FUNCTIONS OF THE CEPHALIC NERVES. 173 



of fibres takes place between it and the Spinal Accessory ; but many more 

 fibres can be traced from the latter into the former than from the former into 

 the latter. Hence it results that, of the branches into which the Par Vagum 

 subsequently divides, many enjoy a high degree of motor power; whilst those 

 of the Spinal Accessory do not appear to possess any great share of sensibility. 

 The pharyngeal branches are among the most decidedly motor of all those 

 given off from the Pn.eumogastric ; and these may in great part be traced 

 backwards into the Spinal Accessory. Hence the idea of Arnold and Scarpa, 

 that the Par Vagum and Spinal Accessory are together analogous to a 

 spinal nerve, the former answering to the posterior roots, and the latter to the 

 anterior, appears sufficiently probable. In regard to its trunk, however, the 

 Par Vagum must of course be considered as a nerve of double endowments. 

 The chief function of its afferent portion is to convey to the Medulla Oblongata 

 the impression produced by venous blood in the capillaries of the lungs, or of 

 carbonic acid in the air-cells ; this impression may give rise, as we have seen, 

 to the respiratory movements, without producing sensation ; but, if it be from 

 any cause stronger than usual, the sense of uneasiness which it occasions is 

 very distressing. This impression may be imitated by pressure on the nerve, 

 which induces an immediate inspiratory movement. That the nerve is capa- 

 ble of conveying those impressions, which become sensations when, communi- 

 cated to the sensorium, is further evident from the fact that, when its trunk 

 is pinched, the animal gives signs of acute pain. Besides the pulmonary 

 impressions, this nerve also conveys to the Medulla Oblongata those which 

 originate in the mucous surface of the larynx, trachea and bronchi, as well as 

 on the lower part of the ossophagus and on the walls of the stomach. The 

 purpose of these is to stimulate various movements, which are performed 

 through the motor portion of the nerve ; this excites the actions of the muscles 

 of the pharynx and larynx, of the oesophagus, and, in some degree, of the 

 stomach and respiratory tubes. 



230. The section of the Par Vagum produces, as would readily be expected, 

 great disorder of the functions of Respiration and Digestion, to which it minis- 

 ters. It is an operation which has been very frequently performed; and the 

 statements of its results vary considerably amongst each other, being generally 

 influenced, in some degree, by the preconceived views of the experimenter.* 

 The section of the Par Vagum, when practised with the view of ascertaining 

 the influence of the nerve upon the lungs and stomach, is usually made in 

 the neck, between the origins of the superior and inferior (or recurrent) 

 laryngeal branches. Hence the muscles of the larynx are paralyzed; 'and, if 

 the animal should struggle violently, the ingress of air is likely to be ob- 

 structed by the flapping down of the arytenoid cartilages, and by the closure 

 of the glottis. This is especially the case in young animals, in which the 

 larynx is small. But in those that are full grown, and ha've a large larynx, an 

 adequate quantity of air may still find its way through the aperture, if the 

 animal refrain from any violent effort. In a considerable number of Dr. Reid's 

 experiments, therefore, he did not find it necessary to introduce the trachea- 

 tube, which other experimenters have generally employed. An opening was 

 made into the trachea, however, in those instances in which, from any cause, 

 the entrance of air was obstructed. 



231. The real character of the morbid changes in the lungs which are 

 induced by cutting the Par Vagum, the order in which they arise, and the causes 

 to which they are immediately due, constitute very interesting subjects of 



* The Author employs, as in his opinion the most worthy of confidence, the experi- 

 ments of Dr-. J. Reid, (Edinb. Med. & Surg. Journ., Vols. xiix. and li.,) on whose accu- 

 racy he has strong personal reasons for placing reliance; and whose anatomical and 

 pathological attainments are such as to render him fully competent to the task. 



15* 



