68 DENNIS, ON THE MICROSCOPIC 



complete change and one anomalous, so far as I have ob- 

 served, in the structure of that bone in birds, takes place ; in 

 the vertical section the Haversian tubes are beautiftdly and 

 regularly reticulated, the reticulations forming circles gene- 

 rally of a very regular form, and the long and fusiform 

 lacunae are replaced by roundish ones; in the transverse 

 section the Haversian tubes appear as truncated branches, 

 and the lacunae are large, which would seem to indicate that 

 their form is pretty circular. The femur of the gannet 

 (fig. 20) very nearly compares with the coracoid, without so 

 much regularity in the Haversian circles ; the lacunae are 

 often roundish oval, sometimes irregular, or long and thickish. 

 In the transverse section (fig. 21) the Haversian tubes appear 

 in parallel rows, Avith the excised ends of others between ; the 

 lacunae have irregular canaliculi, beautifully reticulated. 

 In the tibia (fig. 22) vertical section the Haversian tubes 

 run longitudinally, with numerous transverse, diagonal con- 

 nections, characters not at all uncommon in the same bone 

 in other animals ; the lacunae are small sharp-pointed ovals, 

 and differ little either in the vertical or transverse sections. 

 In the tarsus the Haversian tubes are not so numerous, but 

 very much thicker, and also of variable dimensions ; in the 

 toes the reticulations are numerous, but not circular, and 

 rather appearing as traces to the longitudinal tubes. In the 

 rib (fig. 24) of the gannet there is a striking correspondence 

 in the characters and jiaucity of the Haversian tubes with 

 those of the humerus of the Pteropus ; and we may also 

 notice, in the furcula (fig. 6) of the swift, the same diminu- 

 tion of the Haversian tubes, a bird of very peculiar construc- 

 tion in its wing, with an enormous muscular development. 

 The swifts have by far the shortest humerus, and the longest 

 metacarpal bones and phalanges, of any bird I have dissected. 

 The sternum of the gannet, being exceedingly transparent, 

 affords an excellent \dew of the coarser Haversian tubes 

 without the use of the microscope ; in the keel they radiate 

 from the thick portion, where the coracoids articulate, like the 

 spokes of a wheel to the edge, appearing looped and braced ; 

 in the sternum plate they are bowed outwards from the 

 central line towards the coracoids, being very close together, 

 with numerous cross braces ; in the ilia they run longitudi- 

 nally with much reticulation at the edges ; in the flat 

 sui'face of the back, behind the sockets of the femur, they 

 pass across to the anchylosed vertebrae, being met by others 

 at right angles that extend longitudinally along the narrow 

 part of the bone until they approach the pehds, when they 

 reticulate until they arc lost sight of in the thickness of the 



