564 



THE NERVOUS SYSTEM 



III. THE BRAIN. 



General Arrangement of Parts of the Brain. The great relative and 

 absolute size of the cerebral hemispheres in the adult man and in mammals 

 to a great extent mask the real arrangement of the several parts of the brain. 

 An examination of the accompanying diagram, figures 370, 371, reveals that 

 the parts of the brain are disposed in a linear series, as follows (from before 

 backward): Olfactory lobes, cerebral hemispheres, thalamencephalon 

 (thalami and third ventricle), the mid-brain (corpora quadrigemina and 

 crura cerebri, medulla oblongata, and cerebellum. 



FIG. 372. Diagrammatic Horizontal Section of the Vertebrate Brain. The figures 

 serve both for this and the next diagram. Mb, mid-brain; what lies in front of this is the 

 fore-, and what lies behind the hind-brain; Lt, lamina terminalis; Olf, olfactory lobes; 

 Hemp, hemispheres; Th. E, thalamencephalon; Pn, pineal gland; Py, pituitary body; 

 F. M., foramen of Munro; cs, corpus striatum; Th, optic thalamus, CC; crura cerebri; the 

 mass lying above the canal represents the corpora quadrigemina; Cb, cerebellum; I-IX, the 

 nine pairs of cranial nerves; i, olfactory ventricle; 2, lateral ventricle; 3, third ventricle; 4, 

 fourth ventricle; +, iter a tertio ad quartum ventriculum. (Huxley.) 



The linear arrangement of parts actually occurs in an early stage of the 

 development of the human fetus, and it is permanent in some of the lower 

 vertebrata. In fishes the cerebral hemispheres are represented by a pair 

 of ganglia intervening between the olfactory and the optic lobes, and con- 

 siderably smaller than the latter, their adult development is fairly well repre- 

 sented by the figure 373. In Amphibia the cerebral lobes are further 

 developed, and are larger than any of the other ganglia. 



