664 



THE CEREBRUM 



Comparative Physiology of the Cerebrum. — In the course of verte- 

 brate evolution, the primitive reflex stem of the nervous system event- 

 ually acquires a number of structures which collectively make up the 

 brain. Its constituent elements are the hindbrain (rhombencephalon), 

 midbrain (mesencephalon), tweenbrain (diencephalon) and forebrain 

 (prosencephalon). The one named last is formed by the cerebral 

 hemispheres. Obviously, these complex masses find their origin in 

 the adjuncts to the head ganglia of the lowest forms which, as has 

 previously been pointed out, are primarily concerned with the forma- 

 tion of the sense of smell, sight, touch, etc. The point to be empha- 

 sized is that these areas are developed from small beginnings and that 



Fig. 334.— Diagram- 

 matic Representation of 



THE HeXAJD of a TuRTLE, TO 



Show the Position of the 

 Cerebrum C and Optic 

 Lobes O, 



Fig. 335. — Diagrammatic Representation of the 

 Brain of a Frog {A) and Shark {B). 

 ON, Olfactory nerves; OL, olfactory lobe; C, cere- 

 brum; T, tween brain; OpL, optic lobes; Ce, cerebellum; 

 M, medulla; Co, spinal cord; OC, olfactory capsules; 

 OB, olfactory bulb. The cranial nerves are indicated 

 by Roman numerals. 



their mass and complexity is in accord with the position of the animal 

 in the scale of the Animal Kingdom. 



Thus, we find that the olfactory realm occupies almost the entire 

 cerebrum of the fishes, and forms the most conspicuous part of the brain 

 of the reptilia and amphibia. This condition permits of only one con- 

 clusion, namely, that the sense of smell is especially well developed in 

 these animals, and that their existence is mainly dependent upon ol- 

 faction. Moreover, while their reactions are almost wholly reflex, 

 they must also possess a moderate power of associating these sensa- 

 tions. As we ascend the scale of the Animal Kingdom, this sense 

 becomes retrogressive. A constantly increasing number of other 

 mechanisms are added to the hemispheres which enable the animal 



