| OCR Text |
Show 194 MR. R. I. POCOCK OX HORN-GROWTH [Mar. 21, from the antepenultimate break, tlie other about 1 inch behind the latter and 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 reference may be numbered, from apex to base, 1, 2, 3, 4, 5, 6. No. 6, measuring 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 normal 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, measuring I t, inches, also exhibits on its anterior (inferior) surface, close to the base of no. 4, a small tubercle representing the prong; but there is no sign of any prominence on its opposite side. No. 4, measuring 1 inch, has no tubercles. No. 3 measures l j 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 liorn-slieaths 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 more 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 traversed by a large number of clefts running longitudinally and obliquely backwards 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 parts described above. The appearance of the whole series of clefts, however, forcibly suggests a corresponding number of attempts to get rid of the sheaths at different 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 certain 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 freely movable. On the other hand, the central core that binds the proximal sheath (no. 6) to the previously formed portions of the |
