464 



PEOF. G. ELLIOT SMITH ON THE MOBPHOLOGT OF THE 



body the fibres end in the hippocampus in a manner exactly analogous to that tchich 

 characterizes the cornmissura dorsalis in the most cephalic region of the hijjpocampm. 



In 1S99 I called attention * for the first time to the fact that in the brain of Ornitho- 

 rhynchus the roof {tela) of the forebrain extends forward to be attached to the upper 

 extremity of the lamina terminalis in front of the dorsal commissure. As a result of this 

 an-angement, a small diverticulum — recessus siqjerior — of the third ventricle is formed 

 upon the upper surface of the cornmissura dorsalis. The significance of this arrange- 

 ment may possibly be not very great, but the relations of the parts which stirround this 

 little recess are most instructive when compared with those which are found in certain 

 Reptiles, and especially the Saiirians. 



The surface of the brain of Ch-nithorhynchus, exposed by a mesial sagittal section. ilagniSed '2 diameters. 



I have introduced here a figure from the memoir to which I have jost referred, in 

 order to make the general relations of this peculiar recessus superior clear, and to 

 explain the appearances presented by sections which I intend to discuss. For a full 

 account of the figure the reader is referred to the original description. 



If a section be made through the brain of Ornithorhynchus (in the plane x-y), the cut 

 surface will present an appearance which is reproduced upon a somewhat lai'ger scale in 

 the next illustration (fig. 12). The general features of this section will be familiar to 

 those who are acquainted with the writings of Alexander Hill (Philosophical Transactions, 

 1893), Symington (Journal of Anatomy and Physiology, 1S92), or of the present writer 

 (Journal of Anatomy and Physiology, 1896). I need merely call attention to the two 

 large commissural bands — the cornmissura dorsalis, connecting the typical hippocampal 

 formations which are placed upon its upper surface: and the coniniissura centralis, 

 deriving its fibres from the whole of the other cortical areas of the brain. 



* '•Further Observations ou the Anatomv of the Brain in the ilonotremata,"' Journal of Anatomy and 

 Physiology, toL xxxiii. p. 315. 



