448 HISTORY OF THE HUMAN BODY 



yet, in spite of this, all are agreed that the sensory element 

 originally associated with this has become incorporated with 

 the Trigeminus. The sensory portion of the Abducens is 

 probably also a part, of the Trigeminus, although certain 

 facts indicate an association with the Facialis. 



III. THE TRIGEMINUS-FACIALIS GROUP. (Trigeminus, Fa- 

 cialis, A custicus. ) 



This group and the next are by far the most extensive, and 

 together constitute the main bulk of the nerves of the head. 

 Their relationships differ considerably in fishes and aquatic 

 amphibians on the one hand [Plate VI], and in terrestrial 

 (and secondarily aquatic) vertebrates on the other [Plate 

 VII], a difference largely due to the presence in the one and 

 the suppression in the other of an extensive system of ex- 

 ternal sense-organs of uncertain function but undoubtedly 

 of assistance in an aquatic life. These organs, variously 

 termed " integumental sense-organs" or "dermal canal sys- 

 tem'' are visible externally and are arranged in definite lines 

 running about the head and continued in a single (or double) 

 longitudinal row, the lateral line, down the sides of the body. 

 The system of nerves which supplies these is shown in the 

 first of the accompanying diagrams, and consists of three su- 

 perficial trunks directed forwards and a fourth one directed 

 backwards, the former referred to the Facialis (VII), the lat- 

 ter to the Vagus (X). To each trunk there belongs typi- 

 cally a special ganglionic swelling, placed near its origin; but 

 these are distinct in only a few forms (selachians, dipnoans, a 

 few aquatic amphibians) and in all others become completely 

 fused with the ganglion semilunare of the Trigeminus and 

 are demonstrable only in the embryo. To the compound gan- 

 glion thus resulting, which is found in most amphibians and 

 in all the amniotes, may be applied the term Gasserian, long in 

 use in human anatomy for this organ. 



The most dorsal of the three Facialis branches of this sys- 

 tem is the superficial ophthalmic (ramus ophthalmicus super- 

 ficialis Septimi), and is accompanied by a like-named branch 

 of the Trigeminus (ramus ophthalmicus superficialis Quinti), 



