46 



NERVOUS SYSTEM AND GENERAL SENSATION. 



lumns of the medulla expand in its interior, giving off branches 

 which are covered by grey substance, and forming an arbor 

 vitse. The optic lobes are considerably developed, and seen 

 at b, behind the hemispheres. When these bodies are separated, 

 / we observe the anterior commissure bound- 



ing the third ventricle ; pineal and pituitary 

 bodies are distinct ; the hemispheres are 

 greatly increased in volume in this class ; 

 they are still smooth, without convolutions 

 and' posterior lobes. The absence of the 

 latter permits us, when we open the skull, 

 to see the optic lobes lying behind them. 

 Fig. 27. The brain The olfactory nerves, with their ganglionic 

 of a pigeon. enlargement, are seen in fig. 27, which re- 

 presents the base of the brain of a pigeon, a, is the hemi- 

 spheres ; b, the optic lobes; c, the cerebellum ; 1 to 6, pairs of 

 nerves. The olfactory nerves arise at the an- 

 terior and inferior parts of the anterior lobes 

 7 j of the hemispheres; the corpus callosum is re- 

 presented by a feeble rudiment in this class. 

 [96. The Brain presents many phases of de- 

 velopment in the different orders of the MAM- 

 - MALTA. In the monotremata, and marsupialia, 

 the hemispheres are not much more developed 

 than in birds ; and the corpus callosum is still 

 rudimentary. In the ornithorhyncus, the cere- 

 bellum, like that of birds, is one-lobed, with 

 indications only of the lateral lobes, and the he- 

 mispheres become narrow and pointed as they 

 advance. In the rodentia, as in fig. 28, which 

 represents the brain and spinal cord of a rat 

 (Musdecumanus] the hemispheres, a, are smooth, 

 and without convolutions, and the posterior 

 lobes are undeveloped ; the cerebellum, d, lies 

 free and uncovered, as do also the optic 

 lobes, 6, and pineal gland ; the middle lobe of 

 the cerebellum, c, c, is more highly developed 

 than the lateral lobes, d, d ; the superior 

 enlargement of the spinal cord, e, extends 

 Fi 28 . _ The into the middle swelling ; /, is the inferior 

 brain and spinal enlargement, terminating in the cauda equiua ; 

 coM of P rot 1 i: - -*' fl ->^ ^factory nerves. 



