596 CLASS XVII. 



pair of dorsal nerves a plexus for the anterior limbs ; at tlie origin 

 of these nerves the cord exhibits a swelling ; a second occurs lower 

 down where the lumbar and sacral nerves arise which form the 

 plexus for the hind limbs. The phrenic nerves, peculiar to mammals, 

 have their origin principally from the fourth pair of cervical nerves. 

 The brain is distinguished at its base from that of other verte- 

 brates bj a commissure of the hemispheres of the cerebellum pass- 

 ing across the medulla oblongata; it is named fons Varolii, and is 

 found in all mammals. The great ti-ansverse commissure of the 

 hemispheres of the cerebrum is with the exception of the Mono- 

 tremeS and Marsupiates^ present in all mammals. The corpora 

 quadrigemina form a much smaller mass of the brain than in the 

 lower vertebrates, are not hollow internally, and in many are covered 

 by the hemispheres of the brain. Also the cerebrum forms many 

 convolutions {(jyri) on its surface ; although there are many mammals 

 in which the surface is smooth, as it is in the brain of the oviparous 

 vertebrates. The weight of the brain is to that of the whole body 

 in the ox as 1 : 750 or 1 : 850, in the elephant as 1 : 500, in the 

 sheep as 1 : 350, in the fox as 1 : 205 ; the relative weight of the 

 brain is greater in small animal species ; in the mouse, for instance, 

 it is stated to be jJj of tlie weight of the whole body 2. In man the 

 proportion is as 1 : 40^. Of more interest is the relation of the 

 cerebral mass to the size of the cerebral nerves, and the comparison 

 between the greatest breadth of the brain and the breadth of the 

 spinal cord, behind the bridge of Varolius. The breadth of the spi- 

 nal marrow thus estimated is to that of the brain in man, for ex- 

 ample, as 1 : 7, in the dog as 1 : 2, &c.* 



1 R. Owen On the structure of the Brain in Marsupial Animals; Philos. Transact. 

 1837, Part I, pp. 87—96, PI. 5—7. 



^ Compare on these relations Haller Mem. Physiol, iv. pp. 6, 7 ; Cuvier Zef . 

 d'Anat. camp. 11. pp. 149 — 151. 



^ The weight of the human brain amounts to between three and four pounds ; see 

 TiEDEMANN Das Him des Negers, u. s.w. Heidelberg, 1837, 4to, s. 6, 7. 



•* Cuvier Le^. d'Anat. comp. 11. p. 154 ; J. G. Ebel Ohservationes neurol. ex 

 Anat. comp. Traj. ad Viadi-. 1788, 8vo. The proportion between the breadth of the 

 medulla ohlongata and that of the pons Varolii has also been investigated ; in most 

 mammals this last is not broader, or scarcely broader than the medulla oblongata. 

 Compare F. Tiedemann Icones cerehri simiarum et quorundam Mammalium, Heidel- 

 bergse, 1821, folio, p. 53, which work (with Treviranus Verm. Schr. in. s. 4 — 10, 

 and Gbatiolet in Guerin lievue et Magasin de Zool. 1852, pp. 97 — 113, PI. 4 — 7, 



