14 NERVOUS SYSTEM OF VERTEBRATES. 



of the dorsal root (Fig. i, B). The dorsal rami of the two roots 

 unite to form a common dorsal ramus and the ventral rami unite 

 to form a common ventral ramus. From the ganglion or from the 

 common ventral ramus just beyond the ganglion, there arises a 

 branch w^hich enters a ganglion of the sympathetic system, by way of 

 which the viscera are brought into connection with the central 

 nervous system. The branch to the sympathetic system is known 

 as the ramus communicans. 



In the head region in all true vertebrates the central nen-ous 

 system becomes considerably enlarged to form the brain. The 



Fig. I. — Outlines of the spinal cord with the dorsal and ventral nerve roots; A, 

 in Petromyzon ; B, in a mammal, d. /., dorsal fissure; d. h., dorsal horn; d. w., 

 dorsal ners'e; /. h., lateral horn; v. /., ventral fissure; v. h., ventral horn; v. «., 

 ventral nerve. 



nerves connected with the brain are called cranial nerves. There 

 is no sharp limit between spinal cord and brain or between spinal 

 and cranial nerves, as there is none between trunk and head. 

 The brain presents in the adult five constant and well marked 

 portions which mil serve as a guide in the description of the 

 nervous system of the head. These are known as secondar}' 

 segments of the brain and are named from behind forward: my- 

 elencephalon, metencephalon, mesencephalon, diencephalon and 

 telencephalon (Fig. 2). 



The myelencephalon, or medulla oblongata, appears as a gradual 

 enlargement extending forward from the spinal cord. As the 

 spinal cord merges into the brain the dorsal portions of the cord 

 spread apart and the dorsal fissure seems to widen out into a 



