464 CETACEA. 



internal organ of hearing in Cetacea is largely endowed with nervous supply, 

 whatsoever otherwise may be the faculty of hearing in this aquatic group of 

 the mammaha. The second smaller branch of the nerve M'ill, of course, correspond 

 to the facial. 



The eighth pairs (8) (pneumogastric, glossopharyngeal, and spinal accessory), 

 ninth pairs (hypoglossal), and the roots of the anterior spinal nerves, form a successive 

 series of loose bundles of large roots, issuing in close succession along the lateral 

 borders of the medulla and top of the cord. Their filaments are encased and inter- 

 woven with the thecal coverings of the cord and spinal vascular plexuses, but 

 nevertheless their ganglia are pronounced. The nerve strands of the pneumogastric 

 pair are apparently slightly the thickest ; otherwise, it is difficult to distinguish 

 between them. 



I may here remark that in this preserved brain any division between the pons 

 Varolii {pv) and medulla oblongata {m) is rather indefimte. The obscure line of 

 demarcation, the want of transverse fibres at the uj)per part of the latter, and a kind 

 of intermediate piece between them I jDresume to be Trevu'anus' so-called "trapezium.'''' 

 If so, it is thus quite appreciable in Platanista. Frederick Ouvier states, a 

 " trapezium " is wanting in the dolphin (D. clelphis), himseK quoting Tiedemartn ; 

 on the other hand, Huxley mentions of the porpoise {T?liocaena) , " the medulla 

 oblongata has corpora trapezoidea." 



The cerebral hemispheres are highly convoluted, though questionably if as much 

 as in the porpoise, but, before referring to the convolutions, I shall point out the main 

 fissures and certain of the primary or rather secondary sulci. 



The longitudinal fissure is very deep j)osteriorly, but somewhat less so towards 

 the frontal region, though nevertheless, on pressing aside the lijjs of the fissure, the 

 corpus callosum is by no means near the surface. The Sylvian fissru^e {sy') is also 

 remarkably distinct, runs very much upwards on the lateral face of the cerebrum, or 

 has a pronounced vertical direction, and in its course there is a widish deep valley 

 partitioning the fronto-parietal from the temporo-parietal region. Even in the plaster 

 of Paris cast of the cranial cavity, this median depression, or broad indentation of 

 the parts opposite the lower end of the Sylvian fissure, is strikingly evident. It 

 is this sunken area at the position in question which gives a marked feature to 

 the brain of Flatanista as compared with that of Orcella and such other brains 

 of the Delphinidce that have been figured. I could not satisfy myself of the 

 presence of gyri forming the island of Heil in the Sylvian cleft, but they may 

 be present. 



On the right side, I could readily distinguish what may be regarded as corre- 

 sponding to an antero-parietal sulcus. This commenced on the side of the frontal 

 lobe and above the orbital region, and ran along the upper but outer cerebral face, in 

 a winding manner, but with a tolerably arching curve, across the parietal region and 

 towards the posterior border of the occipital lobe. It is highest just above the top 

 end of the Sylvian fissure. Throughout its course various subsidiary short sulci join 

 it at difi^erent points. 



