130 INNER VATION. [c HAP. XX. 



as ecial nerves of respiration, whilst those of the cervical plexus 

 were nerves of volition. There are indeed no good grounds for 

 coming to any other conclusion than that which Dr. John Reid 

 arrives at; namely, that the external branch of the spinal accessory 

 exactly resembles in its functions, the branches of the cervical 

 plexus with which it so freely anastomoses. 



It may be fairly asked, however, why do the trapezii and sterno- 

 mastoid muscles receive their nerves from a double source? The 

 most reasonable reply to this is, that while the branches of the cer- 

 vical plexus serve to connect these muscles with the centres of 

 volition and sensation in the ordinary way, the external branch of 

 the spinal accessory connects it in a more direct manner with the 

 centre of respiration. Nevertheless, this branch, although especially 

 implanted in that centre, is capable of obeying voluntary impulses, 

 so long as the medulla oblongata maintains its normal relation to 

 the centre of volition. 



Thus, on the whole, we assign motor power to the external branch 



of the spinal accessory, but we see no good reason to subscribe 



to the opinion that its internal branch must be regarded as the 



motor root of the vagus. Indeed, we are much more disposed, for 



anatomical reasons, to regard the office of this branch as totally 



different. None of the views hitherto put forward respecting 



this nerve explain the object of its peculiar and most extensive 



connexion with the nervous centre; a connexion which in the 



larger quadrupeds is still more extensive than in man. Our view 



is as follows : the internal branch of the spinal accessory consists 



of afferent fibres, which, connected with the sensitive surface of the 



respiratory organs, pass towards the centre in the trunk of the vagus, 



but separate from that nerve to be implanted in a large extent of 



the respiratory centre. This mode of implantation of the spinal 



accessory nerve serves to bring the sentient surface of the lungs and 



air passages into immediate relations with the roots of all those 



nerves which animate the great muscles of respiration, the phrenic, 



the external thoracic, the cervical plexus, and the motor fibres of 



the spinal accessory and vagus nerves. 



Respecting the subjects discussed in this chapter, the systematic works on 

 descriptive Anatomy and Physiology may be consulted ; also Sir C. Bell's, and 

 Mayo's works, and Dr. Reid's Essays in the Edinb. Med. and Surg. Journal; 

 Dr. Marshall Hall's writings ; the Article " Par Vagum," in the Cyclopaedia of 

 Anatomy and Physiology. 



