92 
MAMMALIAN ANATOMY 
Draw transverse sections at various levels, showing the structures 
and relationships which have been worked out, and a medial view of 
the interior of the nasal cavity. 
Dissect out the maxillary concha from the nasal cavity of the 
half head, leaving intact the surface of its anterior region upon 
which a small pore (the orifice of the naso-lacrimal duct) may be 
seen; cut away also the superficial layer of the nasal concha 
thereby laying open the sinus which it encloses; remove the freely 
projecting ends of the ethmoid conchse, and note the complicated 
character of the sinuses and scrolls thus displayed. In the walls 
of the nasal cavity as thus exposed, look for orifices leading into 
the frontal, nasal, and maxillary sinuses. Pass probes of fine 
wire, or other suitably flexible material, as far as possible into all 
probable orifices of this kind, including that of the naso-lacrimal 
duct, and after recording the location of each of these upon a drawing 
of the lateral wall of the cavity, complete the identification of 
these orifices by dissecting away the bony walls sufficiently to 
follow the probe .previously introduced until the relationships 
have been clearly determined. In the case of the naso-lacrimal 
duct note that the tube is one of considerable size which passes 
very obliquely through the outer plate of the maxillary bone and 
through the lacrimal foramen into the orbit of the eye. By means 
of dotted lines added to the drawing of the lateral wall of the cavity show 
the connections thus worked out between the nasal cavity and the 
various sinuses, and the course of the naso-lacrimal duct. 
E. THE EAR. (Cf. model of the human ear.) 
The internal ear, the real auditory organ, lies within the 
thickened or petrous portion of the temporal bone and consists 
of a thin-walled, complicated structure known as the membran¬ 
ous labyrinth, enclosed within a thin layer of bone known as the 
bony labyrinth. The membranous labyrinth is filled with a 
fluid known as the endolymph, and the space between the mem¬ 
branous and the bony labyrinth is filled with perilymph. Acces¬ 
sory to the internal ear, are the cavum tympani (middle ear) arid 
the external ear, which afford a continuous channel through which 
the sound waves reach the internal ear. 
