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, 
Fic, 262,—Diagram of spinal cord of man, thoracic 
region.—After Johnston, 
S.S., Somatic sensory; V..S., visceral sensory ; S.J7., somatic motor ; 
V.M.and U.M., visceral motor; d@.r., dorsal root; U.&., 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.—lIt 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 
