Prof. Reid on the Voa;marus Islandicus. 477 



^to 



neural spines this tube was filled up with another texture pre- 

 senting an indistinct cellular appearance having some pretty 

 large and distinct nucleated cells dispersed through it (fig. 7), 

 approaching the cartilaginous texture, yet decidedly diff"ering 

 from it*. The walls of this tube were thinner in the superior spi- 

 nous processes than in the interneural spines, and in both (figs. 7 

 and 8) bands passed outwards from its circumference, which in 

 the interneural spines sometimes assumed the form of the single 

 chain, similar to what has been represented in fig. 4. Quadri- 

 lateral and hexagonal hyaline tubes, combined with a greater or 

 less proportion of the fibrous and nucleated tissue which fills up 

 the intervals between the rays in the body of the vertebrse, enter 

 into the formation of many of the bones of the head, while por- 

 tions of some of them, as the nasal processes of the intermax- 

 illaries, and portions of the body of the sphenoid, and the ante- 

 rior sphenoid, and some parts of the superior maxillary, are en- 

 tirely composed of these tubes closely aggregated together. Se- 

 veral bones which consist internally of true cartilage, as those 

 composing the posterior, lateral and superior walls of the cranium, 

 the hyoid and inferior maxilla, are covered externally by a layer 

 of these tubes intermixed with the fibrous and nucleated tissue. 

 In the bones of the cranium this covering is confined to the ex- 

 ternal surface. The branches of the superior maxilla which go 

 to join the inferior maxilla are not covered by skin, but by a dense 

 firm layer made up of these tubes. This tubular structure also 

 abounds in the branchial arches, in the scapular arch, in the 

 opercular bones, and in the branchiostegous rays, all of which 

 structures are destitute of true cartilaginous and osseous tissues. 



EXPLANATION OF PLATE XVL 

 Fig- 1. Part of the digestive tube slit open. The commencement of the 

 large intestine has been detached and separated from the posterior 

 end of the stomach : a, pyloric portion of stomach ; b, cul de sac 

 of stomach ; c c, second portion of small intestine ; d, caeca sur- 

 rounding duodenum or first portion of the small intestine ; /, duo- 

 denum ; g, lower portion of oesophagus ; h, valve at the termina- 

 tion of the small in the large intestine ; m, commencement of large 

 intestine; o, spleen. 

 Fig. 2. Fibres, greatly magnified, from the external surface of the skin, upon 



which its silvery lustre depends. 

 Fig. 3. Transverse section of an abdominal vertebra. 

 Figs. 4, 5 and 6. Representations greatly magnified of a peculiar hyaline 



tissue entering largely into the formation of the skeleton. 

 Fig. 7. Peculiar structure seen in a transverse section of the intersjiinous 



bones. 

 Fig. 8. Peculiar structure seen in a transverse section of the spinous pro- 

 cesses of the vertebree. 



* The smaller tubes formed by this dense hyaline substance, such as those 

 in the bodies of the vertebrae, in some instances, if not in all, contain more 

 or less of a soft substance without any distinct structure in which some gra- 

 nules are imbedded. 



