



THE HIND-BRAIN OR RHOMBENCEPHALON 



Sylvius), which serves as a tubular communication between the third and fourth 

 ventricles; while its walls are thickened to form the corpora quadrigemina and 

 cerebral peduncles. The fore-brain undergoes great modification : its anterior part 

 or telencephalon expands laterally in the form of two hollow vesicles, the cavities 

 of which become the lateral ventricles, while the surrounding walls form the cere- 

 bral hemispheres and their commissures ; the cavity of the posterior part or dien- 

 cephalon forms the greater part of the third ventricle, and from its walls are devel- 

 oped most of the structures which bound that cavity. 



THE HIND-BRAIN OR RHOMBENCEPHALON. 





The hind-brain or rhombencephalon occupies the posterior fossa of the cranial 

 cavity and lies below a fold of dura mater, the tentorium cerebelli. It consists 

 of (a) the myelencephalon, comprising the medulla oblongata and the lower part 

 of the fourth ventricle; (b) the metencephalon, consisting of the pons, cerebellum, 

 and the intermediate part of the fourth ventricle; and (c) the isthmus rhomben- 

 cephali, a constricted portion immediately adjoining the mid-brain and includ- 

 ing the superior peduncles of the cerebellum, the anterior medullary velum, and 

 the upper part of the fourth ventricle. 



The Medulla Oblongata (spinal bulb). The medulla oblongata extends from 

 the lower margin of the pons to a plane passing transversely below the pyram- 

 idal decussation and above the first pair of cervical nerves; this plane corre- 

 sponds with the upper border of the atlas behind, and the middle of the odontoid 

 process of the axis in front; at this level the medulla oblongata is continuous 

 with the medulla spinalis. Its anterior surface is separated from the basilar part of 

 the occipital bone and the upper part of the odontoid process by the membranes 

 of the brain and the occipitoaxial ligaments. Its posterior surface is received into 

 the fossa between the hemispheres of the cerebellum, and the upper portion of it 

 forms the lower part of the floor of the fourth ventricle. 



The medulla oblongata is pyramidal in shape, its broad extremity being directed 

 upward toward the pons, while its narrow, lower end is continuous with the medulla 

 spinalis. It measures about 3 cm. in length, about 2 cm. in breadth at its widest 

 part, and about 1.25 cm. in thickness. The central canal of the medulla spinalis 

 is prolonged into its lower half, and then opens into the cavity of the fourth ven- 

 tricle; the medulla oblongata may therefore be divided into a lower closed part 

 containing the central canal, and an upper open part corresponding with the lower 

 portion of the fourth ventricle. 



The Anterior Median Fissure (fissura mediana anterior; ventral or venlromedian 

 fissure) contains a fold of pia mater, and extends along the entire length of the 

 medulla oblongata: it ends at the lower border of the pons in a small triangular 

 expansion, termed the foramen cecum. Its lower part is interrupted by bundles 

 of fibers which cross obliquely from one side to the other, and constitute the pyram- 

 idal decussation. Some fibers, termed the anterior external arcuate fibers, emerge 

 from the fissure above this decussation and curve lateralward and upward over 

 the surface of the medulla oblongata to join the inferior peduncle. 



The Posterior Median Fissure (fissura mediana posterior; dorsal or dorsomedian 

 fissure) is a narrow groove; and exists only in the closed part of the medulla oblon- 

 gata; it becomes gradually shallower from below upward, and finally ends about 

 the middle of the medulla oblongata, where the central canal expands into the 

 cavity of the fourth ventricle. 



These two fissures divide the closed part of the medulla oblongata into sym- 

 metrical halves, each presenting elongated eminences which, on surface view, 

 are continuous with the funiculi of the medulla spinalis. In the open part the 

 halves are separated by the anterior median fissure, and by a median raphe which 



