Description of the Brain of the Opossum. By Jeffries Wyman, M.D. 



The most striking feature of the brain of the opossum, when compared with that of pla- 

 cental mammals, is the large size of the olfactory lobes, and their position in front of the 

 cerebral hemispheres, the shortness of these last, the considerable interval between them 

 and the cerebellum, and the consequent exposure of the optic lobes. When the skull is 

 viewed as a whole, the cranial portion, which envelopes the brain, is remarkable for its 

 diminutive size, and in this particular reminds one of the skull of a reptUe. 



Three specimens of adult brain which had been hardened in alcohol, and were therefore 

 somewhat smaller than natural, displaced respectively 4.4, 4.4 and 4.5 cub. cent, of water, 

 and one of them measured 40 mm. in length and 20 mm. in breadth. 



I, upper, II, under, and iii, side view of brain. iv, horizontal section, showing the ventricle of the olfactory lobe, 

 corpus striatum, hippocampus, and the smaller convolution, also the optic lobes and a part of the cerebellum, v, transverse 

 section behind the anterior commissure, vi, optic lobes, optic thalami and optic nerve. 



A, Olfactory lobes. B, Cerebral lobes, or nates and testes. C, Optic lobes. D, Cerebellum, b, transverse furrow; 

 d, large internal convolution, or hippocampus; /, optic thalamus and optic nerve; g, corpus striatum; h, bulging of the sur- 

 face surrounded by well marked furrows. 



Olfactory lobes. — Seen from above, these form two egg-shaped masses (figs, i, ii, iii, iv, 

 A), flattened against each other, somewhat higher than broad, from 9 to 10 mm. long, 

 from 5 to 6 mm. high, and merging without any very distinct line of separation in the 

 lower half of the fore part of the cerebral lobes. Beneath they are from 12 to 13 mm. 

 in length, underlap the cerebral lobes and here as well as on the sides are distinctly circum- 

 scribed. Each lobe is hollow, and if a cross vertical section is made each ventricle is seen 

 as a nearly vertical slit inclined slightly outwards. It is largest in the middle of the lobe, 

 contracts toward either end, but as it approaches the cerebral lobe connects with the ven- 

 tricle of this, as seen in the horizontal section fig. iv, by an almost capillary opening. 



