2 24 Journal of Comparative Neurology and Psychology. 



in the light of the general considerations contained in the follow- 

 ing section. 



12. TJie occipital region and the postenor limit of the head. 

 The constitjition of the vagus. Shifting of organs. 



In the occipital region of the head we find the following 

 peculiar condition. (i) A number of myotomes (beginning 

 with somite lO in Petromyzon, somite 9 or 8 in selachians) im- 

 mediately behind the gills send ventral buds downward behind 

 and then forward beneath the gill sacs to form hypobranchial 

 muscles. These myotomes and their ventral derivatives are 

 supplied by a series of ventral nerve roots from the medulla 

 oblongata which usually join into a plexus known as the 

 hypoglossal nerve. 



(2) A variable number of myotomes following the ear 

 (beginning with somite 4) have disappeared in gnathostomes and 

 with them have disappeared the somatic motor nerves. 



(3) Throughout this region the general cutaneous, splanch- 

 nic sensory and splanchnic motor components are wholly separ- 

 ate from the somatic motor component. This is the condition 

 which persists throughout life in the whole body in Amphioxus. 



(4) Usuall}^ these three components are united into a com- 

 plex known as the vagus or vago-accessory nerve, which sup- 

 plies the skin, mucosa, and visceral (lateral plate) musculature 

 of the region following somite 4 and the first branchial arch, 

 including trapezius musculature, stomach, etc. 



(5) The relative position of myotomes and gills has been 

 so much changed by growth and expansion of one part and 

 shifting of another that it has been difficult or impossible to 

 trace any fundamental relationship between the myomerism 

 and branchiomerism. 



(6) The above conditions which obtain in embryos are 

 further complicated in the adults of many vertebrates by the 

 secondary addition of vertebral elements to the skull. 



Before undertaking a general account of this region it will be 

 an advantage to give a summary of the conditions in Petromyzon, 

 of which we have a clear and judicial account by Koltzoff. 



