THE RHOMBENCEPHALON 



655 



lobe, the vermis, and two lateral hemispheres. It is marked by numerous gyri and 

 narrow sulci which have in general a transverse direction. The posterior third of 

 the medulla oblongata is not covered by the cerebellum. 



The brain is developed from the expanded cephalic part of the neural tube of the embryo. 

 The process comprises a series of tliickenings, fiexures, and une(]ual f2;ro\vth and expansion of 

 various parts of the tulie. In the higher animals the resuU is (hat tiie tubuUir cliaracter of the 

 brain is not very evident, since tiie himen comes to consist of four irregular cavities, the ventricles, 

 which are connected 1:)}' narrow passages. The tube is first sui)di\ided t)y two constrictions into 

 three brain vesicles, termed respectively the hind-brain or riiombenceplmlon, the mid-brain or 

 mesencephalon, and the fore-brain or prosencephalon. The hind-brain gives rise to three secon- 

 dary segments and the fore-l)rain to two. The annexed table indicates the origin of the princii)al 

 structures of the fully developed brain from the primitive vesicles. It has become customary to 

 describe the brain with reference to its embryologic^al relations. 



TABLE INDICATING THE DERIVATION OF THE PRINCIPAL PARTS OF THE 



Primary Segments. 



Rhombencephalon (pos- 

 terior vesicle) 



Mesencephalon (raidtlle 

 vesicle) 



Prosencephalon (ante- 

 rior vesicle) 



BRAIN 



Secondary Segments. 



Myelencephalon 



Metencephalon 



Isthmus rhombencephali. . . 

 Mc.sencei)halon 



Diencephalon . 



Telencephalon. 



Derivatives. 



Medulla oblongata. . 

 fPc 

 tCe 



ons 



^'erebellum . 



f Anterior cerebellar peduncles 

 1 Anterior medullary velum. . 



( Corjjora qvia<lii(ieniina 



i Cerebral peduncles 



f Optic thalami 



■| Subthalamic tegmenta . . . 

 I Pineal botly 



Cavities. 



Fourth ventricle 



Cerebral aqueduct 



Posterior part 

 third ventricle 



of 



Pituitary body 



Optic nerves and retina?. . 

 Cerebral hemispheres . . . . 

 Olfactory tracts and bulb. 



Anterior part of third 

 ventricle. Lateral 

 ventricles and olfac- 

 tory continuations. 



THE RHOMBENCEPHALON 

 The Medulla Oblongata 



The medulla oblongata lies on the basilar part of the occipital bone. It is 

 quadrilateral in outline, but much wider in front than behind, and compressed 

 dorso-ventrally. Its length, measured from the root of the first cervical nerve to 

 the pons, is al)out two inches (ca. 5 cm.). 



Its ventral surface is convex in the transverse direction. It is marked by a 

 median fissure (Fissura mediana ventralis) which is continuous behind with the 

 similar fissure of the spinal cord. The posterior part of the fissure is faintly marked, 

 but in front it becomes deeper and ends in a small depression (Foramen caecum) 

 behind the central part of a transverse band, tlie corpus trapezoidetun. On either 

 side of the fissure is a rounded tract, the pyramid (Pyramis), which is bounded 

 externally by a faint lateral groove (Sulcus intermedins ventralis). The pj^ramids 

 join the pons in front; behind they become narrower and disappear into the sub- 

 stance of the medulla, in which their fibers intercross, forming the decussation of 

 the pyramids (Decussatio pyramidum).' 



The superficial origin of the sixth cranial nerve (X. oculomotorius) is just 

 lateral to the anterior end of the pyramid. The corpus trapezoideum is a trans- 

 verse band which extends across the surface immediately behind the pons. It is 

 crossed by the pyramids, which cut off a small central part. The lateral part 

 extends out to the roots of the seventh and eighth nerves on either side. Behind the 

 outer part of the corpus trapezoideum there is a variably developed rounded 

 eminence, the tuberculum faciale. The root-fibers of the twelfth or hypoglossal 

 nerve form an oblique linear series lateral to the posterior part of the pyramid. 



The dorsal surface is largely concealed by the cerebellum and forms the greater 

 part of the floor of the fourth ventricle. The dorsal median fissure (Fissura 

 mediana dorsalis), the direct continuation of the corresponding groove of the spinal 



' The decussation varies superficially in different specimens. In some there is a distinct 

 superficial crossing of fibers so that the median fissure is practically effaced at this point. 



