NERVOUS SYSTEM OF VERTEBRATES. 



77 



Owen's Classification of Mammals. — Professor Owen 

 classified mammals by this test into four groups : i. 

 Archencephala, or ruling brain. In 

 this subclass he placed man alone. 

 2. Gyrencephala, or convoluted 

 brains. In this he placed all the 

 larger mammals. 3. Lissencephala, 

 or smooth brains. In this he 

 placed all the rodents and some 

 other small mammals. 4. Lyen- 

 cephala, or separated brains. The 

 lowest mammals, such as insecti- 

 vores and marsupials, he placed 

 in this. This attempt is interest- 

 ing, but the classification can not 

 be regarded as natural, for the 

 earliest animals of all these sub- 

 classes, except one — man — have 

 smooth brains. 



In the average bird (Fig. 43) not only is the olfac- 

 tory lobe wholly uncovered in front and the cerebellum 

 behind, but also the optic lobes are at least half un- 

 covered. 



B 



A 



Fig. 42. — Brain of Dasyurus 

 ursinus, showing expo- 

 sure of the optic lobes, ol. 

 (From Owen.) 



Fig. 43. — Bird brain : A, side view ; B, top view.' 



In the reptile (Fig. 44) all the lobes are fully ex- 

 posed, and the brain becomes a succession of lobes in a 



* In all these figures to 54: m, medulla; ei, cerebellum; of, 

 optic lobes ; cr, cerebrum ; of, olfactory lobes, 



