IV. OF RANA PIPIENS. 21 



are visible on each side of the fourth ventricle, and two pyramidal tracts are con- 

 tinued from them as far as the posterior bulging of the chord, dilating in this last 

 as well as in the brachial enlargement ; in comparing the posterior pyramids with 

 the other bulgings of the pyramidal tracts, I am led to the conclusion that they 

 are I'epetitions of similar parts, and that each tract enlarges in the medulla oblon- 

 gata, just as it enlarges in the brachial or crural bulging. 



The fourth ventricle results from the separation, or perhaps more correctly from 

 the want of union, of the lateral portions of the chord on the back. By a comparison 

 of the adjoining Figs. 1 and 3, it will 

 be seen that the section of the chord 

 and that of the medulla are reducible to 

 the same typical structure. In Fig. 3 



the white substance is still presented in ,^^'"\_'^' K^^^_^ V_]j^_f'^ x^'^ 



the form of two crescents, a, but their 

 concavities, instead of being turned towards each other, are directed ixpwards, and 

 within them the gray substance, i, is spread out, and is covered by an epithelium 

 layer, c. This last is continuous with, and is an expansion of, the layer of epithe- 

 lium which lines the central canal of the chord, and the gray substance is likewise 

 continuous with that of the chord, but is fissured as far as the central canal. 



Opposite the origins of the vagus and trigeminus nerves, there are slight gan- 

 glionic projections into the cavity of the ventricle; but they are very indistinct, 

 and can only be seen after close examination. 



As has already been stated, the fourth ventricle is closed over by a membranous 

 covering, which in its minute structure is reducible to a vascular plexus, like that 

 of the pineal body, or like the plexus choroides of higher animals ; the vessels 

 form a series of loops connected with a central vessel, and projecting to the right 

 and left ; it is covered with ciliated epithelium. The existence of vibrating cilia 

 on the ventricular side, and of a fluid in the cavity of the ventricle, iuAolves 

 the necessity of the fluid being kept in constant motion. From what has been 

 before stated with regard to vibrating cilia in the cavity of the ventricles of 

 the brain and on the pituitary body, it is rendered highly probable that a cir- 

 culation of serum exists through the cavities of the ventricles, passing through the 

 passage from the third to the fourth ventricle. If vibrating cilia should hereafter 

 be detected in the central canal of the spinal chord, this last would also be the 

 seat of a similar movement. 



The change of form which the spinal chord undergoes during the progress of 

 development is one of its most interesting features, and one which long since at- 

 tracted attention. The phases are the same that are met with in other Vertebrates 

 in which limbs are developed ; but while in these the changes take place with 

 great rapidity, in Frogs generally the irhthyic condition of the chord in which 

 there are no enlargements or bulgings continues for several months, the eggs being 

 hatched in the spring and the complete development taking place in the latter part 

 of the summer. In Bull-frogs, in this latitude at least (42° North), development 

 lasts not less than a year, as the tadpoles hatched in the spring pass the following 

 4 



