MARSUPIALIA. 



295 



Mammalia, and which exists in addition to the 

 hippocampal commissure, is wanting in the 

 brain of the Wombat ; and as the same defi- 

 ciency exists in the brain of the Great and Bush 

 Kangaroos, the Vulpine Phalanger, the Pera- 

 meles lagotis, the Ursine and Mange's Dasy- 

 ures, and the Virginian Opossum, it is most 

 probably the cerebral characteristic of the mar- 

 supial division of Mammalia. 



In the modification of the commissural ap- 

 paratus above described, the Marsupial ia pre- 

 sent a structure of brain which is intermediate 

 between that of the placental Mammalia and 

 Birds; as in the latterclass the great commissure 

 is wholly wanting, and the hemispheres, though 

 comparatively larger than in many of the Mam- 

 malia, are brought into communication only by 

 means of the anterior, posterior, and soft com- 

 missures, and by a slight trace of die fornix or 

 hippocampal commissure. 



Of the other peculiarities of the marsupial 

 brain, the relatively large size of the anterior 

 commissure ( c , fig. 1 18) is most worthy of 

 notice; its development corresponds with 

 the large size of the cerebral ganglion, which 



Fin 118. 



Didelphys Viryiniiena . 



forms the chief origin of the olfactory nerve, 

 and some of the anterior fibres of this com- 

 missure arch forwards, and are directly con- 

 tinued into those nerves. 



In the position, superficial transverse fissure, 

 and solidity of the bigeminal bodies, the mar- 

 supial brain adheres to the Mammiferous type, 

 as also in the exterior transverse fibres of the 

 commissure of the cerebellum, forming the 

 pons Varolii, the presence of which relates to 

 the development of the lateral lobes of the 

 cerebellum. 



In one of the latest published treatises on 

 comparative anatomy, the Lehrbuch der Ver- 

 gleichenden Zootomie, zweite auflage, 1834, 

 of Cams, the first and chief structural charac- 

 teristic of the brain in Mammalia is stated, as 

 in the Lecons d'Anatomie Comparee of Cuvier, 

 to be the presence of the corpus callosum.* 

 The brain of the Rodentia is cited as the 

 example of the transitional condition of this 

 organ from Mammalia to Birds.f Besides the 



* ' Theils und vorziiglich werden sie in den 

 durch cine neue grosse Commissur vereinigten,' 

 $ 122, B. i. p. 77. " Die einzelnen Hirnmassen 

 bctrcftend, so aiissert sich, \vie schon bemerkt, das 

 Eigenthiiinliche der ersten, der Hemisph';iren, vor- 

 ziiglich durch die Erscheinung des Balkens (corpus 

 callosum) und des Geivolbes (fornix)." 



t " Im allgemeinen bildet zur Hirnfonn dioser 

 Klasse von der der vorigen (derVogel) die Gehirn- 

 bildung, vvie sie in den Nagetfiicrcit ( Kodcmia) 

 beobachtetwird, dend'eutliehsienUebergang." Ibid. 

 p. 77. 



Rodentw, Carus afterwards states that the 

 Monotremata, Marsupialia, and Inscctivora 

 (shrews, moles, and bats) also present the more 

 simple form of brain among the Mammalia, 

 " the hemispheres being of an ovate form con- 

 tracted anteriorly, and their surface perfectly 

 smooth, as in Birds, not extending over the 

 cerebellum, and sometimes not over the corpora 

 quadrigemina. Internally the great commissure 

 is generally very short (in the Bats and Kan- 

 garoo hardly so long as the corpora quadrige- 

 mina, a structure which reminds one of that in 

 Birds). The fold of the corpus callosum and 

 the cornua ammonis (which (Jar us terms the 

 processes of the corpus callosum in the ven- 

 tricles) are commonly broad and large."* In 

 illustration of this simple structure of the 

 mammiferous brain Carus gives a figure of the 

 brain of a Rodent, and from the whole descrip- 

 tion the reader is led to infer that the Rodent 

 and Insectivorous placental Mammalia partici- 

 pate with the marsupial Mammalia in all the 

 characters of the cerebral organization which 

 approximate to those in Birds. Rudolph 

 Wagner also describes the brain in the Roden- 

 tia, Cheiroptera, Edentata, and Marsupialia as 

 being characterised by the small size of the 

 corpus callosum,f which does not extend far 

 back. It must not be supposed that the pre- 

 ceding quotations are adduced to detract from 

 the merit of works whose reputation deservedly 

 stands high. The position of the accomplished 

 and indefatigable authors in a central town of 

 Germany is unfavourable to the acquisition of 

 specimens of animals which, like the Marsu- 

 pialia, are almost confined to one of our most 

 distant colonies; but it would be unpardonable 

 in an English Comparative Anatomist, pos- 

 sessing the requisite opportunities of consult- 

 ing nature, to content himself with copying 

 the generalizations of foreign systematic writers, 

 which, as regards the marsupial Mammalia, 

 are liable to repose on so limited and imperfect 

 an induction. 



The. Spinal Chord. The spinal chord mani- 

 fests all the usual Mammalian characters ; the 

 brachial and pelvic enlargements correspond 

 with the relative size and muscularity of the ex- 

 tremities to which they furnish the nerves, the 

 lumbar or pelvic enlargement is consequently 

 most marked in the Kangaroo (Jig. 119) and 

 Potoroos, but does not exhibit the rhomboidal 

 sinus which characterises this part of the 

 chord in Birds. The disposition of the layer of 



* " Die aussore gcstalt der hemispharen ist in 

 den Nagern, so wie in den Scbnabel-iind Beutcl- 

 thieren, Spitzmausen, Maulwmfen und Flcder- 

 mausen ein vorwHits sich verschmalerndes Kirr.uJ, 

 und ihre Oberflachc, wie in Vogel vollkommen 

 glatt ; hintcrwarts werden dadurcb. weder klciucs 

 Hirn, ja oft nicht einmal die Vierhugol brdeckt ; 

 innerlich istdie grosse Commissur (corpus callosum) 

 gewbhnlich noch st-hr kurz (bei den l-'lrdermiiuM-n 

 und dem Kanguruh kauiii so lang als die Vicrhii^e], 

 eine Bildung, so an die Vii^el orinnert) der I'li:-- 

 chlag des Balkens und Forsetzung dessrlbrn iu die 

 Seitenhohlen (cornua Ammonis) vorxii'jlirh brcit 

 und gros (f. v. c. g.)" I' 1 '''- \ x I'-'-L P- 7!'. 



1 Der Halken ist noch scbinal bci den Nagorn, 

 Fledermausen, Edentaton und Beutelthieren 

 "cht nicht writ nach hiiiien. f.,!ir/ni,/t </<; !</- 

 gleichenden Anatomie, p. (>(>'i, [835, 



