COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 



299 



single spinal nerve which passes out through the intervertebral foramen and almost immedi- 

 ately divides into three branches or rami the dorsal, ventral, and visceral rami. Although, 

 as intimated above, the dorsal root is almost purely sensory, entirely so in the higher verte- 

 brates, and the ventral root purely motor, the rami are mixed. The dorsal and ventral 

 rami both contain somatic motor and sensory fibers passing to and from the body wall and 

 also include a few visceral fibers. The somatic motor fibers of the dorsal rami pass to the 

 epaxial muscles, those of the ventral rami to the hypaxial muscles. The visceral ramus 

 (also called ramus communicans) connects with a sympathetic ganglion. It carries visceral 

 motor fibers from the spinal cord into the sympathetic system, these constituting the white 

 ramus; and also carries visceral sensory fibers from the ganglion into the cord, these forming 

 the gray ramus (Fig. 68). The white and gray rami together form the visceral or com- 

 municating ramus, which serves typically to connect each spinal ganglion with the adjacent 

 ganglion of the sympathetic system. 



The spinal nerves are paired and segmentally arranged. There are in general as many 

 pairs of spinal nerves as body segments below the head. In connection with the paired 

 appendages the ventral rami of several successive spinal nerves form a network or plexus 

 from which the nerves to the appendage arise. This innervation shows: that the limb muscles 

 arise from the hypaxial parts of the myotomes, since the ventral rami supply only hypaxial 

 muscles; that the limb muscles are derived from several myotomes, since there is but one 

 spinal nerve to each segment of the body; and that the muscles of the appendages have under- 

 gone much torsion and change of position, resulting in a crisscross arrangement of their 

 nerves, since each nerve retains its innervation to the muscle which it originally supplied. 



The cranial nerves are much less typical in arrangement than the spinal nerves. Most 

 of them do not contain the four classes of fibers. They are attached to the brain by roots, 

 but these are irregularly arranged. Those cranial nerves that contain somatic sensory 

 fibers bear a ganglion corresponding to a spinal ganglion. The composition and functions 

 of the cranial nerves will be described in the dissections. There are ten cranial nerves in 

 anamniotes and twelve in amniotes, but the additional two are not new formations. The 

 cranial nerves are not segmentally arranged, although probably so arranged in the be- 

 ginning. 



5. The segmentation of the head. It was previously stated thai the vertebrate head was 

 originally segmented, but the segmentation is now much obscured. In attempting to work 

 out the head segmentation, the brain, the cranial nerves, the visceral arches, and the true 

 head muscles (not visceral muscles) have been studied. The brain is plainly segmented, 

 particularly in its posterior part ; these segments are called neuromeres. Probably the poste- 

 rior neuromeres are true segmental divisions. The cranial nerves are now much altered 

 from their original condition but they were formerly segmental nerves, and a certain amount 

 of evidence has consequently been obtained from a study of them. The visceral arches 

 are plainly segmentally arranged, but the relation of their segmentation to that of the head 

 as a whole is not entirely clear. It is generally believed that the gill slits are intersegmental 

 in position, i.e., occur at the myosepta. The best evidence has been obtained from the study 

 of the myotomes of the head. In cyclostomes all of these form muscles of the adult, but in 

 other vertebrates many of them disappear in embryonic stages. Those which persist form 

 the muscles of the eyeball and the intrinsic musculature of the tongue. (The student should 

 recollect that the apparent head muscles are visceral muscles, derived from the hypomeres, 

 and not from the myotomes.) The various lines of evidence lead to the following conclusions: 

 that there are four head segments in front of the ear of which the first (anterior head cavity) 

 is evanescent, while the next three (named i, 2, and 3) give rise to the muscles of the eyeball; 

 behind the ear the number of head segments appears to be variable, about 6-8 in forms above 

 cyclostomes, more in cyclostomes. These postotic segments develop no muscles above 



