MOTOR NUCLEI IN PHYLOGENY 415 



expiration (Gaupp). These muscles are innervated from the 

 III and IV spinal segments and are ordinarily not called into 

 special play during respiration, since the tonicity of the muscles 

 of the body wall together with the natural elasticity of the 

 lungs is sufficient to expel the pulmonary air when the glottis 

 is opened. The mucosa overlying the functional vocal cords 

 is innervated by sensory branches of the R. laryngeus longus 

 and brevis X, while the mucosa of the bucco-pharyngeal cavity 

 with its accessory vocal sacs is innervated by visceral sensory 

 branches of both VII and IX nerves. Since the degree of tension 

 of the walls of the resonator cavity, as well as that of the vocal 

 cords must be factors of great importance in regulating the 

 action of the accessory expiratory muscles during sound pro- 

 duction, the descent of fibers in the fasciculus solitarius to the 

 level of the III cervical segment may well have been determined 

 during the perfection of this method of sound production, whose 

 principle has been universally adopted by all air breathing 

 vertebrates. 



The probability of the importance of such a factor in deter- 

 mining the descending course of certain elements of the fasciculus 

 solitarius is increased when it is realized that the evolution of 

 a cer\'ical motor center of a purely respiratory function must 

 have occcurred in phylogeny after the stage represented by 

 modern amphibians. 



N. accessorius and m. trapezius. In the descriptive part of 

 this communication attention has been drawn to the fact that 

 the most caudal vagus rootlets in R. catesbeana are made up 

 entirely of motor fibers, and emerge from the medulla on a more 

 ventral plane than the rostral rootlets of this series. According 

 to Strong (I.e.) a similar condition obtains in the tadpole. In 

 Rana the motor fibers of the caudal vagus rootlets are distrib- 

 uted by two branches (R. accessorius and R. scapularis) respec- 

 tively to the m. cucularis and m. interscapularis (Gaupp, 26). 

 Among urodeles also the accumulated e\ddence goes to show^ that 

 the caudal vagus rootlets are chiefly if not entirely motor, and 

 that from these roots the innervation of the m. cucularis is de- 

 rived (v. Coghill, 16, 17; Norris, 45, 46; Kingsbury, 40). 



