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. 



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

 The process comprises a series of thickenings, fiexiu-es, and unequal growth and expansion of 

 various parts of the tube. In the higher animals the result is that the tubular character of the 

 brain is not \('ry evident, since the lumen comes to consist of four irregular cavities, the ventricles 

 which are connected by narrow passages. The tube is first subdividetl l)v two constrictions into 

 three brain vesicles, termed respecti\ely the hind-brain or rhombencephalon, the mid-brain or 

 mesencephalon, and the fore-brain or prosencephalon. The hind-brain gi\c.s rise to three secon- 

 dary .segments and the fore-l)rain to two. The annexed tal)le indicates the origin of the principal 

 structures of the fully developed i)rain from the primitive vesicles. It has become customary to 

 describe the brain with reference to its eml)ryological relations. 



TABLE INDICATING THE DERIVATION OF THE PRINCIPAL PARTS OF THE 



Primary Segments. 



Rhoinbencephalon (pos- 

 terior vesicle) 



Mesencephalon (middle 

 vesicle) 



Prosencephalon (ante- 

 rior vesicle) 



BRAIN 



SKroNDARV .Segmknts. 



Myeleneejihalon 



.Metencephalon 



Isthmus rhombencephali. .. 

 .Mesencei)lialon 



Diencephaloii 



Telencephalon . 



Derivatives. 



Medulla oblongata 



Pons 



Cerebellum 



Anterior cerebellar peduncles 

 Anterior iiiedullarx- velum. . 



Corpora quadrifreniina 



Cerebral pcduiicle.s 



Oi)tie thalami 



.Subthalamic tegmenta 



Pineal bodv 



Pituitary botly 



()))tic nerves and retina>. . 

 Cerebral hemispheres . . . . 

 Olfactory tracts and bulb. 



Cavities. 



Fourth ventricle 



i Cerebral aqueduct 



1 Posterior part 

 / third ventricle 



of 



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, nu^asured from the root of the first cervical nerve to 

 the pons, is about 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 sjiinal cord. The posterior part of the fissure is faintly marked, 

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

 behind the central part of a transverse band, th(^ corpus trapezoideum. 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 pyramids 

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

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

 the pyramids (Decussatio pyramidum).^ 



The superficial origin of the sixth cranial nerve (N. 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. 



