514 COMPARATIVE ANATOMY. 



a) Spinal Nerves. 

 § 386. 



The metamerism of the vertebrate body, which is first seen in 

 the formation of primitive vertebras, is quite as much expressed by 

 the characters of the spinal nerves, and by their mode of distribu- 

 tion. A pair of nerves corresponds to each vertebral segment. 

 Each nerve is formed by the union of two roots which have their 

 origin in the lateral halves of the spinal chord. The superior or 

 sensitive root forms a ganglion, before it unites with the inferior or 

 motor root; and the fibres thus formed mix with those of the lower 

 root, and form the trunk of a spinal nerve. In the Selachii the 

 inferior and superior roots pass through separate foramina in the 

 spinal canal. As a rule, the nerves leave the spinal canal between 

 two arches. 



Each spinal nerve divides into two chief branches — a ramus 

 dorsalis supplies the muscles and skin of the back, a ramus ventralis 

 goes to the sides, and ventral wall of the body, and gives off a ramus 

 visceralis to the viscera. This latter forms the connection between 

 the so-called sympathetic, and the cerebro-spinal nervous system. 



In Fishes we always find the spinal nerves at the intermuscular 

 ligaments. They follow exactly the metamerism of the body, so 

 long as this continues to be expressed. 



The size of the nerves is correlated with the development of the 

 parts which they supply. When the extremities are formed, their 

 rami ventrales become very large. A number of rami ventrales of 

 the anterior spinal nerves (cervical nerves) are then formed into a 

 plexus (plexus brachialis), from which the nerves for the anterior 

 extremities are given off ; in the same way, the nerves for the hinder 

 extremities are given off: from a plexus (plexus lumbalis and p. sac- 

 ralis), which is formed more posteriorly, either iu front of, or in the 

 pelvis. These plexuses change their position when the limbs do so 

 (cf. pp. 473, 474). 



The brachial plexus of the Amphibia is formed of three or four 

 nerves (in the Frog, the 2nd, 3rd, and 4th spinal nerves). In the 

 Reptilia the brachial plexus is generally made up by the cervical 

 nerves from the sixth to the ninth ; in Varanus, from the seventh to 

 the tenth ; and in the Alligator the first thoracic nerve also enters 

 the plexus. In Birds it is formed from the last cervical and 

 first thoracic nerve, or from the 11th and 12th cervicals, or 1st and 

 2nd thoracic nerves. In the Mammalia, the last 3, 4, or 5 cervical 

 nerves, and the 1st, and sometimes also the 2nd, thoracic nerves, 

 contribute to form the plexus. 



The nerves for the hinder extremities are given off, in the 

 Amphibia, from a plexus which is generally formed by three nerves. 

 The anterior of the nerves thus formed is the crural ; the sciatic is 



