THE KERVOUS SYSTEM. 611 



This difference is, that the yielding of the cartilages of the larynx in 

 young animals permits the glottis to be closed by the atmospheric pres- 

 sure in inspiration, and so they are quickly suffocated unless tracheotomy 

 be performed. In old animals, the rigidity and prominence of the aryt- 

 enoid cartilages prevent the glottis from being completely closed by the 

 atmospheric pressure; even when all the muscles are paralyzed, a por- 

 tion at its posterior part remains open, and through this the animal 

 continues to breathe. 



In the case of slower death, after division of both the vagi, the lungs 

 are commonly found gorged with blood, cedematous, or nearly solid, 

 from a kind of low pneumonia, and the bronchial tubes full of frothy 

 bloody fluid and mucus, to which, in general, the death may be ascribed. 

 These changes are due, in part, to the passage of food and of the various 

 secretions of the mouth and fauces through the glottis, which, being 

 deprived of its sensibility, is no longer stimulated or closed in conse- 

 quence of their contact. 



The Xlth Nerve (Spinal Accessory). 



Origin and Connections. The nerve arises by two distinct origins 

 one from a centre in the floor of the fourth ventricle, partly but chiefly 

 in the medulla, and connected with the glosso-pharyngeal-vagus-nucleus ; 

 the other, from the outer side of the anterior cornu of the spinal cord 

 as low down as the fifth or sixth cervical nerve. The fibres from the 

 two origins come together at the jugular foramen, but separate again 

 into two branches, the inner of which, arising from the medulla, joins 

 the vagus, to which it supplies its motor fibres, consisting of small me- 

 dullated or visceral nerve-fibres, while the outer consisting of large 

 medullated fibres, supplies the trapezius and sterno-mastoid muscles. 

 The small-fibred branch is said to arise from a nucleus corresponding to 

 the posterior vesicular column of Clarke. 



The principal branch of the accessory nerve, its external branch, 

 then supplies the sterno-mastoid and trapezius muscles; and, though 

 pain is produced by irritating it, is composed almost exclusively of 

 motor fibres. The internal branch of the accessory nerve supplies chiefly 

 viscero-motor filaments to the vagus. The muscles of the larynx, all of 

 which, as already stated, are supplied, apparently, by branches of the 

 vagus, are said to derive their motor nerves from the accessory; and 

 (which is a very significant fact) Vrolik states that in the chimpanzee 

 the internal branch of the accessory does not join the vagus at all, but 

 goes direct to the larynx. 



Among the roots of the accessory nerve, the lower or external, aris- 

 ing from the spinal cord, appears to be composed exclusively of motor 



