PYRAMID TRACT IN SQUIRREL AND CHIPMUNK 139 



In the monotremes and marsupials the fibers are said to decus- 

 sate into the dorsal columns. Ziehen ('99) found it to be so 

 in the ring-tailed phalanger (Pseudochirus peregrinus). In 

 another species, the koala (Phascolarctus cinereus), he was 

 unable to say whether the fibers, after crossing, pass into the 

 dorsal or lateral column. Edinger ('11) describes complete decus- 

 sation into the dorsal column in one of the marsupials (Halma- 

 turus giganteus). 



The rodents form a large order and comparatively few species 

 have been examined, but here there is a distinct decussation of 

 the pyramids in the lower part of the medulla oblongata, the 

 crossed fibers passing into the dorsal columns. There is one 

 notable exception, however, and that is the Leporidae, including 

 the rabbits and hares. In this family the fibers, after decussat- 

 ing, pass into the lateral column. In the rabbit the crossing is 

 not complete; a few fibers can be traced into the lateral column of 

 the same side. The pyramid tract in this animal, therefore, appears 

 to agree in every respect with that in the cat, dog, and monkey, 

 except that its cortico-spinal fibers are more scanty. 



In the rat the pyramid tract was found by Stieda ('69) to 

 . decussate into the posterior column, and this has been confirmed 

 by Spitzka ('86), Goldstein ('03), Van der Vloet ('06), King ('10) 

 and Ranson ('13). Most of these observers believe that the de- 

 cussation is complete. 



A complete crossing of the cortico-spinal fibers into the pos- 

 terior column has also been described by Koteenberg ('99) in 

 the marmot, and by Bechterew ('90), Wallenberg (cited by 

 Goldstein '03) and Reveley and Simpson ('09) in the guinea-pig. 



In the Canadian porcupine (Erethizon dorsatus, Linn.) the 

 condition is unique (Simpson '12). The fibers of the anterior 

 pyramid, on entering the cord, divide into fom' fasciculi, two 

 crossed and two direct. Of the crossed fibers, the greater number 

 pass into the dorsal column (crossed dorsal tract) , but a few can 

 be followed into the lateral column (crossed lateral tract). A 

 very considerable number of fibers remain uncrossed and are 

 continued into the spijial cord, forming a comparatively large 

 and compact bundle in the ventral column extending along 



