FIFTH PAIR OF NERVES. 



2C9 



be, the data, from which it is professedly 

 drawn, may be reasonably objected to. In 

 the first place the volume of the organ cannot 

 be assumed as being alone the measure of that 

 of the nerve supplying it the degree of ner- 

 vous endowment, whether general or special, 

 which the organ enjoys, must be also taken 

 into account ; and in the second, the extent 

 of the organs of the senses cannot be admitted 

 as a measure of the volume of the fifth nerve, 

 which is not connected with them all ; thus 

 the greater part of the organs of touch is inde- 

 pendent of that nerve. It appears to me 

 that the extent of distribution and amount of 

 endowment conjointly determine the volume 

 of the nerve, and that the latter cannot be 

 inferred a priori. 



Each nerve is composed of two portions, 

 which are remarkable for particular characters, 

 and have received distinct names ; they differ 

 from each other in size, in anatomical disposi- 

 tion, and in function ; one of them, larger than 

 the other, is provided with a ganglion, and dif- 

 fers in its distribution ; it also differs in proper- 

 ties, being subservient to sensation ; the other 

 is small, has no ganglion, and is destined to 

 volition ; they are hence denominated, the 

 former the larger, the ganglionic or the sentient 

 portion, the latter the smaller, the non-ganglio- 

 nic or the voluntary portion. 



The distinction of the nerve into two por- 

 tions appears to prevail uniformly throughout 

 the animal series. According to M. Serres, it 

 is to be observed in all the classes of the ver- 

 tebrate animals except the Reptiles ; but in 

 them, according to him, the lateral fasciculi* 

 are wanting. The latter assertion, however, is 

 incorrect, the distinction being to be observed 

 as satisfactorily in that class as in any other.f 

 Again, the distinction is not equally remarkable 

 in all ; in some it is still more so than in man ; 

 in others it is less ; and according to the same 

 authority, it is to be observed among Mam- 

 malia the more easily as we pass from Man to 

 the Rodentia. Among the Cetacea it is divi- 

 ded throughout into two separate fasciculi. J 



Each of the two portions of which the nerve 

 consists is a packet containing numerous fas- 

 ciculi, which are again divisible into filaments. 

 The fasciculi, of which the packets are com- 

 posed, are differently circumstanced in different 

 stages of the course of the nerve ; in one part 

 they are bound up so closely together that they 

 cannot without difficulty be separated from each 

 other and disentangled, while in another they 

 are but loosely connected and are easily sepa- 

 rated. 



The two packets are associated together more 

 or less intimately throughout their course ; but 

 inasmuch as they present remarkable varieties 

 in their disposition and mutual relations at dif- 

 ferent parts, it may be advantageous to divide 

 the nerve, through its course, into three por- 

 tions or stages ; one from the ganglion to the 

 connexion of the nerve with the brain, which 



* The uame by which he designates the lesser 

 portion of the nerve. 



-f See sketch of fifth nerve in the Turtle,^. 143. 

 j Op. cit. 



may be denominated its internal or encephalic 

 portion; a second from the ganglion to its ulti- 

 mate distribution, its external or peripheric 

 portion ; and, thirdly, its ganglion. Such a 

 distinction may not be free from objection, but 

 being adopted for the convenience of descrip- 

 tion, it possesses at least the recommendation 

 that there exist well-defined points of demar- 

 cation, whether there exist or not any difference 

 in the properties of those several portions. The 

 nerve, in its encephalic portion, is partly within 

 and partly superficial to the substance of the 

 brain. The superficial part is from one-half 

 to three-fourths of an inch in length, of a 

 flattened form, and of very considerable size. 

 It presents a loose fascicular texture, and is 

 enclosed within a prolongation of the arachnoid 

 membrane sent off upon it from the surface of 

 the brain ; this prolongation is, as in the case 

 of all those sent upon the vessels or nerves, in 

 their passage from that organ to the parietes of 

 the cranium, a cylindrical sheath, within which 

 the nerve is enclosed ; it is at first remarkably 

 loose, but as the nerve recedes from the brain, 

 the membrane invests it more closely, and is 

 continued upon it as far as the ganglion, from 

 which it is reflected to the surface of the canal 

 in which the nerve is contained. In the last 

 particular the disposition of the membrane is 

 subject to variety, for it is at times continued 

 beneath the ganglion, and partially invests the 

 trunks proceeding from this body before it is 

 reflected to line the canal. 



Throughout this part of the nerve the two 

 packets composing it are connected by cellular 

 structure and vessels, and are enclosed within 

 the prolongation of arachnoid membrane just 

 described ; but there does not appear to be any 

 interchange of nervous filaments between them, 

 and they are connected so loosely that they can 

 be separated from each other with great facility. 

 They "consist each of numerous fasciculi held 

 together, like the packets themselves, so loosely 

 that the latter can be easily opened out and 

 decomposed. The fasciculi of both packets 

 are irregular in size, some large, others small ; 

 those of the larger are for the most part some- 

 what smaller than those of the lesser, but they 

 are much more numerous, amounting, accord- 

 ing to J. F. Meckel,* to thirty or forty; while 

 those of the lesser amount, according to the same 

 authority, only to from nine to fourteen. The 

 fasciculi again are composed of numerous and 

 delicate filaments. The number of the fila- 

 ments is very great, but differently estimated 

 by different authorities; according to Meckel 

 those of the greater packet amount to about 

 one hundred, collected into thirty or forty 

 fasciculi ; while, according to Cloquet,f the 

 total number of filaments contained by both 

 packets varies from seventy to one hundred, 

 of which he allots five or six to the smaller, 

 and the remainder to the larger packet. This 

 difference of opinion Meckel explains by sup- 

 posing that fasciculi have been taken for fila- 

 ments and not decomposed, and this appears 



* Manuel d'Anatomie. 

 t Anatomic Descriptive. 



