THE SYMPATHETIC NERVES. 71 



canal of the Myxinoid fishes, which would be simply the in- 

 terspace, or passage, between the trabeculae (which must have 

 origiDally existed if ever they were distinct visceral arches) 

 not yet tilled up ; and the anomalous process of the roof of the 

 oral cavity, which extends toward the pituitary body in thei 

 embryos of the Vertehrata in general, might be regarded as the 

 remains of this passage. 



On this hypothesis, six pair of inferior arches belong to 

 the skull — namely, the trabecular and maxillo-palatine, in 

 front of the mouth ; the mandibular, the hj^oidean, and two 

 others (first and second branchial), behind it. For, as there 

 are three cranial nerves embracing the first three visceral clefts 

 which lie behind the mouth, there must be four post-oral, cra- 

 nial, visceral arches. 



Supposing that the occipital segment in the brain-case an- 

 swers to the hindermost, or second branchial, cranial, visceral 

 arch, the invariable attachment of the proximal ends of the 

 mandibular and hyoidean arches to the auditory capsule leads 

 rne to assign the parietal and the frontal segments to the max- 

 illo-palatine and trabecular visceral arches. And thus the os- 

 sifications of the auditory capsule, alone, are left as possible 

 representatives of the neural arches of the three anterior post- 

 oral visceral arches. 



But these speculations upon the primitive composition of 

 the skull, however interesting, must not, as yet, be placed 

 upon the same footing as the doctrine of its segmentation, 

 which is simply a generalization of anatomical facts. 



The Sympathetic. — A Sympathetic Nervous System has 

 been observed in all the Vertehrata except A.mphioxus and 

 the MarsipohranQliU. It consists, essentially, of two longi- 

 tudinal cords, placed one upon each side of the inferior face 

 of the cranio-spinal axis. Each cord receives communicating 

 fibres from the spinal nerves of its own side, and, when com- 

 plete, from all the cranial nerves except those of the special 

 senses of hearing, sight, and smell — the Vidian nerves consti- 

 tuting the anterior terminations of the vsympathetic cords. -At 

 the points of communication ganglia are developed, and the 

 nerves which emerge from these ganglia are distributed to the 

 muscles of the heart and vessels, and to those of the viscera. 

 These peripheral nerves of the sympathetic system frequently 

 present small ganglionic enlargements. 



In the 3Iarsipohranchii^ the place of the sympathetic ap- 

 pears to be taken, to a great extent, by the pneumogastric j 



