62O 



THE BRAIN. 



[CH. XLIII. 



Cb 



hanging cerebellum (fig. 459), partly by pia mater. This piece 

 of pia mater is pierced by a hole (Foramen of Magendie}, and so 

 the cerebro-spinal fluid in the interior 

 2 a*, of the cerebro-spinal cavity is con- 



tinuous with that which bathes the 

 external surface of brain and cord in 

 the sub-arachnoid space. The fourth 

 ventricle leads into the central canal 

 of the spinal cord. The fifth ventricle 

 in the central structures of the brain 

 does not communicate with the others. 

 The cerebro-spinal fluid is a thin watery 

 fluid, containing a small quantity of 

 salts and proteids in solution, and a 

 substance which gives Trommer's test 

 for sugar ; it is, however, not sugar, 

 but a substance of the aromatic group, 

 allied to pyrocatechin. 



Before passing on to describe these 

 portions of the brain one by one, it 

 will be convenient to state first a few 

 general facts. 



(i.) In the bulb ; at the lower part, the 

 distribution of grey matter follows 

 that which prevails in the cord. Higher 

 up the chief part is found towards the 

 posterior or dorsal aspect, surrounding 

 the central canal. When the central 

 canal opens out into the fourth ven- 

 tricle, the grey matter comes to that 

 surface chiefly, and is found to consist 

 more particularly, on either side, of the 

 nuclei of origin of the cranial nerves, 

 viz., the 1 2th, nth, loth, gth, and 

 8th, and more externally of the nucleus 

 gracilis and nucleus cuneatus. In ad- 

 dition to these masses of grey matter 

 there are the olivary bodies towards 

 the ventral surface with the accessory 



Mo.- 



Fig. 460. Diagrammatic hori- 

 zontal section of a vertebrate 

 brain . The figures serve both 

 for this-and thenext diagram. 

 Mb, mid-brain : what lies in 

 front of this is the fore-, and 

 what lies behind, the hind- 

 brain; Lt, lamina termina- 

 lis ; Olj, olfactory lobes ; 

 Hmp, hemispheres ; Th. E, 

 thalamencephalon; Pn, pineal 

 gland ; Py, pituitary body ; 

 F.H., foramen of Munro ; cs, 

 corpus striatum ; Th, optic 

 thalamus ; CC, crura cerebri : 

 the mass lying above the 

 canal represents the corpora 

 quadrigemina ; Cb, cerebel- 

 lum ; M.o., medulla oblon- 

 gata ; / IX, the nine pairs 

 of cranial nerves ; i, olfactory 

 ventricle ; 2, lateral ventricle ; 

 3, third ventricle ; 4, fourth 

 ventricle ; +, iter a tertio 

 ad qnartum ventriculum, or 

 aqueduct of Sylvius. 



(Huxley.) 



olives and the external arcuate nuclei, 

 placed at the tip of the anterior fissure on either side on 

 the ventral surface of the anterior pyramids. 



(ii.) In the pons Varolii. In addition to the origins of nerves 

 in the floor of the fourth ventricle on the dorsal aspect of the 



