532 TPIE NERVOUS SYSTEM 



nerve substance are revealed: the optic thalami, the corpora quadrigemina, 

 andthecn/s cerebri. These structures, together with the pons and the medulla, 

 form a direct continuation forward of the spinal cord and sometimes are desig- 

 nated under the general term of the brain stem. 



For convenience of description, the physiology of the brain will be presented 

 by discussing the three main subdivisions: the brain stem, the cerebral hemi- 

 spheres, and the cerebellum. 



The human brain on superficial examination does not seem to follow the 

 general plan outlined above, but when the cerebral hemispheres and the 



FIG. 370. 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; Hmp, hemispheres; 

 Th. E, thalamencephalon; Pn, pineal gland; Py, pituitary body; P.M., foramen of Munro; cs, 

 corpus striatum; Th, optic thalamus; CC, crura cerebri; the mass lying above the canal rep- 

 resents the corpora quadrigemina; Cb, cerebellum; I-IX, the nine pairs of cranial nerves; i, 

 olfactory ventricle; 2 V lateral ventricle; 3, third ventricle; 4, fourth ventricle; -\-, iter a tertio 

 ad quartum ventriculum. (Huxley.) 



cerebellum are removed then it is found that what remains closely follows 

 the plan presented. This central axis, or brain stem, is shown in part in 

 figure 3 7 7. 



The morphological parts of the brain usually given are : 



1. The /ore-brain, which consists of the corpora striata and the cerebral 

 hemispheres. 



2. The inner-brain, which consists of the optic thalamiand the parts en- 

 closing the greater part of the third ventricle. 



