SEVENTH PAIR OF NERVES. 



543 



but is absent in many Marsupials. In Birds 

 it is usually, but not invariably, present ; there 

 are even sometimes two, one placed above 

 another. In those aquatic birds which have 

 the tuberosity of the tibia prolonged upwards 

 as a large process, a patella is always found 

 placed just behind it*, sometimes closely 

 adapted to it, and extending beyond it so as 

 apparently to form its summit.-]- No patella 

 has been met with in any reptile. 



Other sesamoid bones. Opposite the plan- 

 tar aspect of the distal joint of the fore and 

 hind foot of Solipedes, there is a long super- 

 numerary bone, called by farriers the shuttle 

 bone, placed transversely. This, like the 

 sesamoid bones above described, enters into 

 the composition of the subjacent joint ; a 

 broad slip of the perforans tendon is inserted 

 into its proximal side, whilst on the distal 

 side a portion of the synovial capsule alone, 

 and that not so strong as one would expect, 

 attaches it to the Lingual phalanx ; the main 

 part of the tendon passes over it to be inserted 

 into the ungual phalanx, leaving a cavity lined 

 by a synovial membrane between itself and the 

 sesamoid bone in question. This bone re- 

 minds one of that which, as above mentioned, 

 is occasionally found at the distal joint of the 

 thumb and great toe in the human subject. 



Small bones are found in one or both heads 

 of the gastrocnemius in all Mammalia except 

 Man, the Seals, Pig, and all Ruminants but 

 Cervus, in which genus they are found, yet 

 only in the external head of the muscle. 



A sesamoid bone is met with in the inser- 

 tion of the tendo Achillis of certain Birds, as 

 the capercailzie J ; and the tendons of the legs 

 of birds are very commonly ossified, not, how- 

 ever, where they correspond to the joints. 



Use. Sesamoid bones serve much the 

 same purpose as processes for the muscles that 

 are inserted into them, without the incon- 

 venience inseparable from a process, of giving 

 an angular form to the joint. They also pro- 

 tect the long flexor tendons at points where 

 perhaps they might be injured. But after all, 

 taking into consideration all the facts related 

 above, and many others that have presented 

 themselves to me in the course of this inquiry, 

 I cannot but believe, that some higher law 

 than that of adaptation concurs in deter- 

 mining the presence, if not the size, of even 

 these little bones. 



(S. R. Pitlard.) 



SEVENTH PAIR OF NERVES (Sie- 



benter Nerv, Germ. ; Le Scptieme Nerf, Fr.). 

 In laying the foundations of the natural sci- 

 ences, few circumstances would seem to have 

 occasioned more serious and permanent embar- 

 rassments than the immediate necessity of in- 

 dicating the various new objects which they 

 presented by specific names, and the difficulty 

 of finding suitable ones. A nomenclature 

 based upon their properties, would, perhaps, 

 readily have suggested itself; but, generally 

 speaking, the recognition of the object so 



* Vol. I. p. 286. 

 J Vol. I. p. 288. 



f Meckel. 



greatly preceded the discovery of its proper- 

 ties, that this attempt was almost impossible. 

 In the science of anatomy, this was especially 

 the case ; and a large proportion of the human 

 structures were named, either according to 

 their form, or, if this was not sufficiently de- 

 fined, by their real or fancied resemblance to 

 some previously known object; or failing this, 

 by the proper names of their discoverers, 

 however polysyllabic or uncouth they might 

 happen to be. 



In some one or other of these modes, the 

 different parts of the complicated nervous 

 centre received their various designations. 

 But the cerebral nerves, although very similar 

 to each other in the outward properties of their 

 shape, size, and appearance, yet offered, by 

 their fewness, a sufficient ground of distinction 

 in the application of the ordinal numbers. 

 By denying the claims of the olfactory lobes, 

 and overlooking the fourth and sixth, the 

 earlier anatomists made a smaller number ; 

 but the arrangement of Willis, which is more 

 usually adopted in the present day, counts 

 nine of these soft round cords, and reckons 

 them from before backwards. 



Yet even this apparently simple method of 

 distinction comes far short of real accuracy. 

 Thus, some of the so-called nerves offer the 

 essential structure of nervous centres ; and 

 include, in addition to the ordinary nerve- 

 fibres, those globular vesicles which modern 

 physiology recognizes as the generators of the 

 nervous force. In others of them, the limited 

 resemblance implied by this numerical ar- 

 rangement is modified by their arising as two 

 or more roots, which subsequently, by their 

 junction, form one nerve. While in the seventh 

 nerve, which forms the subject of the present 

 article, the close proximity of two such cords 

 during a part of their course has led to their 

 being united under one name ; although in their 

 distribution, properties, and functions, they 

 present a marked contrast with each other. 



The facial and auditory nerves proceed to- 

 gether from the medulla oblongata to the 

 bottom of the meatus auditorius internus in 

 the petrous portion of the temporal bone. 

 Up to this point they are included in the 

 term seventh nerve ; but beyond this situation 

 their courses widely diverge. In conformity 

 with these differences, the following short 

 account will describe them separately from 

 each other. It will first mention such of their 

 anatomical features as are manifest on simply 

 laying bare their surface, and will afterwards 

 refer to the appearances afforded by a more 

 artificial dissection or separation of their fibres. 

 Subsequently we shall briefly examine the 

 bearing of these their structural peculiari- 

 ties, and the effect of their morbid changes, 

 with a view to attempting the deduction of 

 their function. 



The auditory nerve is of a considerably softer 

 texture than the facial ; a difference which is 

 in great part attributable to the much more 

 delicate neurilemma by which it is enveloped, 

 but which is, no doubt, to some extent the 

 result of a peculiarity of its constituent nerve- 



