194 MR. R. I. POCOCK ON HORN-GROWTH [Mar. 21, 



from the antepenultimate break, the other about 1 inch behind 

 the latter ancl 1^ inches in front of the proximal break. These 

 breaks divide the sheath superficially into six pieces — a proximal, 

 a distal, and four intermediates, which for convenience of refer- 

 ence may be numbered, from apex to base, 1, 2, 3, 4, 5, 6. 



No. 6, measuixng about 1 inch in length, presents near the 

 middle of its anterior (inferior) surface a very distinct blunt 

 tubercle, representing, I believe, the prong of the noi-mal horn. 

 A similar but smaller boss is also present on its posterior (superior) 

 side, and corresponds in all probability to the posterior angular 

 prominence visible in the proximal half of the normal horn. 



No. 5, measui'ing 1| inches, also exhibits on its anterior (in- 

 ferior) surface, close to the base of no. 4, a small tubercle 

 representing the prong ; bvit there is no sign of any prominence 

 on its opposite side. 



No, 4, measuring 1 inch, has no tubercles. 



No. 3 measures 1^ inches, and shows a very slight eminence 

 near its distal extremity. 



No. 2, also measuring 1| inches, has no trace of the prong. 



No. 1, measuring 2| inches, tapers to a point and forms a 

 semicrescentic curve. There is no trace of a prong. 



The measurements above given represent the lengths of the 

 portions of the individual horn-sheaths left uncovered by the 

 previously formed sheath, with the exception only of the 1st or 

 distal sheath, the first formed of the series. This sufficiently 

 explains the greater apparent length of the latter. 



In longitudinal section, the true length of the separate sheaths 

 is moi"e clearly shown. The composite sheath may thus be seen 

 to consist (1) of a solid horny central core extending from its tip 

 to the apex of the cavity of the proximal piece, which fitted over 

 the bony process of the skull, and serving to bind together the 

 separate sheaths ; and (2) of a cortical layer which is ti-aversed 

 by a large number of clefts running longitudinally and obliquely 

 backv/ards and slightly outwards from the central core towards 

 the periphery of the cortical layer. Of these clefts five only 

 reach the surface, and, being larger than the rest, divide the 

 sheath into the six component j^arts described above. The 

 appearance of the whole series of clefts, however, forcibly suggests 

 a corresponding number of attemjDts to get rid of the sheaths at 

 diflferent periods. 



As already explained, a complete interruption of the continuity 

 of the cortical tissue extending all round the sheath occurs only 

 in two places, and admits of a cei-tain freedom of movement 

 between the adjacent portions of the composite sheath. The 

 cleft defining the distal sheath extends so deeply as almost to 

 sever this sheath from the apex of the antecedent or 2nd sheath, 

 the portion of the central core that binds the two together being 

 only about 2 mm. in thickness. Hence the distal sheath is fi'eely 

 movable. On the other hand, the central core that binds the 

 proximal sheath (no. 6) to the previously formed portions of the 



