120 ME. W. K. PAEKEE ON THE STEUCTUEE AND 



When, however, a sufficient mass of these has been formed in each region, then, 

 whether the tract be of fibrous tissue or of hyaline cartilage, the bony metamorphosis 

 takes place indifferently, and the distinction is lost again ; the two temporary tissues, 

 notwithstanding their very different attributes, giving place to a uniform bony structure, 

 which, when completely formed, tells no tale of its former history. 



The next parts to be described are the facial arches — visceral arches of the head ; 

 and here it will be well to attend with the utmost care to what may be really seen, 

 rather than to what has been written upon the subject. The first poststomal arch is 

 the most highly developed in the Ostrich, in typical birds, and in the Vertebrata gene- 

 rally, it may therefore be our starting-point ; stiU it will be necessary, more or less, to 

 study all the four arches in connexion. 



In Struthio, A., the proximal part — quadrate cartilage, incus — (Plate VII. figs. 3-6, q.) 

 is already completely segmented from that part of the investing mass which is in 

 front of the periotic capsule, and which is indeed the basal part of its investment ; for 

 the cartilage which covers in the simple auditory sac, and that which grows round the 

 " medulla oblongata," are both continuous with that which invests the cephalic part of 

 the " chorda." The mandibular pedicel has its true origin along the outside of the base 

 of the auditory sac ; the pedicel of the hyoid ramus arises from the posterior part of its 

 base, somewhat behind and below the quadrate cartilage ; the second ^r^stomal arch 

 (palatine) arises from the base of the posterior wall of the nasal sac, and the first 

 prestomal (intermaxillary) never becomes perfectly segmented from the base of the 

 internasal septum ; the latter part, being the extreme of the series, thus keeps to a low 

 type of growth like, but still more simple than, the mandibular arch of the Chimseroid 

 fishes ; the Lepidosiren also, and the Batrachia generally. The mandible of the Ostrich 

 (Plate VII. fig. 3, q.-d.) does not show the typical form of a visceral arch so well as 

 many of the higher birds, especially the Gallinae ; the truest form being a descending 

 proximal part, to which is swung a long bar projecting both backwards and forwards, 

 and which lies nearly in the same plane as the cranio-facial axis. The high development 

 of this arch is shown in its complete separation by a synovial joint from the end of the 

 bar in front of it — the pterygo-palatine arcade ; and also in as perfect a division between 

 the suspending portion from the long, almost horizontal bar. The long anterior part 

 (Meckel's cartilage) has grown, as it were, at the expense of the posterior more incurved 

 rod (the internal angular process, manubrium mallei of Mammals) ; and the very free 

 joint between the quadrate and the articular portion allows this arch to form a large 

 angle with the palatine (the rudimentary second prestomal arch) when the mouth is 

 opened. The quadrate cartilage is large, and rather clumsily formed; its head or 

 suspending part shows no trace of subdivision into an anterior and a posterior incudal 

 ci-us; and its orbital or metapterygoid portion is large and thick. A film of bone 

 (Plate VII. figs. 3 & 4, q.) has appeared under the perichondrium at the postero-extemal 

 part of the shaft : on the outside this lamella is seen to be creeping on to the metapte- 

 rygoid process, which is ossified continuously with the main part of the bone. 



