16 ME. W. P. PTCEAFT ON THE 



The Cartilage-bones . 



The basioccijntal, seen ventrally, is more or less liuguiform in shape, and has a 

 straight anterior border overlapped by the basitemporal plate. Its lateral borders are 

 convex, and approach one another towards the middle line, reducing the posterior border 

 to a small segment forming the centre of the occipital condyle, and forcing its way 

 between the exoccipitals to bound the occii^ital foramen. Its lateral boundaries are 

 formed by the exoccipitals only. Seen dorsally, it is found that its lateral boundaries 

 are formed by the pro-otic and exoccipital; the former occupying the anterior and 

 the latter the posterior half of the border. 



The exoccij^ital is large and irregular in shape. Ventrally its mesial border, which is 

 concave, bounds the basioccipital ; its anterior border is convex and overlapped by the 

 wings of the basitemporal plate, whilst its external lateral border bounds the tympanic 

 cavity, the posterior half of the border being produced to form the floor of the trumpet- 

 shaped mouth of that cavity. Its posterior border is more or less triangular in shape, 

 the outer face of the triangle being bounded by the squamosal and the inner face by 

 the supraoccipital. Dorsally, the exoccipital is entirely concealed by the pro- and epi- 

 and opisthotic bones, only a small portion of its mesial border being visible. 



The exoccipital seen ventrally is of considerable size, yet of smaller extent than in the 

 adult. Mesially, of course, it bounds the basioccipital and lateral portions of the 

 foramen magnum ; the increase in size which takes place as growth proceeds is due to 

 additions to the external lateral border forming the tympanic wing. In the nestling 

 one-quarter grown this external border is divided into two regions, an anterior and a 

 posterior, by a deep notch. The anterior segment is represented by an almost quadrate 

 plate bounding the basioccipital and forming the floor of the pro-otic ; the posterior segment 

 belongs to that portion of the plate which bounds the foramen magnum. The external 

 border of this plate extends outwards to form the lower portion of the trumpet- shaped 

 mouth of the tympanic cavity. This posterior segment is bounded superiorly by deep 

 grooves filled by cartilage, covering the pro-otic, the groove being bounded on the other side 

 by the squamosal. In the adult this groove is completely obliterated, the exoccipital 

 passing insensibly into the tympanic wing of the squamosal, to be described presently. 

 Within the cranial cavity the exoccipital is almost entirely concealed by the pro-otic 

 and opisthotic, only an extremely small portion being visible, mesiad of the opisthotic. 



The supraoccipital is a relatively small bone, deeply cleft in the median line from 

 the crest of its superior border downwards to the middle of the bone. On either 

 side of the median cleft lies a long crescentic groove, which later becomes closed by 

 the meeting of the edges of the groove and converted into the channel for the vena 

 cephalica posterior. Between the supraoccipital and the squamosal is a large space 

 filled by cartilage, which, in the dried skin, shrinks and reveals the pro-otic. In the 

 adult this space becomes filled in by the approximation of the edges of the squamosal and 

 supraoccipital. 



The pro-otic appears externally as an oblong mass of cartilage, of small extent, 

 between the supraoccipital, squamosal, parietal, and exoccipital bones. Internally it is 



