SENSORY ORGANS 18& 



systems of organs, forming numerous plexuses. Peripheral ganglia 

 are also present in the viscera. 



The sympathetic extends not only along the vertebral column, 

 but passes anteriorly into the skull, where it comes into relations 

 with a series of the cerebral nerves (comp. pp. 184, 185 and Fig. 

 149) similar to those which it forms further back with the spinal 

 nerves. 



The original segmental character frequently disappears later on 

 and this is especially the case in those regions where marked 

 modifications of the earlier metameric arrangement of the body 

 have taken place — viz., in the neck and certain regions of the 

 trunk, especially towards the tail : thus there are never more than 

 three cervical ganglia in Mammals. 



A sympathetic is not known to exist in Amphioxus, and in 

 Petromyzon it appears to be rudimentary. In Fishes proper, it is 

 more highly differentiated, especially in the head region, while in 

 Dipnoans it has not been observed. In Amphibians the 

 sympathetic is well developed, especially in the higher forms 

 (Fig. 121). In the Myctodera it extends anteriorly to the vagus 

 ganglion and posteriorly through the trunk and haemal canal 

 almost to the apex of the tail, as is the case also in Teleostei. 



In the Sauropsida the cervical portion of the sympathetic is 

 usually uonble, one part running within the vertebrarterial canal 

 alongside the vertebral artery. In all oth er Vertebrates the whole 

 cord lies along the ventral and lateral region of the vertebral 

 column : it is generally situated close to the latter, and overlies 

 the vertebral ends of the ribs. 



III. SENSORY ORGANS. 



The specific elements of the sensory organs originate, like the 

 nervous system in general, from the epiblast ; the peripheral ter- 

 minations of the sensory nerves are thus always to be found in 

 relation with cells of ectodermic origin, which become secondarily 

 connected by means of nerve-fibres with the central nervous 

 system. 



The sensory apparatus was primarily situated on a level with 

 the epidermis and served to receive sensory impressions of but 

 slightly specialised kinds ; but in the course of phylogeny parts of it 

 passed inwards beneath the epidermis, and certain of these became 

 differentiated into organs of a higher physiological order, viz., 

 those connected with smell, sight, hearing, and taste. These are 

 situated in the head, and except the last mentioned, become 

 enclosed in definite sense-capsules (p. 68) ; they must be dis- 

 tino-uished from the simpler integumentary sense-organs, which are 

 concerned with the senses of touch, temperature, &c. 



In many, and more especially in the higher sensory organs, 



