338 HISTOLOGY. 



the optic tract, and gives rise to some which connect with the motor cells 

 of the nerves to the eye muscles; others pass down the spinal cord close 

 beside the median ventral fissure. Thus the anterior corpora are centers 

 of optic reflexes. The posterior or inferior corpora, which are smaller, are 

 in relation through an intervening group of neurones, with the acoustic 

 nerves; thus they are centers of auditory reflexes. The mesencephalon 

 gives rise to the ventral root which forms the oculomotor nerve. The 

 cerebral peduncles which begin in the isthmus extend under the mesen- 

 cephalon. 



The diencephalon has on its median dorsal surface the pineal body 

 [epiphysis]. This is a small nodular structure which is thought to represent 

 a rudimentary median eye, such as is more clearly indicated in reptiles. 

 The upper part of the lateral walls of the diencephalon are each thickened 

 by a mass of nerve tissue called the thalamus. The thalami of the two 

 sides bulge inward so that their most prominent parts adhere across the 

 third ventricle. Fibers from the retina connect with nerve cells in the 

 thalamus, the latter sending their processes to the hemispheres; thus the 

 thalami have an important relation with the optic tracts. The walls of the 

 diencephalon below the thalamus form the pars mamillaris hypothalami. 

 This part of the hypothalamus includes the two mammillary bodies found 

 side by side on the ventral wall of the diencephalon (Fig. 391, B). 



Telencephalon. The fibers from the posterior part of the brain pass 

 outside of the thalami to terminate in the dorso-lateral walls of the telen- 

 cephalon. As seen in Fig. 390, B, this part of the fore-brain forms a 

 hemispherical outpocketing on either side, into each of which a pro- 

 longation of the third ventricle extends; the ' extensions are called lateral 

 ventricles (and are counted as the first two). The hemispheres enlarge, 

 growing back so as to cover the posterior portion of the brain. Their walls, 

 which externally are subdivided by grooves into convolutions, constitute 

 the pallium of the hemispheres. The olfactory bulb is the expanded 

 termination of the part of the hemispheres which receives the olfactory 

 nerves. The entire olfactory tract is called the rhinencephalon. The 

 corpus striatum is a deep portion of the hemisphere found outside of the 

 thalamus; anteriorly it forms the outer wall of the beginning of the 

 lateral ventricle. The hemispheres are connected with one another by a 

 great transverse commissure, the corpus callosum, through which fibers 

 pass from one to the other. The principal subdivisions of the hemisphere 

 are therefore the pallium, rhinencephalon, corpus striatum and corpus 

 callosum. 



Besides the hemispheres, the telencephalon forms the pars optica 

 hypothalami. This includes the optic recess in front on either side, and 



