OF DENISON UNIVERSITY. 27 



The brains were hardened for 24 hours in weak chrome acetic 

 acid, then passed gradually from 50 per cent, to commercial alcohol, 

 as usual. All sections were stained by hand with Grenacher's haema- 

 toxylin. The tracts were brought out better by using the concentrated 

 stain, making it quite intense. For the minute histology, however, 

 much better results were secured by diluting the stain from four to six 

 times its volume and applying it long enough to give only a faint 

 color. 



EXTERNAL FORM AND MEASUREMENTS. 



Erethizon dorsahis. This brain differs greatly in general appear- 

 ance from the usual rodent type. The narrow anterior extremity, 

 passing into the olfactories, the compression at the Sylvian region, and 

 the widely diverging fiocculi, which give to the rodent brain its char- 

 acteristic appearance are not obvious here. Judging from Owen's fig- 

 ures,* there is a resemblance in these points to the beaver. The porcu- 

 pine brain is much wider in proportion to the length of the hemisphere, 

 but both are well filled out in front. The European porcupine, 

 Hystrix crista, has these characteristics much more pronounced. If 

 Owen's figures'!* be correct, the cerebrum is considerably wider in pro- 

 portion to its length than that of Erethizon. The common American 

 muskrat, Fiber zibethicus, also has a cerebrum which much resembles 

 that of Erethizon. 



The Rhinencephalon. The olfactories appear rather smaller than 

 in most rodents and project only about three mm. beyond the hemi- 

 spheres. This is due in part to the unusual development of the an- 

 terior portion of the hemispheres themselves, in part to the fact that 

 the olfactory lobes are closely appressed to the hemispheres. The ol- 

 factory crus is flexed dorsad and laterad, so that the two lobes are flat 

 tened against the cephalo-ventral aspect of the hemispheres, which 

 latter are also flattened at the same region. The two olfactory crura 

 are in close contact, but the lobes diverge dorsallv. This appression 

 of the olfactory lobes may be due to the enormous development of the 

 front part of the skull. The inflation of the skull in the region of the 

 nasal cavities and above them could readily crowd so closely upon 



R. Owen, " On the Structure of the Brain in Marsupials," Philosophical 

 Transactions, MDCCCVII, p. 93, Plate V. 



t" Comparative Anatomy and Physiology of Verterbrates," Vol. Ill, p. no 



