THE MEDULLA OBLONG AT A 307 



The tentorium covers the upper surface of the cerebellum. 

 Its posterior border, where it is attached to the transverse 

 ridges of the occipital bone, encloses the lateral sinuses; along 

 the superior border of the petrous portion it forms the superior 

 petrosal sinus, and at the junction of its upper surface with 

 the falx cerebri is the straight sinus. Besides these points, 

 it is attached to the anterior and posterior clinoid processes. 

 Its anterior concave edge is free, and with the dorsum sellse 

 forms a large oval opening. This is called the incisura tentorii 

 and transmits the mesencephalon. 



The pia is a very vascular delicate membrane which dips 

 into the sulci and forms the various choroid plexuses and also 

 the velum of the third ventricle. The vessels of the brain 

 run in the pia mater before entering the brain. 



The arachnoid is a similar membrane to that of the cord, 

 and is separated, as in the cord, by the subarachnoid fluid 

 from the pia. It does not dip into the sulci. In front it leaves 

 a space between it and the pia mater, viz., along the pons and 

 interpeduncular region, the anterior subarachnoidean space; 

 and behind, between the medulla and the cerebellum, is a 

 second interval called the posterior subarachnoidean space. 

 Both are connected with the ventricles of the brain by the 

 foramen of Magendie in the pia mater covering the fourth 

 ventricle. 



The subarachnoid fluid is a clear alkaline fluid containing 

 1.5 per cent, of solids, animal and mineral. 



Parts Derived from the Hind-brain (Rhombencephalon) 



The hind-brain and the parts included therein the medulla 

 oblongata, the pons, and the cerebellum. 



The Medulla Oblongata 



The medulla oblongata is a pyramidal body, f to 1 inch 

 long, along its ventral surface, and f inch thick. Its larger 

 extremity is continuous with the pons; its smaller extremity, 

 directed downward and backward, blends with the spinal 

 cord. Tlie anterior surface lies on the basilar groove of the 

 occipital bone. 



