SPINAL NERVES 547 



The hyoid primitive segment (III.) was probably supplied by the 

 abducens (ventral) and the facial (dorsal). The auditory, glosso- 

 pharyngeal, and vagus nerves have no ventral roots. 



Spinal nerves. — Each spinal nerve has two roots — a 

 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 appar- 

 ently 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 dorsal line of the cord. As the cord grows the nerve roots 

 of each side become separated. They shift sidewards and downw^ards 

 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. 



According to most authorities, the sympathetic ganglia are offshoots 

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

 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. 



Sense organs. — The ectoderm 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 ectoderm, 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 

 anterior nares to mouth are arched over and open posteriorly 

 into the front of the mouth. In Amphibians, and in all 

 the higher Vertebrates, the nasal chambers open posteriorly 



