]50 ANATOMY OF VERTEBRATES. 



the * fourth,' fig. 128, c/, have the same origin, distribution, and 

 connections with the sympathetic, as in Man. The branch of the 

 ' third ' nerve, which runs along the lower part of the eye-ball, 

 between the 'inferior' and ' external' rectus muscles, and supplies 

 the ' obliquus inferior,' is connected, usually by a short thick 

 cord, with a ' lenticular ganglion ; ' but this is not so well defined 

 in some Mammals, and the ciliary nerves are usually feAver than 

 in Man. The ( fourth ' nerve supplies the ' obliquus superior ' 

 muscle. In the Sheep this nerve receives some branches from the 

 ophthalmic division of the ( fifth ' nerve. Besides the ( rectus ex- 

 ternus,' the sixth nerve, fig. 128, f, in most Mammals, supplies 

 an additional muscle, the ( retractor oculi.' The ( fifth ' or ( tri- 

 geminal' nerve, fig. 128, e, e', is commonly the largest of the 

 cerebral nerves, and resembles the myelonal nerves, fig. 136, in 

 having a gaiiglionic, fig. 230, 9, 10, and a non-ganglionic, ib. n, 

 portion, the latter being ( motorj,' supplying muscles, the former 

 distributed to sensitive and secerning surfaces. This distinc- 

 tion is better marked in Mammals than in Birds and Reptiles : 

 like which, however, the ganglion is single, not divided, as 

 in most Fishes (vol. i. figs. 201, 202). The size of the 'fifth' 

 nerve relates to the perfection or sensitiveness and application 

 of those surfaces, not to the proportion of the facial to the cranial 

 part of the head. Thus we find the fifth or trigeminal nerve of 

 largest relative size in the Ornithorhynchus paradoxus, which 

 uses, like the duck, its beak as a tactile instrument in the detec- 

 tion of its food. Emerging from the ganglion, fig. 51, o ', anterior 

 to the pons, ib. c, it soon divides into three branches, the first 

 and second appearing as one. The first and smallest division 

 divides into two equal branches : the superior or ethmoidal branch 

 enters the nose, combines, in part, with the olfactory, for the 

 service of the pituitary membrane ; but mainly emerges from the 

 nasal cavity, supplies the skin at the upper part of the face, and, 

 by a branch continued from between the nasal and premaxillary 

 bones, is distributed to the nostrils and contiguous integument. 

 The second division of the fifth is two lines broad and one line 

 and a half thick : after emerging by the foramen rotundum, the 

 chief part of it passes through the ant-orbital canal, and divides 

 into two branches, distributed, the one to the nasal or upper 

 parietes of the face, the other to the lateral or labial integuments. 

 The palatine branch divides into a posterior smaller nerve, which 

 passes through the posterior palatine foramen : the anterior and 

 larger branch emerges from the anterior palatine canal, and supplies 

 Jacobson's organ at the floor of the nose and the palatine membrane. 



