37 !• PROF. G. ELLIOT SMITH ON THE 



the I'rosimiiX', for the feAv writers * who refer to any region other than the cerebral 

 hemispheres do not record anything worth recalling in reference to the hrain-stem. 



There is nothing in the walls of the third ventricle, the pineal body, the ganglion 

 habcnnlae, the habennlar commissure (commissura superior), the hypophysis, corpora 

 mammillaria, the interpeduncular ganglion, the crura cerebri, pons Varolii, and corpus 

 trapezoidea to distiuguish them from tlie great majority of mammals and from the 

 smaller Apes. Many of the structiu'es eniuuerated in this list are found to be consider- 

 ably modified in Man and the liigher Apes in comparison with the majority of mammals. 

 Tims the corpus trapezoides becomes gradually covered by the pons Varolii ; the inter- 

 peduncular ganglion becomes deeply buried between the crura cerebri and pons, and is 

 unrecognizable as a distinct ganglionic mass; the corpora mammillaria become more 

 distinctly separated the one from the other. But these changes occur in the transition 

 from the smaller to the larger Apes, and hence it is not surprising that, like the former, 

 the Lemurs should preserve the common mammalian features. 



There are only three features of the Prosimian l)iain-stem that call for special note. 

 Tliese are the relative size of the corpora quadrigeaiina ; the projiortions of the optic 

 thalamus and its external (anterior) geniculate liody ; and the olivary body in the 

 medtdla ol)longata. 



Tlie general featitres of the corpora quadrigeraina are remarkably constant in all the 

 ]\Ieta- and Eutheria. The features which are sttbject to most variation are the relative 

 proportions of the two pairs of collicttli or qtiadrigeminal bodies, and the size of the 

 ti-actus pedttncularis trausversus. 



Gustav Retzius has recently published some excellent representations of this region 

 in the brains of Homo, Aiitliropopithecus, Sim.ia, Ursus, Lnlra, Phoca, Bos, Equus, Ovis, 

 S'lis, Lepus, and Macropns f ; Ziehen has described its features in the Marsupialia and 

 IMonotremata % ; and I have briefly described its appearance in the Edentata § and 

 Monotremata ||. Since then 1 have carefully examined this region in representatives 

 of every mammalian Order with the special purpose of comparing it with the Prosimian 

 mesencephalon. The most important result of this study is graphically demonstrated in 

 the acconijmnying series of diagrams, which represent the dorsal aspect of the thalamic 

 and mid-brain regions in Lemur, Tarsim, and a heterogeneous collection of mammals 

 including Cercopithecna, Fclis, Procavia, Lepus, Lasijpus, and Macroscelides, which 

 r(>present every phase of the changing jiroportions of these ])odies in the whole 

 Mammalia. All the diagrams are ma<?nihed in the same desTree. 



The most striking feature of the diagram representing the condition in Lemur is the 

 relatively enormous size of the optic thalamus and the smalhiess of the corpora quadri- 

 gemina. As we descend the mammalian scale there is a progressive diminution in tlic 



* Burmeister (2'a/-s;((4-), Owen ^C7(i/-ojn(/sX Milne-Kdwnrds ( liidrisiuie), Ondi-muns ((,'/(!/-07(iy6'), and Flatau and 

 Jacobsohn {Jjemxu- macaco). 



t Biolog. Unters., N. F. Bd. viii. uo. 5, Taf. IG & 17. 

 J Jenaische Denkschrift, 1897. 

 § Tran.i. Linnean Soc. ser. 2, Zool. vol. vii. ISfl'. 

 II Journ. Anat. and Phvs. vol. xxxiii. 



