SCIU'LTE, SET AVHALE. 493 



surface of the membrana this fold forms a free edge, which is the base of the triangle. The 

 apex is at the outer border of the tympanic cavity where malleus and membrana approach one 

 another though not in contact. The sides of the triangle lie along the attachment of the fold 

 to malleus and membrana respectively. This attachment of malleus to the membrana tympani 

 corresponds to that found in adults of this species, except that in them the fold is greatly elon- 

 gated, owing to the outward projection of the membrane. 



Cavum tijnt/>(ini: - - The cavum tympani is a bowl shaped cavity, having dorsal and ventral 

 walls which meet in a sharp angle at their margins. The ventral wall contains in the lateral 

 area the membrana tympani, surrounded by the crescentic os tympanicum. The rest of the 

 ventral wall is made up of the fibrous bulla which fills in the space between os tympanicum and 

 the marginal attachment of the bulla. 



The dorsal wall of the tympanic cavity is made up of otic capsule centrally placed, and 

 circumferentially of a ring of fibrous tissue which connects the petrosum to the surrounding 

 bones. As it presents itself after removal of the os tympanicum and bulla, it is seen to be cov- 

 ered by a layer of thick tissue which completely conceals the underlying cartilage and almost 

 fills the cavity. In the outer area of the cavity this structure throws folds about the ossicles, 

 and only on its removal can they be examined. The formation of this tissue shows it to be of a 

 cavernous nature (Beauregard) and its function is variously stated as hydrostatic or auditory. 

 Tuba auditiva: - - The tuba auditiva passes from the choana laterad between hamular 

 and vaginal processes of the internal pterygoid and penetrates the wall of the bulla obliquely. 

 Its entrance into the tympanic cavity is at the rostral circumference of the same, in the angle 

 formed by the meeting of ventral and dorsal walls. The opening is a crescentic slit capable of 

 valve-like closure. The tube is very short, merely an oblique passage through the fibrous wall 

 of the bulla. The expansion of its distal end into the scaphoid fossa as found in the adult is not 

 yet indicated. 



Ossicula auditus: - - The ossicles are typically mammalian in their arrangement. Meckel's 

 cartilage passes beneath the edge of the tympanic bulla, caudad and slightly dorso-laterad, 

 closely roofed over by the tegmen tympani. Mesad to its shaft is the belly and tendon of the 

 tensor tympani muscle. Within the tympanic cavity, the cartilage expands into a fairly large 

 caput mallei, and forms a mesal projection, the manubrium, to the base of which is attached the 

 tensor tympani. A groove in its surface completely encircles the caput, close to the edge of 

 the articulation with the incus. The border itself expands, thus increasing the depth of the 

 groove. 



The incus has a triradiate form. On the well developed body is a saddle-shaped articular 

 surface for the malleus. The processus brevis is stout and of large size. Its extremity is in 

 contact with the crista parotica, to which it is firmly attached by a ligament. The processus 

 longus, which actually is shorter than the brevis, is also bulky in form. It is directed ventrad 

 to articulate through an os lenticulare with the apex of the arch of the stapes. 



The stapes which is lodged in a deep fossula, does not fill with its foot plate the large fenestra, 

 but is united to its circumference by a rather wide annular ligament. The bone closely agrees 

 with the adult form; its arch is high and narrow, the limbs thick, the foramen small. 



Capsula otica: The otic capsule, in contrast to its relatively small size in the adult, is here 

 a large cartilage forming a considerable portion of the floor and lateral wall of the posterior 

 fossa. 



Caudad the capsule is in -contact with the exoccipital, a thick layer of perichondrium being 



