770 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



capable of great variations in development in different members of this 

 group. In fishes and reptiles the brain is less developed than in higher 

 members, no convolutions are found on its surface, and, while the cere- 

 bral hemispheres are not highly marked, the optic and olfactory lobules 

 are usually comparatively voluminous, while the cerebellum of reptiles 

 and fishes is reduced to a single lobe (Figs. 325 and 326). 



In birds the hemispheres or cerebral lobes are still, as in the mam- 

 mals, the most voluminous portions of the brain, but here, also, no con- 

 volutions are found and the}^ are not closely united to each other, since 

 the corpus callosnm, as well as the pons varolii, is absent (Fig. 327). 



In birds the analogue of the tubercula quadrigemina, which are four 

 in number in the mammals, are here reduced to two, and, therefore, receive 

 the name of the tubercula bigemina or optic lobes, and are visible at each 

 side of the brain when looked at from above. 



T- 

 -cM 



I. II. 



FIG. 327. BRAIN OF BIRD (Falco buteo). (Nuhn.) 



I, view of upper surface. II, view of lower surface : cbr, cerebrum ; </, corpora quadrigemina, or 

 bigemina; cbll, cerebellum; nbl, medulla oblongata; //, hypophysis; opt, optic nerve. 



In birds the cerebellum is likewise reduced to a single median lobe, 

 and is entirely uncovered \)j the cerebrum, and being single it possesses 

 no lateral hemispheres; the pons varolii, or the transverse fibres which 

 serve as a commissure for the cerebellar hemispheres, as in mammals, is 

 likewise absent. 



The characteristics of the mammalian brain will be subsequently 

 alluded to. 



The nervous system is composed of central masses which are in 

 constant communication with different parts of the body by means of 

 peripheral prolongations which act as organs of conduction. The 

 nervous system exists, therefore, under two forms the central, composed 

 largely of the so-called nervous ganglia, or nerve-cells, and the organs of 

 conduction, or nerve-fibres. As already indicated, the peripheral termi- 

 nations of the nerves are likewise in connection with corpuscular bodies, 



