446 STRUCTURE OF VERTEBRATA. 



dorsal, posterior, or sensory, and a ventral, anterior, or 

 motor. These arise separately and independently, but 

 combine in the vicinity of the cord to form a single nerve. 

 The dorsal root exhibits at an early period a large ganglionic 

 swelling — the spinal ganglion ; the ventral root is apparently 

 non-ganglionated. Moreover, the dorsal root has typically 

 a single origin (as in the cranial nerves), while that of the 

 ventral root is often multiple. 



The dorsal roots are outgrowths of a continuous ridge or crest along 

 the median doisal line of the cord. As the cord grows the nerve roots 

 of each side become separated. They shift sidewards and downwards 

 to the sides of the cord. The ventral roots are later in arising ; they 

 spring as outgrowths from the latero- ventral angle of the cord. 



Beard maintains that the spinal ganglia do not arise from the spinal 

 cord, but have an independent origin from the deeper layers of the 

 epiblast. 



According to most authorities, the sympathetic ganglia are offshoots 

 from the same rudiment as that from which the dorsal ganglia arise, 

 and it is possible that they are the more or less vagrant ganglia of the 

 ventral roots, with which they are connected by small fibres. On this 

 view (Gaskell's) both roots may be said to be ganglionated. But the 

 ganglion of the dorsal root is stationary in position, and the nerve-fibres 

 which pass through it come both from the visceral (splanchnic) and 

 from the peripheral somatic parts, separating from one another within 

 the cord. On the other hand, the supposed ganglion (sympathetic) of 

 the ventral root is more or less vagrant, and off the main line of the 

 root, from which it receives small fibres passing to splanchnic or visceral 

 structures. 



Sense organs. — The central nervous system has doubtless 

 arisen in the course of history from the insinking of external 

 nerve cells ; it does arise in development as an involution 

 of ectoderm or epiblast. The same layer 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 likelihood 

 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 are two branchial sense organs. 



