IV. 



OF RANA PIPIENS. 

 CENTRAL AXIS. — Plate 1. 



SPINAL CHORD. — Fig. 1. 



I. Medulla Oblongata I. 



II. Brachial Enlargement L. 



III. Crural Enlargement N. 



IV. Coccygeal Portion O. 



With the exception of the corpora striata, which are lodged and concealed in the 

 canities of the cerebral lobes, all the parts above enumerated may be seen without 

 dissection, from a merely superficial examination. When stripped of their mem- 

 branes, and examined in a recent state, there is a striking contrast between the brain 

 and chord as regards color ; the first having a grayish semitransparent, almost gelat- 

 inous appearance, whilst the second is opaque and whitish, the latter color, however, 

 being traceable to a certain extent in the interior of the cerebral masses. Black 

 pigment-cells are to be seen scattered over the general surface of the brain and spi- 

 nal chord, but are especially abundant about the optic lobes, which are much darker 

 than the cerebral lobes. 



Section I. — The Brain. 



I. Olfactory Lohes. (Plate I. Figs. 1-9, A.) — These form the anterior por- 

 tion of the cerebral mass, but ai'e so intimately connected with the cerebral lobes, 

 that they might be supposed to form with them one and the same pair of organs, 

 and have been sometimes described as such. The only mark of separation from 

 the cerebral lobes is a slight constriction at the line of contact, visible both on the 

 under and upper surface, though farther back in the latter tlian in the former ; con- 

 sequently, when seen from below, the olfactory lobes appear relatively larger than 

 from above. A condition of things rarely met with, perhaps only in a few Anou- 

 rous Batrachians, is the fusion of the right and left olfactory masses, with scarce a 

 trace of any indication that they are double organs. In Toads they are likewise 

 fused, but are separate in Siren and Menohranchus. (Fig. 5.) This union of the 

 olfactory lobes, however, is analogous to what occurs in the cerebral lobes of Sharks 

 and other Plagiostome Fishes, and in the optic lobes of the Lepiilosiren, and as it 

 seems in 3Ienobranchus. Their fusion is a subject of additional interest, since it 

 tends to show that they are developed from a single embryonic vesicle, and not from 

 a pair of vesicles. If this statement be true, then we have the olfactory lobes ar- 

 rested in their development, previous to the division of this vesicle. An analogous 

 state of things is easily showii in a chick of the fourth day, where the optic lobes 

 form a single vesicle, though they subsequently become double and widely sepa- 

 rated from each other. In Lepidosireti, according to Owen, there is but one optic 

 lobe, and that on the median line ; we may therefore regard this last as the vesicle 



