Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/189

177 ANATOMY.] HORSE 177 broader, and it ends posteriorly in a rounded excavated border opposite the hinder border of the penultimate molar tooth. It is mainly formed by the maxillae, as the palatines are very narrow. The pterygoids are delicate slender slips of bone attached to the hinder border of the palatines, and supported externally by, and generally ankylosed to, the rough pterygoid plates of the alisphenoid, with no pterygoid fossa between. They slope very obliquely forwards, and end in curved, compressed, hamular processes. There is a distinct alisphenoid canal for the passage of the internal maxillary artery. The base of the cranium is long and narrow; the alisphenoid is very obliquely perforated by the foramen rotundum, but the foramen ovale is confluent with the large foramen lacerum medium behind. The glenoid surface for the articulation of the mandible is greatly ex tended transversely, concave from side to side, convex from before backwards in front, and hollow behind, and is bounded posteriorly at its inner part by a prominent post- glenoid process. The squamosal enters considerably into the formation of the temporal fossa, and, besides sending the zygomatic process forwards, it sends down behind the meatus auditorius a post-tympanic process which aids to hold in place the otherwise loose tympano-periotic bone. Behind this the execcipital gives off a very long paroccipital process. The periotic and tympanic are ankylosed together, but not with the squamosal. The former has a wide but shallow floecular fossa on its inner side, and sends backwards a con siderable &quot; pars mastoidea,&quot; which appears on the outer surface of the skull between the post-tympanic process of the squamosal and the exoccipital. The tympanic forms a tubular meatus auditorius exteruus directed outwards and slightly backwards. It is not dilated into a distinct bull a, but ends in front in a pointed styliform process. It com pletely embraces the truncated cylindrical tympanohyal, which is of great size, corresponding with the large develop ment of the whole anterior arch of the hyoid. This con sists mainly of a long and compressed stylohyal, expanded at the upper end, where it sends off a triangular posterior process. The basi-hyal is remarkable for the long, median, pointed, compressed &quot;glossohyal&quot; process, which it sends forward from its anterior border into the base of the tongue. A similar but less developed process is found in the rhinoceros and tapir. The mandible is largely developed, especially the region of the angle, which is ex panded and flattened, giving great surface for the attach ment of the masseter muscle. The condyle is greatly elevated above the alveolar border ; its articular surface is very wide transversely, and narrow and convex from before backwards. The coronoid process is slender, straight, and inclined backwards. The horizontal ramus, long, straight, and compressed, gradually narrows towards the symphysis, where it expands laterally to form with the ankylosed opposite ramus the wide, semicircular, shallow alveolar border for the incisor teeth. The vertebral column consists of seven cervical, eighteen dorsal, six lumbar, five sacral, and fifteen to eighteen caudal vertebrae. There maybe nineteen rib- bearing vertebrae, in which case five only will be reckoned as belonging to the lumbar series. The odontoid process of the atlas is wide, flat, and hollowed above, as in the ruminants. The bodies of the cervical vertebra are elongated, strongly keeled, and markedly opisthocoelous, or concave behind and convex in front. The neural laminae are very broad, the spines almost obsolete, except in the seventh, and the transverse processes not largely developed. In the trunk vertebra the opistho coelous character of the centrum gradually diminishes. The spinous processes of the anterior thoracic region are high and compressed. To these is attached the powerful elastic ligament, ligamentum mtc/ue, or &quot; paxwax,&quot; which passing forwards in the middle line of the neck above the neural arches of the cervical vertebrae, to which it is also connected, is attached to the occiput and supports the weight of the head. The transverse processes of the lumbar vertebrae are long, flattened, and project horizontally out wards or slightly forward from the arch. The metapo- physes are moderately developed, and there are no anapo- physes. The caudal vertebrae, except those quite at the base, are slender and cylindrical, without processes and without chevron bones beneath. The ribs are eighteen or nineteen in number on each, side, flattened, and united to the sternum by short, stout, tolerably well ossified sternal ribs. The sternum consists of six pieces ; the anterior or pnesternum is extremely compressed, and projects forwards like the prow of a boat. The segments which follow gradually widen, and the hinder part of the sternum is broad and flat. As in all other ungulates, there are no clavicles. The scapula is long and slender; the supra-scapular border is rounded, and slowly and imperfectly ossified. The spine is very slightly developed ; rather above the middle its edge is thickened and somewhat turned backwards, but it gradually subsides at the lower extremity without forming any acromial process. The coracoid is a prominent rounded nodule. The humerus is stout and rather short. The ulna is quite rudimentary, being only represented by little more than the olecranon. The shaft gradually tapers below and is firmly ankylosed to the radius. The latter bone is of nearly equal width throughout. The three bones of the first row of the carpus (the scaphoid, lunar, and cuneiform) are subequal in size. The second row consists of a very broad and flat magnum, supporting the great third metacarpal, having to its radial side the trapezoid, and to its ulnar side the unciform, which are both small, and articulate distally with the rudimentary second and fourth metacarpals. The pisiform is large and prominent, flattened, and curved ; it articulates partly with the cuneiform and partly with the lower end of the radius. The large metacarpal is called in veterinary anatomy &quot;cannon bone&quot;; the small lateral metacarpals, which gradually taper towards their lower extremities, and lie in close contact v ith the large one, are called &quot;splint bones.&quot; The single digit consists of a moderate-sized proximal (os siiffraginis,oi large pastern), a very short middle (os coronce, or small pastern), and a wide, semi-lunar, ungual phalanx (os pedis, or coffin bone). There is a pair of large nodular sesamoids behind the metacarpo-phalangeal articulation, and a single large transversely-extended sesamoid behind the joint between the second and third phalanx, called the &quot;navicular bone.&quot; The carpal joint, corresponding to the wrist of man, is commonly called the &quot;knee&quot; of the horse, the joint between the metacarpal and the first phalanx the &quot;fetlock,&quot; that between the first and second phalanges the &quot;pastern,&quot; and that between the second and third phalanges the &quot; coffin joint.&quot; In the hinder limb the femur is marked, as in all other known perfssodactyles, by the presence of a &quot; third trochanter,&quot; a flattened process, curving forwards, arising from the outer side of the bone, about one-third of tlie distance from the upper end. The fibula is reduced to a mere styliform rudiment of the upper end. The lower pait is absent or completely fused with the tibia. r lhe cs calci.s has a long and compressed calcaneal process. The astragal us has a large flat articular surface in front for the navicular, and a very small one for the cuboid. The navicular and the external cuneiform bones are very broad and flat. The cuboid is small, and the internal and middle cunei form bones are small and united together. The metapodals and phalanges resemble very closely those of the fore limb, but the principal metatarsal is more laterally compressed at its upper end than is the corresponding metacarpal. YTT _ -&amp;gt;