corpus caliosum 

 septum lucidum 

 invagination of roof into ventricle of cerebrum 



sulcus 

 gyrus (cingulate) 



fornix 



anterior 

 commissure 



lamina terminolis 



olfactory bulb 

 anterior perforate substance 



temporal lobe 



foramen of Monroe 

 intermediate moss of thalamus 

 roof of diencepholon 



hippocompal commissure 

 third ventricle 



habenular commissure 

 pineal 



posterior 



commissure 



corpora quodrigemina 

 (midbrain) 



cerebellum 

 dentate nucleus 



optic nerve 

 optic chiasmo' 

 pituitary 



tuber cinereum 

 oculomotor nerve (111) 



red nucleus 

 substantia niger 



reticular body with cranial nerve 

 nuclei 



chorioid plexus over fourth ventricle 



spinal cord 



Figure 13-1. Semidiogrommatic medial aspect of right half of a human brain. 



The two hemispheres are joined across the midline above 

 the level of the diencephalon by a corpus caliosum formed 

 of transverse fibers. This structure is characteristic of the 

 placental brain and is derived from the pallial commissure 

 of the opossum, the monotreme, or the reptile (Figures 13- 

 2, 13-4). Hanging down from the corpus caliosum are bilat- 

 eral tracts identified as the fornix. These are suspended by 

 a septum lucidum. Below the corpus caliosum, in the ante- 

 rior wall of the brain (the lamina terminalis) is the anterior 

 commissure. Below this and anterior to it are the olfactory 

 bulbs and nerve. 



The posterior limit of the telencephalon is marked ven- 

 trally by the preoptic recess, a thinning of the floor just 

 anterior to the optic chiasma. Dorsally there is no marker, 

 but the ingrowing chorioid tissue probably represents the 

 velum transversum which separates the telencephalon and 

 diencephalon of the embryo. 



The roof of the diencephalon is modified into a chorioid 

 plexus which extends down into the third ventricle and 

 through the foramina of Monroe into the lateral ventricles 

 of the cerebral lobes. Behind this membranous roof is a 



pineal organ. This organ is derived from the epiphyseal 

 diverticulum. Ventrally and anteriorly is the optic chiasma 

 where part of the fibers of the optic nerves decussate (cross). 

 Behind this and suspended by a thin stalk is the pituitary 

 body or hypophysis. In the posterior wall of the infundibu- 

 lum, or hypothalamus, are a tuber cinereum and above 

 this paired mammillary bodies. Connecting across the cavity 

 of the diencephalon is the intermediate mass which devel- 

 ops from bilateral ingrowth of the thickened walls, the 

 thalami. The intermediate mass does not contain cross-con- 

 necting fibers. The supraoptic and paraventricular nuclei, 

 mentioned in relation to the pituitary gland, lie in the dien- 

 cephalon. 



The mesencephalon consists of the corpora quadrigemina 

 above. These are two pairs of lumps: the anterior pair, the 

 superior or anterior colliculi, are visual relay centers; the 

 posterior pair, or inferior colliculi, are auditory reflex cen- 

 ters. The side walls and floor of the mesencephalon, the 

 tegmentum, contain the nuclei for the third and fourth 

 nerves which serve the eye muscles. Nucleus here refers to a 

 clump of nerve-cell bodies, or ganglion, within the brain 



384 



THE NERVOUS SYSTEM 



