A STUDY OF THE SUPERIOR OLIVE. 163 



that it completely overhangs the medial portion of the lateral mass. Figure 7, 

 plate 2, shows the relative positions of the nuclear complex in transverse section, 

 and figures 2 and 3, plate 1, show the model of the entire nucleus, the two portions 

 having been pinned together in the piling, in order that their proper relations might 

 be retained. The model of the medial mass shows an antero-posterior concavity 

 laterally directed, which corresponds to the rounded medial portion of the lateral 

 mass, the two being separated by a narrow interval occupied by the fiber bundles, 

 all of which appear to run parallel to the long diameter of the nuclear masses that 

 bound them in transsection. 



The laterally placed mass is more complex and of considerably greater bulk 

 than is the medial portion. Beginning caudal ly as an irregular cell mass, this 

 lateral portion soon assumes the typical S-shaped double curve, the medial bar of 

 the S pointing ventralward, the lateral bar dorsalward. Thus there are twohila, a 

 medial one opening ventrally , a lateral one opening dorsally, the lateral limb of the S 

 and its ventral coil being shorter and more robust than the medial one, the bar of 

 which is slender and elongated. Its ventral tips in successive sections constitute 

 the most irregular portion of the entire nuclear mass. At certain levels, corre- 

 sponding to the more atypical portions to be found at the caudal pole of the mass, 

 there are numerous small, irregularly shaped cell-masses, situated in the area 

 between the ventral extremities of the medial bar and the medial limb of the lateral 

 mass, giving the impression that there had at one time been a continuity of struc- 

 ture which had been severed by the fiber-bundles so numerous in this area. The 

 surface of the lateral mass is smooth and is more regular in outline than that of 

 the medial bar. The thick lateral coil, which is ventrally directed, ends abruptly as a 

 wide cell-mass at a point somewhat cephalad to the mid-point of the general nuclear 

 mass; whereas the less robust medial coil, which is dorsally directed, extends farther 

 cerebralward, becoming progressively smaller and ending in a blunt, rounded point 

 almost in contact with the shelving lateral surface of the medial bar at the junction 

 of its caudal three-fourths and cerebral one-fourth. This medial portion of the 

 lateral mass is completely roofed in by the broad upper portion of the medial mass. 



The superior olivary nucleus in the dog presents the same general features 

 as those found in the cat, though lacking in some degree the symmetrical outline 

 which is so marked in the feline nucleus. The canine superior olive also consists 

 of two portions, a medial and a lateral portion, which are continuous for a short 

 distance toward the caudal pole of the nucleus, where there is less regularity than 

 is noted elsewhere in the nuclear mass. The medial mass begins caudally at a 

 somewhat lower level than the lateral, usually one or two sections. Comma-shaped 

 at first, it speedily becomes racquet-shaped, the handle of the racquet pointing 

 lateralward ( fig. 8, plate 2), and the larger portion thinning out in the center until 

 it consists of a central space surrounded by a narrow coil of cells. The lateral 

 portion of the coil in turn thins out and disappears as the summit of the mass is 

 approached; thus the coil becomes U-shaped in its cerebral one-fifth, the hilum of 

 the U looking lateralward. The central cavity extends fully half-way down the 

 cell-mass. The medial mass is perfectly smooth and regular in its upper one-fourth ; 



