748 



SPINAL ACCESSORY NERVE. 



that the communication of the spinal accessory 

 with the posterior root of the first, or with 

 any other spinal nerve, will be an equivalent 

 to it for a posterior root." We have already 

 seen that Stilling concludes, on anatomical 

 grounds, that those filaments of the accessory 

 that come from the medulla oblongata con- 

 tain centripetal filaments.* 



The spinal accessory passes through the 

 foramen lacerum posterius in a canal formed 

 by the dura mater, common to it and the 

 vagus, but they are occasionally separated 

 from each other as they enter this canal by a 

 bridle of arachnoid, or of the dura mater. 

 Soemmering has pointed out that the acces- 

 sory does not perforate the dura mater like 

 the other nerves, but is, as it were, insensibly 

 surrounded by this membrane.f 



One or two filaments generally pass be- 

 tween the accessory and the superior gang- 

 lion or ganglion jugular e of the vagus, as they 

 lie in the foramen lacerum posterius. Hein 

 states that he has more than once distinctly 

 observed, as also Krause has remarked, the 

 superior five, or even six filaments of the root 

 of the accessory approximate very closely to 

 the ganglion jugulare of the vagus, and partly 

 enter into its formation, so that a junction 

 between the vagus and accessory had already 

 taken place in this ganglion, before the fila- 

 ments of the accessory had been fully collected 

 to form together the trunk of this nerve.J 



As the spinal accessory is passing through 

 the foramen lacerum, it is in close proximity 

 to the posterior surface of the par vagum, and 

 it there divides into its two branches its 

 internal and external branches. 



The former, or the internal, is composed 

 of the filaments forming the upper roots of the 

 nerve (fig. 521, 11.), and entirely, or almost 

 entirely, of those coming from the medulla 

 oblongata; and it joins itself to the vagus 

 immediately below the ganglion jugulare of 

 that nerve. The passage of the accessory 

 through the foramen lacerum posterius, its 

 division into two branches, and the distri- 

 bution of the internal branch as far as it is 

 known, have been already described in the 

 art. PAR VAGUM, vol. iii. pp. 883. and 890 , 

 and need not be repeated here. 



The external branch, composed of those 

 fibres which arise from the spinal chord 

 (Jig- 521, 12.), proceeds down wards, outwards, 

 and backwards behind the internal jugular 

 vein, in front of the occipital artery, and be- 

 hind the posterior belly of the digastric and 

 stylo-hyoid muscle, and reaches the inner sur- 

 face of the sterno-cleido-mastoid muscle at 

 the lower part of its upper third. In con- 



* Stilling further states (p. 59) that in an ana- 

 tomical point of view we may regard the upper 

 roots of the accessory forming the internal branch 

 of that nerve as being composed of centripetal and 

 centrifugal filaments, exactly like the vagus. 



f " Non reliquorum nervorum more, sub arcu 

 durse membranse fertur, sed insensili quasi modo a 

 dura membrana obducitur." De Basi Encephali, 

 &c., p. 104, reprinted in Ludwig's Scrip. Nevr. Min. 

 Sel., torn. ii. 



J Muller's Archiv. 1844, p. 337. 



tinuing its course downwards and outwards, 

 it here generally perforates the sterno-cleido- 

 mastoid ; at other times it is only closely con- 

 nected to it by cellular tissue ; but in both 

 cases it gives branches to this muscle. In 

 this part of its course it is strengthened and 

 anastomoses with twigs of the third and 

 second cervical nerves. Continuing its pro- 

 gress downwards and backwards it anastomo- 

 ses with twigs of the fourth and fifth cervical 

 nerves, and throws itself into the inner surface 

 of the trapezius muscle, among whose fibres 

 it is ultimately lost. 



Comparative anatomy of the spinal accessory. 

 The origin and distribution of this nerve 

 in the mammalia does not essentially differ 

 from what is found in the human species.* 

 Willis states that this nerve is not only pre- 

 sent in the mammalia, but also in birds and 

 fishes f ; but the existence of it in the two 

 latter divisions of the vertebrata has been 

 subsequently denied by many excellent ana- 

 tomists. " If an animal," says Mr. Shaw, 

 " does not perform part of the act of respira- 

 tion by muscles which run from the skull to 

 the chest, no spinal accessory is found. The 

 truth of this observation may be shown by the' 

 dissection of any of the larger birds, but the 

 most extraordinary proof is to be found in the 

 neck of the camel. The constitution of the 

 neck of this animal is like that of birds ; there 

 being a succession of short muscles along the 

 side of the neck, and attached to the vertebrae, 

 but no long muscle passing from the jaw to the 

 sternum to assist in breathing, as in other 

 quadrupeds." J It appears, however, that in 

 the camel this nerve is present, but it is smaller 

 and differently distributed from what it is in 

 the horse.$ Serres found it in three of the 

 larger birds, Weber in some fishes, and Bis- 

 choff has given descriptions and representations 

 of it in several birds, reptiles, and fishes. In 

 these animals the upper part only of this nerve 

 seems to be present, for it does not stretch 

 downwards along the spinal chord to the 

 same extent in them as in the mammalia. 

 The whole of this nerve, in these animals, 

 throws itself into the vagus, while a branch 

 leaves the vagus after it has escaped from the 

 cranium, and taking the place of the external 

 branch of the accessory is distributed to the 

 muscles of the neck in birds and in reptiles, 

 and to the muscles which move the pectoral 

 fin in fishes. || In the chimpanzee, the spinal 

 accessory, after passing through the foramen 

 lacerum, divides into two branches. The in- 

 ternal runs towards the larynx, into which it 

 penetrates above the os-hyoid. It is placed 

 between the superior laryngeal nerve and 

 stylo-hyoid ligament, and passes behind the 

 internal carotid artery to the superior hyoi- 



* Dissections of this nerve upon several mammalia 

 are given in detail by Bischoff and Bendz. 



f Opus cit. p. 295. 



t London Medical and Physical Journal, vol. xlix. 

 p. 458, 1823. 



Vide note by Defennon, at p. 527 of torn. ii. of 

 the Archives GAie'rales de Me'decine, 1823. 



|| A full account of the comparative anatomy of 

 this nerve is given by Bischoff. 



