwg, a fsw Cheiroptera, 

 and the Monotrematous Echidna. 



does not extend as far as Sacrum 



in some Cheiroptera, the Hedgehog (Insectivora), and the Echidna, 

 (Monotremata); the myelon being concentrated into the dorsal region. 



presents only the anterior enlargement 

 in the Cete, and Sirenia. 



2. Brain, or EncephaJon. 



i. Bimana. Membranes. In Man (as also in most Quadrumana) 

 the sole ossification co-extended with any part of the dura mater is 

 that called * crista galli' (in Anthro.). An unossified process from 

 the middle of the posterior border of the tentorium, extending from 

 the internal occipital crest, projects into the notch between the 

 hemispheres of the Cerebellum, and is termed falx minor, or falx 

 cerebelli. Epenceplmlon. A thin layer of superficial fibres, which 

 in lower Mammals with non-prominent * olivary' bodies pass out- 

 ward as a ' trapezoid' layer, in Man curve round the exterior of the 

 olivary prominences and constitute the arciform fibres. The ' corpus 

 dentatum' is well marked. The transverse section of the medulla 

 oblongata is nearly circular. The lateral lobes of the cerebellum 

 acquire the largest proportions (as compared with other Mammals), 

 and reduce the middle lobe to the semblance of a subordinate ad- 

 junct, called * vermiform process.' Mesencephalon. The corpora 

 quadrigemina are small in comparison with those of the lower Mam- 

 malia. Prosencephalon. The Human Brain surpasses that of all 

 Mammalia in its greater proportional bulk to that of the body, and 



