Mesencephalon. 



Metencephalon. fcSp Cerebellum. 



AA 



Myelencephalon. i l,g | Medulla oblongata. 



Medulla spinalis. 



DIAGRAM FROM J. B. JOHNSTON'S Nervous System of Vertebrates. 

 FIG. 13. A, the spinal cord, is prolonged upwards into B, the 

 medulla oblongata (myelencephalon). In this region the spinal 

 cord seems to widen out into a thin membranous roof covering the 

 central canal (fourth ventricle). The lateral walls of the medulla 

 oblongata contain aggregations of ganglionic nerve cells, which 

 form the nuclei or place of origin of some of the most important 

 nerves of the body. C, the metencephalon, forms a short section 

 of the brain, its roof is known as the cerebellum, the arms of 

 which encircle this section of the brain (Pons Varolii). D, the 

 Mid-brain (mesencephalon), consists of ventral and lateral walls 

 of nervous matter enclosing a narrow canal (Aqueduct of Sylvius). 

 Its dorsal walls form the two optic lobes. E, the Inter-brain 

 (diencephalon) constitutes, by means of its thickened lateral walls, 

 the optic thalami ; the dorsal wall is also thickened at one point 

 by fibres which connect the two knot-like portions of nervous 

 substance known as the nuclei trabeculae. F, the Fore-brain (telen- 

 cephalon) consists of paired lateral lobes from the anterior end of 

 which processes pass to the olfactory lobes. A membranous roof 

 connects and covers the lateral lobes of the Fore-brain ventricle, 

 which divides anteriorly into a Y-shaped cavity and is continued 

 into the olfactory bulbs. The common cavity with that of the 

 inter-brain (diencephalon) constitute the third ventricle of the 

 brain ; the lateral or olfactory portions are comparable with the 

 lateral ventricles of the mammalian brain. 

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