19 
the Insects, the external longitudinal fibres serving only as commis- 
sural or communicating portions. 
3. Those ganglia of the Insects which are perfectly separate in the 
larval condition often exhibit a tendency to fusion in the perfect con- 
dition (Blanchard wt antea). Thus in the Coleoptera the last abdo- 
minal ganglion is always formed by a fusion of several original ones ; 
the first and second abdominal often form a single mass with the 
metathoracic, whilst in the Chafer this last is united with the meso- 
thoracic (idem). In like manner the fourth and fifth segments in 
the perfect insect are fused together. In the Polydesmide, the two 
first segments which bear legs unite their nervous centres with the 
first subcesophageal, 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 enea all the abdominal ganglia have united to form a short 
cord (Blanchard wé 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 (édem, 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 vertebre and the formation of a cauda 
equina, as may be clearly seen in Blanchard’s plates (wt antea, e.g. 
in the Nitidula enea, the Calandra palmarum, and the Dyticus mar- 
ginalis). 
5. In the Chilognatha, or higher order of the Myriapoda, the 
ganglia coalesce so as to form a uniform spinal cord, the commissural 
fibres no longer occupying intervening spaces as in the Chilipoda, 
but 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 Ayrsaki). 
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, Report of Brit. Assoc. 
. 242). 
1. The number of vertebra constituting the spinal cord always 
corresponds with the number of segments in the cord as indicated by 
