656 



SHELL. 



centres, of which they form such large and 

 important roots : while, allowing them to be 

 affected in their functions, we are at least not 

 justified in calling them " sensitive branches 

 of the fifth." 



By this elision of one sensitive anastomosis 

 after another, sensibility still remaining, we 

 have been led, in a retrograde course, to the 

 ganglion at the hiatus Fallopii : at and above 

 this point the evidence afforded by experi- 

 ment fails us. 



Magendie* states himself to have succeeded, 

 in one instance, in exposing tlie facial nerve 

 within the skull, or where it enters the 

 auditory nieatus ; and adds that it was in- 

 sensible to irritation. But the experiment 

 stands alone, and it appears doubtful whether 

 the portio intermedia was included in the 

 irritation. 



But the anatomical discoveries of Morganti 

 may somewhat supply the deficiency of direct 

 evidence. He has pointed out the complete 

 analogy of the facial to a spinal nerve ; and 

 hence deduces the probability, that the portio 

 intermedia, which exclusively forms the geni- 

 culate ganglion, is, like the posterior or gan- 

 gliform root of the spinal nerve, the source of 

 sensitive fibres to the compound nerve. 



The observed insensibility of the facial after 

 section of the fifth militates strongly against 

 this view. But it will be recollected that 

 although affirmed by some, it has been denied 

 by others. And even granting it to be as 

 complete as Longet states it, yet possibly it 

 would constitute a less conclusive objection 

 than might at first appear. For when we 

 consider the violent nervous shock which 

 division of the important trifacial must pro- 

 duce on the parts in the immediate neigh- 

 bourhood of its origin, and the close prox- 

 imity of the two nerves at their roots, we are 

 perhaps justified in considering the result an 

 insufficient testimony to their more imme- 

 diate connection. A comparison of the lesion 

 and symptoms in many cases of cerebral 

 hemorrhage would almost parallel the occur- 

 rences of such an anaesthesia ; while, in such 

 an instance, a direct continuity would scarcely 

 be admitted. 



But even granting that the facial nerve, 

 as thus constituted, possesses an inherent 

 sensibility, it is still probable, from its nu- 

 merous anastomoses, both with the fifth and 

 with the cervical nerves, that it subsequently 

 receives many additional sensory filaments. 

 These junctions differ from a plexus like the 

 cervical, or brachial, in the fact that, in place 

 of forming communications between the mixed 

 nerves of different segments of the body, they 

 connect nerves of different endowments. The 

 exchange appears to be at the expense of the 

 sensitive nerve ; that is, more filaments seem 

 to pass from the fifth to the facial than from 

 the latter to the former. The amount of 

 these filaments given to the different branches 

 of the seventh is said to differ : thus, Longet 

 affirms the insensibility of the " mentonnier," 

 or supra-maxillary portion. 



* Loc. cit. 



Little can here be said of the more minute 

 ramifications of Morganti's theory.* But 

 nothing that is known at present, either of 

 the facial generally, or of the chorda tympani, 

 or superficial petrosal branches, is absolutely 

 incompatible with his views. All of these 

 branches, except the lesser petrosal, he 

 shows to be mixed nerves : the experiments 

 and observations above quoted tend to indi- 

 cate all as more or less directly subservient 

 to motion ; and in none are we able to deny 

 the possibility of sensation. But the petrosal 

 nerves would probably be likened to the 

 branches which connect the roots of the 

 spinal nerves to the sympathetic ganglia on 

 the side of the spine ; and the obscure and un- 

 certain results obtained by experiment on these 

 filaments of the facial are closely analogous to 

 the effects of similar experiments on the spinal 

 nerves in connection with the sympathetic of 

 the trunk. But the anomaly of the tensor 

 tympani being apparently supplied solely from 

 the sensitive portion of the facial nerve, is very 

 much weakened by the physiological facts of 

 the involuntary character of its movements, 

 and the interposition of a second ganglion. 



The more important effects of disease of 

 the facial nerve have already been spoken of 

 in treating of its functions. For its morbid 

 anatomy, in which it offers no especial pecu- 

 liarity, reference is made to the article 

 NERVE. 



BIBLIOGRAPHY. See NERVOUS SYSTEM. 



( William Brinton.) 



SHELL. This term is commonly employed 

 to designate the hard envelopes in which the 

 bodies and members of many animals be- 

 longing to the Radiated, Molluscous, and 

 Articulated sub-kingdoms are enclosed. Ge- 

 nerally speaking, it is applied to those only 

 into whose composition mineral matter enters : 

 thus, we speak of the shell of a Crustacean, 

 whilst we do not give that appellation to the 

 dermo-skeleton of an Insect or Myriapod. 

 Still this rule is not strictly observed ; for 

 there are many Crustacea and Mollusca which 

 are commonly spoken of as possessing shells 

 although these bodies are entirely destitute 

 of calcareous matter, being as horny in their 

 texture as the envelope of a beetle or a centi- 

 pede. Among radiated animals, the class of 

 ECHINODERMATA is the only one in which 

 shells are met with ; and these are by no 

 means universally present throughout the 

 group. In the molluscous series, we meet 

 with shells in every class save the TUNICATA ; 

 all the animals of the class CONCHIFERA, \V\IQ- 

 iher lamella-branchiate or pallia-branchiate, being 

 furnished with them ; a considerable propor- 

 tion of GASTKROPODA (all of them, it would 

 seem, in the embryonic state) possessing them ; 



* The comparative anatomy of the geniculate 

 ganglion seems to show that its position is much 

 more closely related to the orifice of the bone than 

 to the motor nerve. Ib this any analogy to the 

 similar close relation (to a more external aperture) 

 of the ganglion on the posterior root of a spinal 

 nerve ? 



