xm 



PHYLUM CHORDATA 



103 



inclusive. Each branchial nerve behaves in exactly the same 

 way as the glossopharyngeal ; it forks over the gill-pouch to 

 which it belongs, sending one branch to the anterior, another to 

 the posterior wall of the pouch. Thus each gill-pouch has its 

 own nerve while each gill receives its supply from two sources ; for 

 instance, the gill of the second branchial arch has its anterior 

 hemibranch innervated from the first, its posterior hemibranch 

 from the second branchial branch of the vagus. The vagus also 

 gives of? a cardiac nerve (X. c.) to the heart, a gastric nerve (X. g.) to 

 the stomach. In the air-breathing Craniata there are, of course, no 

 branchial nerves ; but the vagus still retains control of the 

 respiratory organs by giving origin to pulmonary nerves to the 

 lungs and laryngeal nerves to the larynx. 



B C 



FIG. 789. A, tactile spot from skin of Frog. a. touch-cells ; b. epidermis ; N, nerve. B. tactile 

 corpuscle from dermal papilla of human hand. a. connective-tissue investment ; b. touch- 

 cells ; n, n', n", n"', nerve. C, Pacinian corpuscle from back of Duck. A, A 1 , neuraxis ; 

 JK. central knob and surrounding cells ; L,Q. investing layers ; NS. medullary sheath of 

 nerve-fibre. (From Wiedersheim's Vertebrata.) 



The eleventh or accessory nerve (XL), which is recognisable in 

 some Fishes as a part of the vagus, is a distinct nerve in higher 

 forms, and consists of cerebral and spinal portions, so that it 

 occupies an intermediate position between the purely cerebral 

 and the purely spinal nerves. It acts in higher Craniates mainly 

 as the motor nerve for certain muscles of the shoulder. 



The twelfth or hypoglossal (XII.) arises from the ventral aspect 

 of the medulla oblongata, after the manner of the ventral root of 

 a spinal nerve. It is purely motor, and supplies the muscles of 

 the tongue and certain neck-muscles. In the Amphibia its place is 

 taken by the first spinal nerve, and there is no doubt that it is 



