Lewis, Mixed Cerebral Nerves. 1 79 



Fig. I, A, a modification of Johnston's diagram based chiefly 

 upon Petromyzon.^ The trigeminus is here represented by two 

 separate nerves, the ophthalmic in front, and the maxillo-mandi- 

 bular behind. The latter divides into the maxillary branch, in 

 front of the oral cleft, and the mandibular branch, posterior to the 

 cleft. The ophthalmic nerve is purely sensory. The maxillo- 

 mandibular is mixed; its minor portion, in fishes generally, ac- 

 companies the mandibular branch exclusively.^ 



The intermedins, after uniting with its facial portion, divides 

 into three branches. The post branchial passes down behind the 

 first branchial or spiracular cleft, in the hyoid arch. The supra- 

 branchial runs forward above the cleft toward the palatal region. 

 From it the third or praebranchial branch passes down in front of 

 the first cleft, in the mandibular arch. Of these three branches 

 the suprabranchial and praebranchial are sensory and the post- 

 branchial is mixed. 



The glossopharyngeus is similarly related to the second cleft. 

 It also has sensory suprabranchial and praebranchial divisions, 

 and a mixed postbranchial. The same is true of the vagus, but 

 this nerve supplies not only the third cleft but all those which 

 follow it. It is assumed that each of these once had its separate 

 nerve. By the formation of a collecting commissure along the 

 brain, and of a distributing commissure above the branchial clefts, 

 and by the disappearance of such portions of the nerves as ex- 

 tended between these commissures, the posterior branchial nerves 

 have become a part of the vagus. This process would lead to such 

 a vagus as is shown in the diagram. 



In mammals the branches of the mixed cerebral nerves have 

 been carefully studied by Froriep,^ in cow embryos. A recon- 

 struction of a 12 mm. pig, from which the diagram. Fig. i, B, has 

 been drawn, agrees closely with his figures. The trigeminus in 

 these mammals shows no evidence of its double origin, the 

 ophthalmic nerve having fused completely with the maxillo- 

 mandibular. All of the motor fibers go with the mandibular 



'Johnston, J. B., Journ. of Comp. Neurology, 1905, Vol. 15, pp. 175-275, PI. 4. 



"In Bdellostoma some motor fibers enter the maxillary nerve (Worthington, J., ^uart. Journ. 

 of Mic. Science, 1905, Vol. 49, p. 168) and in Petromyzon a large part of this nerve is motor (John- 

 ston, J. B., Morph. Jahrh., 1905, Vol. 34, p. 156). 



^Froriep, a., Arch. f. Anat. u. Entw. 1885, pp. 1-55. 



