IV. 



OF RANA PIPIENS. 15 



it, are all strong indications that it is the homologue of the corpus striatum. Be- 

 hind these bodies is an opening which communicates with the third ventricle (Fig. 

 8, L), and above this another directed towards and above the optic thalami, 

 and which may therefore be regarded as the fissure of Bichat. The third ven- 

 tricle exists in the form of a fissure between the optic thalami (Fig. 8, M), com- 

 municating posteriorly with the fourth ventricle, and above is covered over pos- 

 teriorly only by commissural fibres. The optic lobes (Figs. 6, 7, 8, D) are hollow, 

 and their cavities communicate freely with each other. On removing their upper 

 walls we have brought into view two masses, which are seen from above in Fig. 6, 

 and in section in Fig. 8, which nearly fill the cavity, allowing, however, a passage 

 beneath and between them to the fourth ventricle. 



The ventricles of both cerebral and optic lobes are lined with ciliated epithelium, 

 which doubtless serves to keep in motion the secreted fluids of the ventricular 

 cavities, a function in all probability performed by the cilia of the choroid plexus 

 in the higher Vertebrates. Thus we have suggested to us again the old hypothesis 

 of the motion of the fluids contained in the brain, not moved, however, by the 

 so-called cerebral pulsations, but by an agency which, when the theory was first 

 propounded on a purely hypothetical basis, was wholly unknown. 



One of the more important features indicating perfection in the brain of the 

 higher animals is the existence of commissures between its different segments on 

 opposite sides. The corpus collosum, or great commissure between the cerebral 

 lobes, does not appear to exist in or below the Marsupials, and the pons Varolii or 

 cerebellar commissure is confined to Mammals. The nearly complete separation of 

 the cerebral lobes in Frogs would seem to preclude the possibility of any thing like 

 a corpus collosum passing from the one lobe to the other. The only commissural 

 fibres which I have been able to trace are those between the optic thalami (Fig. 

 6, D), just in front of the optic lobes ; also between the same parts in the base of 

 the brain. 



Although great labor has been expended in attempting to unravel the micro- 

 scopic structure of the brain in Vertebrates, as yet but little has been accomplished 

 beyond the mere demonstration of the histological elements and their general plan 

 of arrangement. The existence of tubular fibres and of vesicular structure is ad- 

 mitted in the nervous system of both Vertebrates and Invertebrates, but the mode 

 of combination, their precise anatomical relationship, is still unsettled ; and until 

 that relationship can be determined, but little progress can be hoped for in the 

 explanation of the mode of the origin and transmission of nervous force. Its 

 determination is no less important to the physiology of the nervous system, than 

 the discovery of the circulation of the blood was to the physiology of respiration 

 or nutrition. Some anatomists, as Hanover, describe the vesicular structure as 

 consisting either wholly or in part of caudate cells, the prolongations of which are 

 either the commencement or termination of nerve tubes ; this view has also been 

 more recently maintained by Wagner and Robin. Bidder has described and fig- 

 ured nerve tubes, which seem to dilate into ganglion corpuscles or nerve cells, and 

 then revert to the condition of nerve tubes beyond the cell, so that the cell, accord- 



