

XIX] CRANIAL NERVES 427 



spiracle. The ninth or glossopharyngeal nerve is similarly 

 forked round th,e first true gill-slit (Fig. 207). 



The tenth or vagus or pneumogastric nerve, which is 

 certainly a compound one, gives oft" a branch to each of the remain- 

 ing slits, to which it bears a relation similar to that borne by the 

 ninth nerve to the second slit. The main stem of the nerve passes 

 along the alimentary canal and sends nerves to its muscles and to 

 those of the heart, all these muscles being developments of the 

 inner or splanchnic wall of the unsegmented coeloin. The tenth 

 nerve has also in the lower Craniata a sensory division. This 

 separates from it soon after it leaves the brain and passes backward, 

 supplying a series of long mucous canals fused together, called the 

 lateral line, which extends from head to tail along the mid-lateral 

 portion of the body and is provided with a series of openings to the 

 exterior. In some forms this sensory branch, the so-called lateralis, 

 appears to originate from the brain as an independent nerve and in 

 all cases its constituent fibres can be traced back through brain 

 substance to a peculiar group of neurons in the side of the hind-brain 

 known as the tu ber acusticum. From this same group the eighth 

 nerve and the sensory branches of the seventh nerve take their origin. 

 On account of its extensive area of distribution the tenth nerve has 

 received the name of vagus (wandering). 



The alimentary canal exhibits a marked difference from the 



condition found in the lower Chordata. The gill-slits are reduced 



in number, there being as a rule not more than eight; it would 



indeed be more correct to speak of them as gill- 



canai tary pouches. In this respect Craniata agree with the 



Hemichorda in contrast to the Cephalochorda and 



Urochorda. No trace of a tongue-bar has however been found in 



any Craniate. 



The endostyle becomes shut off from the pharynx and 

 thus loses entirely its original function ; it branches and forms 

 a mass called the thyroid gland. The evil results attendant on 

 its removal or diseased condition and experiments on living animals 

 show that it secretes into the system a substance which has a 

 beneficial influence on metabolism, especially as regards the " tone " 

 of the nervous system and the growth of connective tissue. 



The subneural gland of the Urochorda, on the other hand, 

 seems to be represented by a structure called the pituitary body. 

 This, like the subneural gland, is a dorsal pocket of the stomodaeum, 

 but it becomes cut off from all connection with the mouth and 



