468 THE NERVOUS SYSTEM. 



of internal strabismus, in man, accompanied by the above symptoms, 

 apparently due to compression of the abducens nerve within the cranial 

 cavity. 



Seventh Pair. The Facial. 



In the innervation of the external parts of the face, this nerve holds 

 an equal rank with the fifth pair, and may be regarded as complementary 

 to it in physiological endowments. As the trigeminus is the nerve of 

 sensation for the integument of this region, the facial is the motor 

 nerve for its superficial muscles. It is the nerve of expression, by 

 which the features are animated in their varying movements, corre- 

 sponding with the different phases of mental or emotional activity. 

 Although at its origin an exclusively motor nerve, it receives, soon after 

 its emergence from the cranium, a communicating branch from the fifth 

 pair, which gives to it, and "to the muscles in which it terminates, a 

 certain share of sensibility. 



The facial nerve has its principal source in a collection of gray sub- 

 stance, already described as giving origin to the abducens (page 467). 

 The fibres of the abducens and facial nerves are given off from its. 

 internal and external borders respectively ; those of the abducens passing 

 directly downward through the tuber annulare, near the median plane, 

 those of the facial first passing outward and then bending downward, 

 to their point of emergence at the -lateral part of the posterior edge of 

 the pons Varolii. 



According to Dean, Meynert, and Henle, a considerable portion of the 

 root fibres of the facial nerve communicate, either directly or through 

 the nucleus, across the median line, with the opposite side of the brain. 



After emerging from the edge of the pons Yarolii, the facial nerve, 

 in company with the auditory, passes into and through the internal 

 auditory meatus. It thence enters the aqueduct of Fallopius, and, fol- 

 lowing the course of this canal through the petrous portion of the 

 temporal bone, comes out at the stylomastoid foramen and turns forward 

 upon the side of the face. It spreads out between the lobules of the 

 parotid gland in a number of branches, which, by mutual interlacement, 

 form the well-known " parotid plexus," or "pesanserinus," of this nerve. 

 Its branches thence diverge upward, forward, and downward, to the 

 superficial muscles of the facial region. It also supplies, by branches 

 given off immediately after its emergence from the stylomastoid fora- 

 men, the muscles of the external ear, as well as the stylohyoid and the 

 posterior belly of the digastric ; and by a twig which descends to the 

 submaxillary region, it supplies filaments to the upper part of the 

 platysma myoides, and communicates with an ascending branch of the 

 superficial cervical nerve from the cervical plexus. 



Physiological Properties of the Facial Nerve. The facial is shown, 

 by the result of abundant investigations, to be, at its origin and in its 

 main physiological characters, a motor nerve. Not only is the tactile 

 sensibility of the facial region completely destroyed by section of the 



