492 STRUCTURE OF VERTEBRATA. 



They are usually connected in a chain, which is linked anteriorly to 

 cranial nerves. They are also connected by fine fibres with the ventral 

 roots. They give off nerves to blood vessels and viscera. 



55 



FIG. 262. Diagram of spinal cord of man, thoracic 

 region. After Johnston. 



S.S., Somatic sensory; V.S. t visceral sensory; S.M., somatic motor; 

 V.M. and (7.M., visceral motor; d.r., dorsal root ; U.R., ventral 

 root. 



Sense organs. The ectoderm or epiblast gives origin to 

 the essential parts of the sense organs. The Vertebrate 

 eye is formed in great part as an outgrowth from the brain, 

 but as the brain is itself an involution of epiblast, the eye 

 may be also referred to external nerve-cells. 



Branchial sense organs. In many Fishes and Amphib- 

 ians there are lateral sense organs which form the "lateral 

 lines," while others lie in the head, and were in all likeli- 

 hood primitively connected with gill-clefts. In Sauropsida 

 and Mammals these branchial sense organs are no longer 

 distinct as such. 



The nose. It is possible that the sensory pits of skin 

 which form the nasal sacs were originally two branchial 

 sense organs. They are lined by epithelium in great part 

 sensory, and innervated by the olfactory nerves. In Fishes 

 the nasal sacs remain blind posteriorly, but there is a 

 peculiar condition in Dipnoi, where the grooves from 



