Zoological Society. 143 



the perfect insect are fused together. In the Polydesmid(e, the two 

 first segments which bear legs unite their nervous centres with the 

 first suboesophageal, so as to form a short cord similar to that of the 

 Ostracion and some other fish (Newport on Myriapoda, Phil. Trans. 

 1843). In the Scorpion the fusion has gone so far as to form a sort 

 of medulla oblongata, giving rise to eight pairs of nerves {idem). In 

 Nitidula cenea all the abdominal ganglia have united to form a short 

 cord (Blanchard ut antea, plates) ; and in Calandra palmarum the 

 ganglia of the whole body have approximated so as to form a conti- 

 nuous moniliform cord (so far ganglionic in appearance as that the 

 distinction between the segments has not been obliterated), which is 

 placed in the anterior portion of the body {idem, plates) . 



4. The ganglionic cord of Insects undergoes the same alteration at 

 its posterior extremity that the spinal cord of the Vertebrata does by 

 its withdrawal from the caudal vertebrae and the formation of a cauda 

 equina, as may be clearly seen in Blanchard's plates {ut antea, e.g. 

 in the Nitidula cenea, the Calandra •palmarum, and the Byticus mar- 

 ginalis) . 



5. In the Chilognatha, or higher order of the Myriapoda, the 

 gangha coalesce so as to form a uniform spinal cord, the commissural 

 fibres no longer occupying intervening spaces as in the Chilipoda, 

 hut forming the external layer of the nervous cord (Newport on My- 

 riapoda, Phil. Trans. 1843): 



6. Whilst the true vertebrate fish Orthagoriscus mola exhibits 

 exactly an opposite character in the ganglionic condition of its myelon 

 (Owen's Lectures, ii. 173, on the authority of Arsaki). 



Section IV. 



A vertebra is the correlative in the osseous of a centre in the 

 nervous system. 



This appears to me to be the most general possible definition of 

 a vertebra, and therefore the most philosophical. The general idea 

 of the relation of the osseous and nervous centres involved in it, 

 though not the relation of the segments of each one to the other, 

 was thus expressed by Oken: "Bones are the earthy, hardened, 

 nervous system ; nerves are the spiritual, soft, osseous system — Con- 

 tinens et contentum" (quoted by Owen, Rep. Brit. Assoc, p. 242). 



1. The number of vertebrae constituting the spinal cord always 

 corresponds with the number of segments in the cord as indicated by 

 the number of pairs of nerves given ofi". When more than one pair 

 perforate one piece of bone, it results from an anchylosis of several 

 vertebrae, as in the sacrum ; and the coccygeal vertebrae, which ap- 

 pear to be an exception to the definition, are not so in reality, the 

 spinal cord passing into them in the foetal condition, and being gra- 

 dually withdrawn just in the same manner as is the case in some of 

 the Coleoptera. As is clearly seen in them, too, the cauda equina 

 represents the nerves of the vertebrae from which the cord has been 

 withdrawn. Some Vertebrata, as e. g. the Python, retain the original 

 relation of the vertebrae and centres throughout the whole of the 

 spinal cord (Owen, Report ut antea, 221). 



