PYRAMID TRACT IN SQUIRREL AND CHIPMUNK 141 



certainly be overlooked since it would be masked by the much 

 larger crossed tract which occupies the same position in the cord. 

 Nowhere in the hterature are to be found entirely satisfac- 

 tory descriptions of the position of the pyramid tract in the 

 brain and spinal cord in the lower orders of mammals. These 

 are confined, for the most part, to the character of the decussa- 

 tion and do not deal with the topographical relationships at the 

 different levels. It seemed desirable, therefore, to investigate 

 further the course of this important tract by the Marchi method 

 in such of the smaller rodents as could be obtained in this country, 

 and the present paper gives the results as found in two closely 

 allied species, namely, the red squirrel and the chipmunk. 



PRESENT INVESTIGATION 



In three red squirrels (Sciurus hudsonius loquax) and three 

 chipmunks (Tamias striatus lysteri), the motor cortex of the 

 left cerebral hemisphere was successfully removed, the operation, 

 in each case being performed under ether anesthesia. After 

 exposing the brain the motor area was located by electrical stim- 

 ulation but it was found impossible to delimit the area with any 

 degree of accuracy on account of the small size of the animal. 

 It was difficult to get exposure close up to the middle hne since 

 the dura mater is very thin and the great longitudinal sinus 

 easily ruptured. After the motor area had been located a shallow 

 incision was made around it with a tenotomy knife and the cor- 

 tex scraped away with a sharp spade within the Umits of this 

 incision. The part of the area extending along the margin of the 

 longitudinal fissure was 'under cut' so as to divide all the nerve 

 fibers coming from it but it was not entirely removed for the rea- 

 son given above, namely, the risk of hemorrhage from the longi- 

 tudinal sinus. 



The animals were kept under observation for periods varying 

 from thirteen to sixteen days after operation; they were then 

 killed by coal gas, the brain and cord removed and placed in 

 3 per cent potassium bichromate solution. After hardening for 

 three weeks in this solution, with frequent changing, the tissue 



