IV, OF RANA PIPIENS. 17 



of the cells. Such, however, cannot be the case in the cerebral lobes, since there 

 are no nerve tubes except at the base of the brain, and these are entirely out of 

 proportion to the cells above them. 



Having failed to demonstrate the intimate connection between the tubes and 

 cells, the following general arrangement of the two was nevertheless easily traced, 

 and the more easily from the general absence of commissural fibres in the en- 

 cephalic masses. The appearance w^hich first of all attracts the eye is the strong 

 contrast of color between the brain and the spinal chord, the first seeming almost 

 gelatinous, and the second of an opaque white, a contrast depending upon the 

 predominance of nerve tubes in the latter and of nerve cells in the former. The 

 white fibres of the chord, as they pass forwards from the medulla oblongata, may 

 be seen entering the difi'erent cerebral masses, as indicated in the plan. (Fig. 9, 

 Plate I.) Some of the fibres forming the edge of the fourth ventricle can be 

 traced into the cerebellum, where they extend towards the median line ; but few of 

 them reach the posterior edge of this organ, which has the gelatinous appearance 

 of those parts which consist wholly or in a great measure of cells. Other fibres 

 from the lateral portions of the chord are traceable to the optic lobes, on entering 

 which they are crossed by the fibres of the optic nerves, coming from the opposite 

 direction and passing them externally. The remaining portion of the fibres which 

 are continued forwards from the chord pass beneath the optic lobes, and divide into 

 the crura cerebri, where they can be more easily followed by slitting the brain longi- 

 tudinally ; some of them terminate in the optic thalami, and others, quite few in 

 number, and only detected by the microscope, are seen to enter the corpora striata 

 and the outer walls of the base of the cerebral lobes, each crus subdividing in order 

 to reach those parts ; finally, the last traces of longitudinal fibres are seen follow- 

 ing the base of the brain as far forwards as the olfactory lobes. No portion of the 

 brain contains so many nerve tubes as the optic lobes ; they are derived from the 

 two sources above mentioned, namely, the chord and the optic nerves, and within 

 the substance of the lobes the two kinds are so completely interwoven and inter- 

 mixed, as to render abortive every attempt to unravel them, and determine the ex- 

 istence of a definite plan. 



The general result, then, which has been obtained from an examination of the 

 minute structure of the brain, is simply this: — 1st. That each of the cerebral 

 masses is in direct connection with the spinal chord, by nerve tubes extending 

 from the one to the other. 2d. All the longitudinal fibres of the brain appear to 

 be accounted for, in the species here described, by the fibres prolonged from the 

 chord. 3d. With the exception of the optic lobes, the cerebral masses consist 

 mainly of cells, none of which are caudate, and none of which have any visible 

 connection with the nerve tubes. The cells are all nearly spherical, and their 

 contents are granular. 4th. No longitudinal commissural fibres exist, nor any 

 thing analogous to a fornix. 5th. As regards the functions of the cerebral lobes, 

 their bases excepted, whatever force they originate, whatever impulses they trans- 

 mit, and whatever impressions tliey receive, all must be effected by the aid of cells 

 alone ; these must be both originators and conductors of nervous force. 



