10 Howard Avers and Julia Worthington 



bundles for varying distances, and are eventually distributed in the 

 cerebellum (Figs. 26 and 30, tr. a. c). In addition to this definite 

 cerebellar tract an occasional fiber may be seen that leaves the central 

 part of the acusticum and runs cephalad into the cerebellum. There 

 is no indication that these fibers cross. 



The Auditory Nerves. 



In Bdellostoma there are two distinct auditory nerves, each with its 

 &wTt ganglion, one for each of the two main divisions of the auditory 

 5ac. These nerves are the ramus acusticus utricularis, and the ramus 

 acusiicus ^accularis. 



The Auditory Ganglia. — These two ganglia lie laterad of the medulla 

 between it and the cranial wall (Figs. 15, 21, 22, 23 and 24) ; they are 

 closely apposed to each other, and the two are wrapped together in a 

 common connective tissue sheath, so that when dissected out they appear 

 as one body, and it requires microscopic study to show their histological 

 separateness. The utricular ganglion (Fig. 47) lies the more cephalad, 

 and is also the larger of the two. It is cone shaped, with its apex 

 directed dorsally. The long axis of its ovoid base runs parallel to the 

 long axis of the lateral face of the medulla. The plane of the base is 

 inclined at an angle of about 15° to the plane of the long axis of the 

 brain. The extreme length of the base of the cone is about 1.37 mm. 

 and its height about 1.25 mm. The caudal face of the cone is not 

 uninterrupted in its slope from apex to base like the other faces. 

 About .9 mm. ventrad of the apex it leaves the saccularis ganglion, to 

 which it has been applied, and its face runs ventro-cephalad from this 

 point of division for about .4 mm. It then makes an angle of 18° and 

 runs ventro-caudad for about .4 mm. At this point there is a space of 

 about .2 mm. between the dorsal and ventral limits of the ganglion, and 

 across this entire face the ramus utricularis is given off (Fig. 47, 

 VIIIu). Through the indentation mentioned above the facialis passes 

 between the cranial wall and the brain (Figs. 21 and 47). 



The utricular fibers pass from the ganglion to the brain in two 

 distinct roots (Fig. 47), a dorsal root, whose emergence is confined to 

 the dorsal third of the ganglion (this root consists exclusively of 

 acusticus fibers), and a ventral root emerging along the mesial face 

 from the indentation dorsad about .3 mm., consisting of general cu- 

 taneous fibers. 



In histolooieal structure the utricular sfanalion is found to consist of 



