Anatomy of the Brain of Bdellostoma Dombeyi 11 



two distinct sizes of nerve cells, large ones measuring from 46 microns 

 to 66 microns and small ones measuring from 16 microns to 26 microns. 

 The large cells are found in all parts of the utricular ganglion, but the 

 small cells are largely confined to its ventral portion, although a fairly 

 large number are found in its dorso-caudal part (Fig. 15). They are 

 particularly numerous in its ventro-caudal part (Fig. 21 and 23), but 

 comparatively few are found in the ventro-cephalic part of the ganglion. 

 The large cells are the ganglion cells of the general cutaneous fibers, 

 which innervate the lining epithelium of the ear apart from the sense 

 organs (Fig. 19) ; the small cells belong to the acusticus fibers. Fig. 23 

 shows how these two sets of fibers cross in the ganglion to reach their 

 proper paths of entry to the brain. 



The general cutaneous fibers enter the brain in numerous bundles 

 arranged in a series along the lateral surface of the medulla and 

 penetrate directly into the general cutaneous nucleus. In Worthing- 

 ton, '05, this is described as an acusticus root, as it was thought at that 

 time that its fibers penetrated to the acusticus nucleus. Golgi sections, 

 which had not been obtained at that time, disprove this completely, as 

 they show that these ventral fibers turn and run cephalo-caudad in the 

 general cutaneous nucleus (Fig. 24). This is not the ventral root 

 mentioned by Sanders, '94, the group of fibers that he calls the ventral 

 root of the acusticus being in reality the motor root of the ti^igeminus. 

 Holm, '01, probably saw the root, but did not attach any significance 

 to it, for he speaks of it as ''a few fibers that leave the ganglion here 

 and there and enter the medulla." 



The saccularis ganglion, when stripped of its nerves, may also be 

 considered as a cone, this time an inverted one, its apex directed ventrad, 

 and having the base cut away at its caudal end (Fig. 47). It is about 

 .37 mm. from the base to the apex of the cone, and its greatest cephalo- 

 caudal length is also about .37 mm. The fibers of ramus saccularis 

 leave the ganglion in two distinct sets, those of ramus saccularis anterior 

 (Fig. 47, VIIIs. a.), leaving from the apex of the cone, and those of 

 ramus saccularis posterior (Fig. 47, VIIIs. p.), leaving from the middle 

 portion of the caudal surface. The cells of the saccularis ganglion are 

 similar in size and character to the acusticus cells of the utricular 

 ganglion. 



The ramus saccularis has but one root, a dorsal one that leaves the 

 ganglion at its dorso-cephalic end and enters the brain caudad and 

 dorsad of the dorsal root of the ramus uiricularis. The saccular nerve 

 does not, apparently, carry an)'' general cutaneous fibers. 



