The portion which represents the tympanic bone (a), and which can be separated from the malleus in theyoungsubject, is a slender osseous filament bent into three-fourths of a circle, and placed upon the inner margin of the tym panic fossa, its concavity looking outwards: this concavity is impressed with a fine groove for the insertion of the membrana tympani : the posterior part of the hoop passes across the commencement of the Eustachian canal, and terminates in a free point upon the pos terior wall of the tympanic fossa : the anterior end of the hoop is applied to and usually anchylosed with the longitudinal bar of the malleus (b).
Only a small portion of this ossicle is con tained within the cavity of the tympanum ; the principal portion forms the external and part of the posterior boundary of the bony meatus auditorius, and is then continued forwards in the form of a slender pointed process; the bone slightly expands as it extends backwards, and its broadest part is abruptly bent inwards until it nearly meets the posterior end of the tympanic hoop. From the extremity of this in flected portion a slender compressed process (c) extends to the centre of the space encircled by the bony hoop ; it is attached by its whole length to the membrana tympani, and repre sents the handle of the malleus. At the pos terior margin of the broad incurved part of the malleus there are two minute tubercles nearly a line apart: the short and simple columelliform stapes (d) ascends vertically from the inner most of these tubercles, with the upper surface of which it is articulated ; its opposite ex tremity closes the foramen ovale in the form of an expanded plate. The membrana tym
pani is concave outwardly at its middle part.
The eighth and ninth pairs of nerves have the usual origins and proportions. The pneumogastric nerve (fig. 180, d) is closely attached, at its origin, to the hypoglossal (fig. 180, b), but is quite distinct from the sympathetic (fig. 180, f): it gives off the superior laryngeal, and then proceeds along the neck to the chest : the right nerve here sends its recurrent branch, in the usual manner, round the arteria innominata; the left branch (fig. 187, k) winds round the aorta : the trunk of the pneumogastric is then expended in the cardiac, pulmonic, cesophageal, and gastric nerves. The spinal accessory nerve (fig. 180, c) is thicker than the pneumogastric, and has the usual distribution.
The brachial plexus is formed by the five posterior cervical and the first dorsal nerves. The third cervical nerve is shown at g, fig. 180. The median nerve perforates the inner condyle of the humerus.
The lumbar plexus is formed by the two posterior dorsal, the two lumbar, and the first sacral nerve.
The great ischiadic nerve divides into the peroneal and tibial branches before it quits the pelvis. The crural nerve is shown at h (fig. 180).