SUMMARY OF THE RHOMBENCEPHALON. To the rhombencephalon or hind-brain belong :.
The isthmus rhombencephali, The metencephalon, The myelencephalon.
It encloses the fourth ventricle.
To the isthmus rhombencephali belong : Dorsal—The brachia conjunctiva cerebelli, The velum medullare anterius, The trigonum lemnisci ; Ventral—The crura cerebri.
To the metencephalon belong : The pons and the cerebellum.
The cerebellum is subdivided into the worm and the hemispheres. More or less deeply penetrating fissures separate the lobes of the hemispheres from one another. The chief segments are the lobus superior, the lobus posterior and the lobus inferior, each of which is made up of lobules. The individual lobes and lobules of the hemispheres corre spond to definite divisions of the worm.
The myelencephalon, or the medulla oblongata, has as its upper boundary, ventrally the lower border of the pons, dorsally the striae medullares fossae rhomboideae. Below, the medulla passes into the spinal cord, its ventral boundary being the lower end of the pyramidal decussation. Dorsal, behind the rhomboid fossa, are the dorsal and lateral columns, with their tubercula, and the restiform bodies. Ventral, lie the pyramids
and the olives.
The fourth ventricle has as its roof the velum medullare anterius, the brachia conjunctiva cerebelli, the velum medullare posterius and the tela chorioidea ; as its floor, the fossa rhomboidea. It is connected with the third ventricle by means of the aquae ductus cerebri, below is continuous with the central canal of the spinal cord, and com municates with the subarachnoid space by means of the apertura mediana (foramen Magendii) and the aperturae laterales (foramina Luschkae).
The most important masses of gray substance within the rhombencephalon are : The nucleus lemnisci, The nucleus pontis, The substantia corticalis cerebelli, The nucleus dentatus The nucleus fastigii The nucleus globosi cerebelli, The nucleus emboliformis The nucleus gracilis, The nucleus cuneatus, The nucleus lateralis, The nucleus arcuatus, The nucleus olivaris inferioris, The nuclei nervorum, within the floor of the fourth ventricle.