SENSORY ORGANS 249 



cord lies along the ventral and lateral region of the vertebral 

 column : it is generally situated close to the latter, overlying the 

 vertebral end of the ribs. 



In Mammals, the cervical portion of the cord may have an 

 independent course from the vagus, or it may be more or less 

 closely applied to the latter nerve, the anterior cervical ganglion of 

 the sympathetic and the vagus-ganglion forming a single mass ; the 

 posterior cervical ganglion commonly fuses with the first thoracic. 

 From the anterior cervical ganglion the sympathetic passes into 

 the skull along with the internal carotid artery, and its cranial 

 portion takes on relations to the cerebral nervesmore particularly 

 the Vth, IXth, and Xth, as in other Vertebrates. Numerous 

 branches also pass from the anterior cervical ganglion to the hypo- 

 glossal, the anterior cervical nerves, and to the pharynx, larynx, &c. 



III. SENSORY ORGANS. 



The specific elements of the sensory organs originate, like the 

 nervous system in general, from the ectoderm ; the peripheral 

 terminations 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. 1 



The sensory apparatus was primarily situated on a level with 

 the epiderm, and served to receive sensory impressions of but 

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

 it passed inwards beneath the epiderm, certain of these becoming 

 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 mesodermic sense-capsules (p. 77) ; they must 

 be distinguished from the simpler integumentary sense-organs, which 

 are concerned with the senses of touch t pressure, and temperature. 

 In addition to free ne,rve-endings in the skin, various specific 

 forms of sensory cells occur, and these may be surrounded by 

 supporting or isolating cells, both kinds, however, being ectodermic. 

 The mesoderm may also take part in the formation of the sensory 

 organs, giving rise not only to the above-mentioned sense-capsules, 

 but also to various protective coverings and canals as well as to 

 contractile and nutritive elements (muscles, blood- and lymph- 

 channels). 



In the sensory organs of the integument of branchiate Verte- 

 brates, as well as in all the higher sensory organs, the surrounding 



1 The vertebrate eye forms an exception to the other sense-organs in that it 

 arises from a part of the ectoderm which has been involuted to form the medullary 



tube. 



