482 GENERAL STRUCTURE OF NERVES. 



the pia mater, investing the brain and the spinal marrow. It 

 is very vascular.* 



The ramification of a nerve is simply the separation of some 

 fibres from the general fasciculus. The branch commonly forms 

 an acute angle with the main trunk. 



The course of these branches from their origin to their ter- 

 mination is generally as straight as possible. 



When the nervous chords are examined in an animal 

 recently dead, there is an appearance of white lines arranged 

 in a transverse or spiral direction. The cause of this appear- 

 ance is not well understood. 



In various parts of the body networks are formed by the 

 combination of different nerves or the branches of nerves. In 

 those instances the branches of one nerve unite with those of 

 another, and form new branches. These new branches again 

 divide, and their ramifications unite with other new ramifi- 

 cations to form other new trunks. These new trunks divide 

 again, and form new combinations in the same way. 



The trunks last formed proceed to the different parts of the 

 body, as other nerves do which arise immediately from the brain. 



These combinations are denominated Plexuses. There are 

 several of them in the cavities of the abdomen and thorax, 

 formed by the ramifications of the par vagurn and the sym- 

 pathetic nerves. The four lower cervical, and the first dorsal 

 nerve, form a very remarkable plexus of this kind, which 

 extends from the side of the neck to the axilla, and forms the 

 nerves of the arm. The lumbar nerves form a similar plexus, 

 although not so complex, from which the crural nerve arises. 

 The anterior nerves of the sacrum also unite for the formation of 

 the great sciatic nerve. 



It appears to be clearly ascertained, that the great object of 

 this peculiar arrangement is the combination of nervous fibres 

 from many different sources, in each of the nerves, which are 



* Several authors have written professedly on the structure of 'the nerves, 

 viz. Monro, in his " Observations on the Structure and Functions of the Nerv- 

 ous System." Bichat, "Anafomie Generate." Fontana, " Treatise on the 

 Poison of the Viper." Reil, " Exercitationes Anatomicse." Scarpa, " Annota- 

 tiones Academics." Prochaska, "De Structura Nervorum." 



I regret that it has not been in my power to procure Reil, Prochaska, or Scarpa. 



