COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 341 



2. The spinal cord and the roots of the spinal nerves. — With the bone scissors 

 cut out a piece of the vertebral column two or three inches long from the posterior 

 thoracic and anterior lumbar region. Remove the epaxial muscles from this 

 piece so as to expose the vertebrae, and with the bone scissors cut off the neural 

 arches of the vertebrae, thus exposing the neural canal. In this canal, but not 

 completely filling it, lies the spinal cord. Note that the spinal cord is loosely 

 inclosed in a tough membrane, the dura mater, from which strands pass to the 

 walls of the neural canal. The space between the dura mater and the spinal 

 cord is the subdural space. Slit open the dura mater. The spinal cord is closely 

 invested by a membrane, the pia mater, which cannot be separated from its 

 surface. Between these two is a delicate membrane, the arachnoid, which is 

 almost impossible to identify in gross dissection. The arachnoid and pia mater 

 of mammals together correspond to the pia mater of lower vertebrates. The 

 spaces around and between these membranes are filled in life with the cerebro- 

 spinal fluid, which is a modified lymph. 



From the sides of the spinal cord observe the roots of the spinal nerves aris- 

 ing in pairs at segmental intervals. They are insheathed in the dura mater which 

 follows them to their exit from the intervertebral foramina and is continuous 

 with their sheaths outside of the vertebral column. Examine one of the roots 

 in detail. Although it appears at first glance to be single, a little gentle picking 

 in the center of the root with the point of a probe will reveal that it is composed 

 of two parts. One of these, the dorsal root, is attached to the dorsolateral region 

 of the cord and near the intervertebral foramen bears a large oval swelling, the 

 dorsal or spinal ganglion. The dorsal root carries sensory fibers only (except 

 in fishes) and the nerve cells from which the sensory fibers originate are 

 located in the spinal ganglion. The other root, the ventral root, arises from the 

 ventrolateral region of the cord by several branches which unite to one trunk. 

 The ventral root carries motor fibers only, arising from motor cells in the cord. 

 The dorsal and ventral roots unite beyond the ganglion to form the spinal nerve, 

 which then exits'through the intervertebral foramen and divides into the dorsal 

 ramus to the epaxial muscles and adjacent skin, the ventral ramus to the hypaxial 

 muscles and adjacent skin, and the communicating rami to the sympathetic 

 system. These rami were already seen. 



Cut through the roots of the spinal nerves and remove a small section of 

 the spinal cord for examination. Identify in the median dorsal line a groove, 

 the dorsal median sulcus; in the median ventral line, another groove, the 

 ventral median fissure. Lateral to the dorsal median sulcus is the dorsolateral 

 sulcus, along which the dorsal roots enter the cord. The region between the 

 dorsal median and dorsolateral sulci is called the dorsal funiculus. The lateral 

 regions of the cord between the dorsolateral sulcus and the line along which 

 the ventral roots emerge is the lateral funiculus. Between this and the ventral 

 median fissure is the ventral funiculus. 



