THE MAMMALIAN NERVOUS SYSTEM 99 



111. The pyramidal trad. Now complete the dissection of 

 the pyramidal tract (tractus cortico-spinalis) from the pons 

 downward and note the decussating fibers of this tract on the 

 cut median surface at the fower end of the oblongata. These 

 crossed fibers form the lateral cortico-spinal tract of the cord. 

 A small part of the pyramidal tract does not decussate, but 

 descends directly and forms the ventral cortico-spinal tract of 

 the cord (cf. Sections 94 and 95). 



10. The Cerebrum 



112. The cerebrum comprises all parts of the brain in front of 

 the isthmus. It is further subdivided into the mesencephalon, 

 diencephalon, and telencephalon. Like the rhombencephalon, 

 the cerebrum includes a stem portion, or segment al apparatus, 

 and a suprasegmental apparatus (the cerebral cortex). 



The brain stem, as a whole, is devoted to the simpler reflex 

 and instinctive activities, while the cerebral cortex serves the 

 higher functions of association. The cerebral hemispheres 

 make up the greater part of the telencephalon. Each cerebral 

 hemisphere comprises cortical and basal or stem portions. 

 The latter includes the olfactory bulb, anterior perforated 

 space (tuberculum olfactorium), septum, corpus striatum, and 

 some other parts. The cortex (pallium) has two great sub- 

 divisions, archipallium (old cortex) and neopallium (new 

 cortex). The archipallium attains its maximum development 

 in lower mammals and is chiefly devoted to olfactory correla- 

 tions. It comprises the hippocampus and part of the gyrus 

 hippocampi (hippocampal lobe or pyriform lobe). The neo- 

 pallium is non-olfactory cortex and attains its highest devel- 

 opment in the human brain. In the sheep also it is more 

 extensive than the archipallium, occupying the convex dorsal 

 surface of the hemisphere. 



113. The midbrain and thalamus. On the medial surfaces 

 of the sheep and human brains review the aqueduct of Sylvius, 

 the boundaries of the third ventricle, and the other landmarks 

 in the midbrain and thalamus (see Section 59) . On the lateral 

 aspect of the specimens locate the colliculus inferior, colliculus 

 superior, corpus geniculatum mediale, corpus geniculatum 

 laterale (medial, or internal, and lateral, or external, geniculate 



