THE SYMPATHETIC NERVES. 77 



and the maxillo-palatine processes may represent pre-oral 

 visceral arches, which are bent forward ; and, in the case of 

 tlie trabeculce, coalesce with one another. Snch an hypothesis 

 would enable ns to understand the signification of the naso- 

 palatine canal of the Myxinoid fishes, which would be simply 

 the interspace, or passage, between the trabeculae (which 

 must have originally existed if ever they were distinct 

 visceral arches) not yet filled tip ; and the anomalous process 

 of the roof of the oral cavity, which extends towards the 

 pituitary body in the embryos of the Verkbrata in general, 

 might be regarded as the remains of this passage. 



On this hypothesis, six pair of inferior arches belong to 

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

 front of the mouth ; the mandibular, the hyoidean, 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, cranial, visceral arches. 



Supposing that the occipital segment in the brain-case 

 answers 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 me to assign the pai'ietal and the frontal 

 segments to the maxillo-palatine and trabecular visceral 

 arches. And thus the ossifications 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 jjlaced 

 upon the same footing as the doctrine of its segmentation, 

 which is simply a generalisation of anatomical facts. 



The Sympathetic. — A Sympathetic Nervous System has 

 been observed in all the Vtrtebrata except Am.phioxus and 

 the Marsipobranchii. 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 commu- 

 nicating fibres from the spinal nerves of its own side, and. 



