112 



THE SKELETON 



The nasal fossae open on the face by means of the apertiira piriformis, a heart-shaped or 

 piriform opening whose long axis is vertical and whose broad end is below. The orifice is 

 bounded above by the lower borders of the nasal bones, laterally by the maxillae, inferiorly 

 by the premaxillarj' portions of the maxillse, and in the recent state the orifice is divided by the 

 septal cartilage. Below, where the lateral margins slope inward to meet in the middle line, 

 is the anterior nasal spine. 



The choanse (posterior nares) are bounded superiorly by the alee of the vomer, the sphe- 

 noidal processes of the palate, and the inferior surface of the body of the sphenoid; laterally 

 by the lateral pterygoid plates; and inferiorly by the posterior edge of the horizontal plates of 

 the palate bones. They are separated from each other by the posterior border of the vomer. 



The nasal fossae communicate with all the more important fossse and the air-sinuses of the 

 skull. By means of the foramina in the roof they are in connection with the cranial cavity; 



Fig. 137. — The Choan^. Veiwed from behind. 

 Pbarvngeai canals 



Pterygoid canal 



Scaphoid fossa 



Pterygoid fossa 

 Lateral pterygoid plate 



Tuberosity of palate bone 



Medial pterygoid plate 



Hamular process 



Spine of palate or posterior 

 nasal spine 



by the infundibulum each fossa is in communication with the frontal and anterior ethmoidal 

 cells; the posterior ethmoidal cells open into the superior meatuses and the sphenoidal sinuses 

 into the recesses above; the spheno-palatine foramina connect them with the pterygo-palatine 

 fossae, and by means of an irregular orifice in each lateral wall they communicate with the max- 

 illary sinuses. The canals for the naso-lacrimal ducts connect them with the orbits,i5and the 

 incisive canals with the oral cavity. 



THE INTERIOR OF THE SKULL 



In order to study the interior of the skull it is necessary to make sections in 

 three directions — sagittal, coronal, and horizontal. This enables the student to 

 examine the various points with facility, and displays the great proportion the 

 brain cavity bears to the rest of the skull. The sagittal section (fig. 138) should 

 be made slightly to one side of the median line, in order to preserve the nasal 

 septum. The black line (fig. 138) drawn from the basion (anterior margin of 

 the foramen magnum) to the gonion (the anterior extremity of the sphenoid) 

 represents the basi-cranial axis ; whilst the line drawn from the gonion to the 

 subnasal point lies in the basi-facial axis. These two axes form an angle termed 

 the cranio -facial, which is useful in making comparative measurements of crania. 

 .\ line prolonged vertically upward from the basion will strike the bregma. This 

 is the basi-bregmatic axis, and gives the greatest height of the cranial cavity. 

 A line drawn from the ophryon to the occipital point indicates the greatest length 

 of the cranium. 



Near its middle, the cranial cavity is encroached upon by the petrous portion 

 of the temporal bone on each side; the walls are channelled vertically by narrow 

 grooves for tlio middle and small meningeal vessels, and toward the base and at 

 the vertex are broader furrows for the venous sinuses. 



The coronal section is most instructive when made in the basi-bregmatic 

 axis. The section will pass through the petrous poition on each side in such a 

 way as to traverse the external auditory passage and expose the tympanum and 

 vestibule, and will also partially traverse the internal auditory meatus. Such 



