116 E. EAMSAY WRI&HT ON THE SKULL AND AUDITORY 



conjoined vertebrte and the basioccipital, a piece of uotochord is to be seen, and resting 

 on the surface of the centrum, are the articular processes (hirgely cartilaginous) of the 

 stapedes and the remains of the median partition of dura mater. This must, consequently, 

 still be the upper surface of the first centrum. 



Although the fovea for the sacculi is not entirely absent in the next section repre- 

 sented (Fig. 15, between which and Fig. 14 twelve sections intervene), yet it is obviously 

 in the region of the third vertebral centrum, for it passes through the points of attach- 

 ment of the mallei to the centrum, and through the escape of the dorsal and ventral 

 divisions of the fourth nerve through the well-developed third neural arch, but in the 

 following figure from the thirtieth section further back (Fig. 16), the large centrum of 

 the fourth vertebra is reached, and there is no trace of any backward prolongation of the 

 basi- and ex-occipitals. Here the dorsal and ventral lamellae of the transverse process may 

 be studied (t"4 and t' 4,) and their relationship to the supraclavicle. It will be seen that 

 the lateral neuromastic canal, although protected on the outside by the supraclavicle, is 

 not surrounded by bone on its medial aspect, but lies free in the interval between the two 

 lamellte of the transverse process referred to. 



A comparison of the number of sections between the emergence of the spinal nerve 

 will be instructive, not only in enabling us to arrive at a conception of the great develop- 

 ment of the fourth vertebra, but of the remarkable reduction affecting those in front of it. 

 Thus thirty-six sections intervene between the escape of the fifth and sixth nerves, that 

 is to say, between the hinder parts of the arch of the fourth vertebra and that of the fifth. 



This would, probably, be the average between similar points of succeeding vertebrae, 

 but 70 sections intervene between the fifth and fourth nerves, 12 between the foi;rth and 

 third, 7 between the third and second, and 6 between the second and first. 



Some further points of interest are to be stiidied in Fig. 16, which represents a section 

 nearer the anterior than the posterior end of the fourth vertebra. This section falls through 

 the nxdimentary air-bladders which lie on each side between the dorsal and ventral 

 lamella^ of the transverse processes. Part of the bladder («&') is membranous, part {ab') is 

 osseous. The latter is continuous with the body and arches of the vertebra, and its pre- 

 sence has profoundly modified the shape of the neural canal, and, consec^uently, of the spinal 

 cord in this region. A communication between the hinder end of the malleus and the 

 membranous portion of the bladder can still be traced in front of this plane. The figure 

 represents only the tunica externa, and it will be observed that the osseous portion is evi- 

 dently formed by ossification of the tunica externa and subsequent fusion with the walls of 

 the neural canals. A similar ossification of a very small part of the tunica externa in 

 Amiurus, gives rise to the sickle-shaped process of bone resting on the side of the fourth 

 vertebra, and continuous in the adult with the malleus. No communication can be de- 

 tected between the two rudimentary air-bladders, nor any between the internal triuics 

 and the œsophagus. 



From the above description it becomes apparent that while the air-bladder and 

 Weberian apparatus of Hypophthalmus are certainly anomalous, yet they are constructed 

 upon precisely the same type as those of the other Nematognathi. Their reduced and 

 functionless condition is probably not more pronounced than in many Hypostomidœ, 

 but some further investigations on the latter are desirable before an attempt is made to 



