80 COMPARATIVE ANATOMY. 



tracts; a conspicuous ganglion (olivary body) is added. The 

 body (vermiform process) of the cerebellum is small and 

 transversely folded. Its lateral appendages (hemispheres) 

 have now a size exceeding that of the body. They are 

 united inferiorly by a band of commissures (Pons Varolii) 

 which traverse the course of fibres going to form the crura 

 cerebri, and, with but few exceptions, by another smaller set 

 (trapezium), which cross immediately in advance of the ante- 

 rior pyramids. The optic lobes (corpora quadrigemina) are 

 small, concealed from above, and are bi-partite, or imper- 

 fectly quadri-partite. The cerebral hemispheres, exceeding 

 in size other portions of the brain, are divided into lobes, 

 most generally convoluted, and, with few exceptions (Impla- 

 centalia), united by commissure (corpus callosum). Corpora 

 striata striated. Optic thalami at times give partial origins 

 to optic nerves. Olfactory lobes rudimentary in man, but 

 large and containing ventricle in Ovis (sheep). As a rule, 

 the relative size of the brain is greater than in that of any 

 other class of Vertebrata, occupying in ox ^J-^, in Elcphas 

 (elephant) 3 -J- , in Vulpes (fox) -J-, and in man ? ! a part of the 

 whole tract. 



Monotremata. Spinal cord in Ornithorhynchus (duck bill) 

 is long, slender, and fills the canal. In Echidna (porcupine 

 ant-eater) it is thicker and shorter, and extends but half 

 the length of the canal. Medulla oblongata with pyramids 

 and olivary bodies defined. Cerebellum large, folded ; me- 

 dian portion with small lobes. Optic lobes quadri-partite 

 (Echidna); median fissure faint, sometimes absent. Cere- 

 brum, with great commissure absent. Hemispheres smooth 

 in Ornithorhynchus; slightly convoluted in Echidna, where 

 three equidistant, transverse folds characterize the posterior 

 portion of each hemisphere. Olfactory lobes relatively very 

 large. In Ornithorhynchus a portion of the nerve arises from 

 the inferior surface of the posterior part of the cerebrum 

 behind the quadrigeminal body. In Echidna the nerve is yet 

 larger; it arises from the anterior part of the cerebral hemi- 

 spheres, and contains a prolongation of the lateral ventricle. 

 Cerebral hemispheres encroach very slightly upon cerebellum. 



