CHAPTER I. 

 Gross Anatomy of the Central Nervous System. 



WHILE the gross features of the brain may be studied 

 a little more conveniently if a larger type than that 

 of the rat is employed, the latter nevertheless 

 shows most of the important structures very satisfactorily. 



The spinal cord extends through the vertebral canal from 

 the base of the skull to the sacral region. It is roughly 

 cylindrical, but presents two slightly swollen regions which 

 are somewhat depressed. These are the cervical and lumbar 

 enlargements, from which arise respectively the nerves for 

 the fore and hind limbs. Posteriorly, it tapers off to end in 

 the sacrum in a fine thread, the filum terminale, which runs 

 back for some distance in the tail. From the spinal cord 

 arises a series of paired spinal nerves, the first of these emerging 

 between the skull and the first vertebra, while the remainder 

 emerge between successive vertebrae. In the cervical region, 

 all the nerves except the last are named from the vertebrae 

 in front of which they emerge, but the nerve behind the last 

 cervical vertebra is also reckoned to this region, so that w^hile 

 there are seven cervical vertebrae, there are eight pairs of 

 cervical nerves. Each of the remaining nerves is named 

 from the vertebra behind which it emerges. Thus there are 

 thirteen pairs of thoracic nerves, six pairs of lumbar nerves, 

 four pairs of sacral nerves, and three pairs of caudal nerves. 



Each spinal nerve is attached to the cord by dorsal and 

 ventral roots, and on the former may be observed a slight 

 enlargement, a spinal ganglion or dorsal root ganglion. Similar 

 masses occur near the bases of some of the cranial nerves, the 



