PARTS OF THE BRAIN. 21 5 



hemispheres " (Figs. 217, c, 218, c). Behind this comes, finally, 

 the fifth and last part, the " neck-marrow " (medulla oblon- 

 gata, Fig. 218, mo), which includes the single fourth brain 

 ventricle and the adjoining processes (pyramids, olives, and 

 restiform bodies). The neck medulla passes directly d^tom 

 into the spinal marrow. The narrow central canal of the 

 spinal marrow extends into the wider " fourth ventricle " of 

 the neck medulla, which is rhomboidal in shape, and the 

 floor of which forms the " rhomboid groove." From this 

 proceeds a narrow duct, called the " aqueduct of Sylvius," 

 which leads through the " four-bulbs " into the third ven- 

 tricle, situated between the two " centres of sight ; " and 

 this cavity in turn is connected with the pair of lateral 

 cavities which lie right and left in the large hemispheres. 

 All the cavities of the central marrow are, therefore, directly 

 connected together. Individually all these parts of the brain 

 which we have enumerated have an infinitely complex, 

 minute structure, which we cannot now study, and which 

 hardly bears on our subject. This wonderful brain-struc- 

 ture, as it occurs only in Man and the higher Vertebrates, is 

 of the highest importance, simply because, in all Skulled 

 Animals (Craniota), it develops from the same simple rudi- 

 ments, from the five brain-bladders already enumerated. 

 (Cf. Plates VI. and VII.) 



Before we direct our attention to the individual develop- 

 ment of the complex brain from this series of simple 

 bladders, we will, in order to understand the matter more 

 clearly, glance for a moment at those lower animals which 

 have no such brain. Even in the skull-less Vertebrates, in 

 the Amphioxus, there is no real brain. In this case the 

 whole central marrow is merely a simple cylindrical cord 



