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 internally by the lamina papyracea of the ethmoid, and its outer wall is crossed obliquely by the lacrimal canal. The roots of the molar teeth do not project up into it. BONES OF THORACIC LIMB The =clavicle= is a small, thin, irregularly-triangular bony or cartilaginous plate. It is embedded in the mastoido-humeralis 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.) [Illustration: FIG. 128.—RIGHT SCAPULA OF DOG, EXTERNAL VIEW. _a_. Supraspinous fossa; _b_, infraspinous fossa; _c_, spine; _d_, upper broad end of spine; _e_, acromion; _f_, glenoid cavity; _g_, tuberosity; _h_, vertebral border; _i_, posterior angle; _k_, scapular notch. (Ellenberger-Baum, Anat. d. Hundes.) ] [Illustration: FIG. 129.—LEFT SCAPULA OF DOG, COSTAL SURFACE. _a_, Subscapular fossa; _b_, _b_, _b_, muscular lines; _c_, _c_, limiting line between subscapular fossa and serratus area; _d_, glenoid cavity; _e_, _f_, tuberosity; _g_, nutrient foramen. (Ellenberger-Baum, Anat. d. Hundes.) ] The =scapula= is relatively long and narrow. The spine increases gradually in height from above downward and divides the external surface into two nearly equal fossæ. 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. The rough area above it for the attachment of the serratus magnus 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 cervical angle is rounded. The dorsal angle is thick and square. The neck is well defined and bears a rough eminence posteriorly. The glenoid cavity is continued forward upon the lower face of the scapular tuberosity, which is blunt and bears no distinct coracoid process. There is a rough eminence on the posterior surface of the neck, from which the long head of the triceps arises. The cervical angle is opposite the first thoracic spine, the dorsal angle lies above the vertebral end of the fourth rib, and the articular angle at a point just in front of the sternal end of the first rib in the ordinary standing position. The shoulder has a great range of movement on the chest-wall.