BONES OF THE THORACIC LIMB 197 



The mastoid foramen is at the junction of the occipital and temporal bones, above 

 the root of the paramastoid process; it opens directly into the cranial cavity. 

 The foramen magnum varies greatly in form; most often the transverse diameter is 

 the greater, but in some skulls it is equaled or exceeded by the vertical diameter. 



The cranial cavity (Fig. 212) corresponds in form and size with the cranium, 

 especially in those breeds in which the various crests are more or less effaced and the 

 frontal sinuses are small. The basi-cranial axis is almost parallel with the palate, 

 and the floor is flattened. The anterior fossa is narrow and is only slightly higher 

 than the middle one. The ethmoidal fossa? are very deep and the crista galli is 

 little developed. The hypophyseal fossa is variable in depth, and the dorsum sella; 

 is relatively high and bears clinoid processes laterally. The cerebral and cerebellar 

 compartments are well marked off laterally by the petrosal crests and dorsally by 

 the tentorium osseum. The base of the latter is traversed by a canal which con- 

 nects the two temporal canals. The anterior angle of the petrous temporal is 

 perforated by a canal for the fifth cranial nerve. 



The nasal cavity (Fig. 212) conforms to the shape of the face. Its anterior 

 aperture is large and nearly circular in most dogs. The complex ventral turbinates 

 occupy the anterior part of the cavity to a large extent, except near the aperture. 

 Behind the ventral turbinate is the large opening of the maxillary sinus. Behind 

 this the cavity is divided by the lamina transversalis into a large upper olfactory 

 region or fundus nasi and a lower naso-pharyngeal meatus. The fundus is occupied 

 largely by the ethmoturbinates. The posterior nares are undivided and are in 

 general long and narrow, but vary with the shape of the skull. 



The frontal sinus is of considerable size in the large breeds, but is confined to 

 the frontal bone. It is usually divided into a small anterior and a much larger 

 posterior compartment, each of which opens into the dorsal ethmoidal meatus. 

 The sinus is very small in extreme brachycephalic types. 



The maxillary sinus is small, and is in such free communication with the nasal 

 cavity as to make it rather a recess than a true sinus. It is bounded medially by 

 the lamina lateralis of the ethmoid, and its lateral wall is crossed obliquely by the 

 naso-lacrimal canal. The roots of the molar teeth do not project up into it. 



BONES OF THE THORACIC LIMB 



The clavicle is a small, thin, irregularly triangular bony or cartilaginous plate. 

 It is embedded in the brachiocephalicus muscle in front of the shoulder-joint and 

 forms no articulation with the rest of the skeleton. (It is nearly an inch long in 

 a large cat and is a slender curved rod.) 



The scapula is relatively long and narrow. The spine increases gradually 

 in height from above downward and divides the lateral surface into two nearly 

 equal fossae. Its free edge is thick and rough above, and at the lower part is thin 

 and bent backward. The acromion is short and blunt and is opposite the rim of 

 the glenoid cavity. The subscapular fossa is very shallow and is marked by rough 

 lines (Linese musculares). The rough area above it for the attachment of the ser- 

 ratus ventrahs is large and quadrilateral in front, narrow and marginal behind. 

 The anterior border is thin, strongly convex, and sinuous. The posterior border 

 is straight and thick. The vertebral border is convex and thick and bears a band 

 of cartilage. The anterior angle is rounded. The posterior angle is thick and square. 

 The neck is well defined and bears a rough eminence posteriorly, from which the 

 long head of the triceps arises. The glenoid cavity is continued forward upon the 

 lower face of the tuber scapulae, which is blunt and bears no coracoid process. The 

 cervical angle is opposite the first thoracic spine; the dorsal angle lies above the ver- 

 tebral end of the fourth rib, and the glenoid angle at a point just in front of the 

 sternal end of the first rib in the ordinary standing position. The tuber scapulae 



