524 PROFESSOR OWEN ON THE EUSTACHIAN CANALS IN CROCODILES. 
median plane. I followed out the further course of these canals in the skull of a Cro- 
codilus acutus of about the same size as the recent Alligator. Figure 7} Plate XLI. 
shows the common median canal extending from e to e', where it divides. Each of 
these branches subdivides, and sends its subdivisions, one to the right the other to 
the left, to communicate with the tympanic cavity. The lateral canals, e/', which com- 
mence below at el, one on each side of the median foramen, communicate with the 
lateral subdivisions, eo, of the posterior or basioccipital branch of the common median 
canal ; a small rhoinboidal sinus, eo', being formed at their point of union, from which 
a short canal is continued to the tympanic cavity. Thus each lateral canal, et, with 
each posterior lateral subdivision, eo, of the basioccipital branch of the median canal, 
has a common opening into the base of the tympanic cavity of its own side. Each 
lateral subdivision, es, fig. 8, Plate XLIL, of the anterior or basisphenoid branch 
of the median canal opens into the tympanic cavity at es, fig. 10, in advance of the 
preceding orifice. The lining membrane of these several canals here becomes con- 
tinuous with that of the tympanic cavity. 
Thus it was seen that no passage from the median orifice or canal in question, e, 
figs. 1 and 7, between the basioccipital and basisphenoid, conducted to the nasal 
passages, but that all the branches from that common orifice opened into the tym- 
panic cavity: at the same time it was demonstrated, that the communication between 
the tympanum and the palate, commonly called the “^Eustachian tube,’ was more 
complex in the Alligator and Crocodile than had been suspected, or than was known 
to exist in any other animal. It may be described as follows: — From each tympanic 
cavity two passages are continued downward, one from the fore part, Plate XLII, 
fig. 10, es, the other from the floor, ih. eo, of the cavity. The anterior canal, es, passes 
downwards and inwards, expands and again contracts before it unites with its fellow 
from the opposite side at es', fig. 8, to form a median canal, es' to e' , which passes 
from the basisphenoid to the space or broad suture between that bone and the basi- 
occipital, where it terminates in the single subvertical canal, e' to e, descending along 
that suture to the median foramen in question, e, fig. 1. 
The opening at the floor of the tympanic cavity, eo, fig. 10, leads to a short canal, 
eo, figs. 7 and 8, which curves towards its fellow from the opposite tympanum, but 
first swells into the rhomboid sinus, fig. 7 , eo', and divides ; one branch descends 
almost vertically, el', and terminates by the small foramen, el, fig. 1, in the osseous 
groove or channel leading to the central aperture and fossa; the other branch, eo to 
eo', fig. 8, continues the course inwards and downwards until it meets its fellow at 
the median line of the basioccipital at eo', and forms the posterior primary division of 
the common median canal, eo to e : this soon joins the anterior division, at e', to form 
that common canal, which then descends and terminates by slightly expanding into 
the foramen, e, at the middle of the fossa between the basioccipital and basisphenoid ; 
which fossa receives also the grooves lodging the membranous canals from the lateral 
fissures. Finally, the three bony canals terminate by their membranous continuations 
