TEE CRANIAL OB ENCEPHALIC NERVES. 841 



this action on the heart, one nerve, usually the right, plays a predominant part 

 (Arloing and Tripier). 



It also transmits to the bulbar vaso-motor centres, through the medium of 

 the depressor nerve of the circulation — Cyon's nerve — which forms part of it, a 

 stimulus that leaves the heart and produces a general vaso-dilatating effect — 

 chiefly, however, in the abdominal cavity. 



11. Eleventh Pair, Spinal Accessory Nerves, or Accessory Nerves of 

 THE Pneumogastrics (Fig. 456, 2, 4). 



The spinal accessory is an exclusively motor nerve, which, at its exit from 

 the cranium, is so intimately connected with the pneumogastric, that we might 

 perhaps follow the example of Miiller, and describe the two as forming one and 

 the same pair. 



Origin. — This nerve exhibits a singular disposition, in that it arises from the 

 "whole extent of the cervical spinal cord, and ascends in the spinal canal to near 

 the pneumogastric, with which it leaves the cranium by the posterior part of 

 the foramen lacerum. It is also described as a cranial nerve, in consequence 

 of the latter peculiarity ; but from its origin it is rather a spinal nerve, a fact 

 which is sufficiently indicated by the name generally given to it. 



In the interior of the spinal canal, it is a long cord measuring from 27 to 31 

 inches in middle-sized animals. It commences, by a very fine point, at the 

 cervical or brachial enlargement of the spinal cord, follows that organ in an 

 ascending course, lying close to its lateral column, and passing between the roots 

 of the two orders of cervical nerves until it arrives at the medulla oblongata, 

 where it is inflected outwards at the foramen lacerum posterius, into which it 

 passes to leave the cranium. 



In this ascending course it gradually increases in volume, as it at intervals 

 receives additional filaments from the lateral column of the spinal cord, like the 

 radicular extremity of the nerve itself. Traced into the spinal cord, the radicular 

 filaments are found to arise from a nucleus situated outside the base of the 

 inferior cornua. Before making its escape from the cranium, it receives, besides, 

 some of the posterior or motor roots of the pneumogastric nerve. In the fora- 

 men lacerum, it becomes applied against the ganglion (jugular) of that nerve, in 

 the manner of motor fibres of mixed nerves, and gives it some of its own filaments, 

 while it also receives others. 



The long cord here described as the root of the spinal accessory, is considered 

 by some authorities as only a portion of this nerve, to which they give the name 

 of external or medullanj root of the spinal accessory. They designate as the 

 internal or bulbar root of that nerve, the anastomosing filaments already described 

 as the motor roots of the pneumogastric. According to them, this internal root 

 only lies beside the pneumogastric for a very short distance, ultimately leaving 

 it and forming the superior laryngeal and pharyngeal nerves, which seem to arise 

 from the vagus rather than from the accessory of Willis. 



Distribution. — Beyond the ganglion of the pneumogastric, the spinal accessory 

 nerve remains beside the trunk of the pneumogastric for scarcely an inch ; it 

 then separates from it at an acute angle, the sinus of which is occupied by the 

 great hypoglossal nerve, is directed backwards, passing beneath the superior 

 extremity of the maxillary gland and mastoido-humeralis muscle, gains the 

 supero-posterior border of that muscle, and follows it to the front of the shoulder. 



