520 THE BRAIN 



part of the brain which connects the lower segment in the posterior fossa 

 with the upper segment in the middle and anterior fossa is called the 

 mesencephalon. The parts above the mesencephalon form collectively 

 the prosencephalon, and the mesencephalon and prosencephalon together 

 constitute the cerebrum. 



The prosencephalon itself is separable into two main parts, the telen- 

 cephalon (end brain), and the diencephalon. The telencephalon 

 includes the cerebral hemispheres with their grey nuclei, the olfactory 

 bulbs and tracts and the associated parts, and the pars optica hypo- 

 thalami; under the latter term are included the tuber cinereum, the 

 infundibulum, the hypophysis, the optic tracts, the optic chiasma, and 

 the lamina terminalis. 



The diencephalon includes two closely associated segments, the pars 

 mamillaris hypothalami and the thalamencephalon. 



The pars mamillaris hypothalami is formed by the mamillary bodies 

 and those portions of the walls of the third ventricle which lie below the 

 sulcus hypothalamicus. The thalamencephalon is separated into the 

 thalamus (O.T. optic thalamus}, the metathalamus, formed by the two 

 geniculate bodies, and the epithalamus, which consists of the pineal body, 

 the habenula, the habenular commissure, and the trigonum habenulae. 



THE PARTS OF THE BRAIN WHICH LIE IN 

 THE POSTERIOR CRANIAL FOSSA. 



The parts which lie below the tentorium cerebelli in the 

 posterior cranial fossa are the medulla oblongata, the pons, 

 and the cerebellum. These are grouped around the fourth 

 ventricle of the brain a cavity which communicates with the 

 central canal of the medulla spinalis below and with the 

 aquaeductus cerebri above ; and they constitute the rhomben- 

 cephalon or hind brain. 



Medulla Oblongata. This is the continuation of the spinal 

 medulla into the brain. It is not more than one inch in length, 

 and may be reckoned as beginning at the level of the foramen 

 magnum. Thence it proceeds upwards, in a very nearly vertical 

 direction, and ends at the lower border of the pons. At first 

 its girth is similar to that of the spinal medulla, but it rapidly 

 expands as it approaches the pons, and consequently it 

 presents a more or less conical appearance. Its anterior 

 surface lies in the groove on the basilar portion of the 

 occipital bone, whilst its posterior aspect is sunk into the 

 vallecula of the cerebellum. 



The medulla oblongata is a bilateral structure, and this is 

 evident even on an inspection of its exterior. The antero- 

 median and postero-median sulci on the surface of the spinal 



