SENSORY ORGANS. 177 



higher vertebrates the same components occur in the pharyngeal, 

 laryngeal, cesophageal, and gastric branches of the intestinalis. The 

 distribution of the vagus shows that the parts supplied are to be re- 

 garded as morphologically derived from the head, though (heart, 

 lungs and stomach) they may be far removed from it in the adult. 

 Although details have been mentioned, some differences between 

 air- and water-breathing vertebrates may be summarized. The lateral 

 line organs are associated with an aquatic life, occurring in the branchiate 

 forms, even of the amphibia. With the assumption of a pulmonate 

 respiration the lateral line organs are lost and with them go the lateralis 

 elements of the seventh and tenth nerves. In the amniotes neither the 

 organs nor the nerves appear, even in development. Also the loss of 

 gills, and the closure of the clefts results in a modification of the nerves 

 of the ventral regions. 



XI. The Accessory Nerve appears as a distinct nerve in the am- 

 niotes, though traces of it appear in the ichthyopsida where the poster- 

 ior roots of the vagus furnish fibres which go to muscles in the pectoral 

 region. In the amniotes the number of these roots is increased (up to 

 seven in mammals), the additions being made to the posterior end of the 

 series. The fibres run forward between the dorsal and ventral roots of 

 the cervical nerves and unite to form a trunk, distinct from the vagus, 

 which bends back to supply muscles connected with the pectoral arch. 

 The components of this accessory nerve belong to the visceral motor 

 system, and the explanation of muscles connected with locomotion being 

 supplied by visceral nerves is not easy. 



XII. The Hypoglossal Nerve of the adult contains only somatic 

 motor fibres, but in the young of several forms, both amniote and 

 ichthyopsidan, two or more ganglionated roots are formed which soon 

 disappear. The roots of the nerve lie at the junction of brain and 

 spinal cord, and hence the nerve lies outside the skull in the lower, in- 

 side it in the higher forms. The nerve contributes to the innervation 

 of the tongue, the trunk and the brachial plexus in the lower vertebrates, 

 while in the higher groups it is more restricted to the tongue and the 

 sternohyoid muscle. 



THE SENSORY ORGANS. 



The sensory organs are to receive information from without and 

 to transform it into stimuli to be carried by the nerves to the ganglia, 

 usually those of the central nervous system. This information varies 



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