A text-book of veterinary anatomy

The roof and lateral walls are marked by digital impressions and vascular grooves.

The inferior wall or floor (Basis cranii interna) may be regarded as forming three fossæ. The anterior fossa (Fossa cranii anterior) supports the frontal and olfactory parts of the cerebrum. It is formed chiefly by the presphenoid, and lies at a higher level than the middle fossa. In front the fossa is divided medially by the crista galli, lateral to which are the deep ethmoidal or olfactory fossæ for the olfactory lobes. The ethmoidal or internal orbital foramen perforates the cranial wall at the outer side of these fossæ. Further back the central part of the surface is slightly elevated, and is flanked by shallow depressions which support the olfactory peduncles. Posteriorly is a bony shelf which covers the entrance to the optic foramina; the edge of this shelf and the posterior borders of the orbital wings of the sphenoid may be taken as the line of demarcation between the anterior and middle fossæ. The middle fossa (Fossa cranii media) is the widest part of the cavity. It extends backward to the internal spheno-occipital and petrosal crests, thus corresponding to the postsphenoid. In its middle is a small fossa, the sella turcica, in which the pituitary body, or hypophysis cerebri, is situated. On either side are two grooves for nerves; the inner one transmits the ophthalmic, third, and sixth nerves to the foramen lacerum orbitale; the outer one leads to the foramen rotundum, and lodges the maxillary nerve. External to the grooves is a depression for the pyriform lobe of the cerebrum. The posterior fossa (Fossa cranii posterior) corresponds to the basilar part of the occipital bone. It contains the medulla, pons, and cerebellum. In front is a median depression (Fossa pontis) for the pons. The surface behind this is concave transversely and slopes gently downward to the foramen magnum; it supports the medulla. On either side are the foramen lacerum basis cranii and the hypoglossal foramen.

The anterior or nasal wall is formed by the cribriform plate of the ethmoid, which separates the cranium from the nasal cavity. It is perforated by numerous foramina for the passage of the olfactory nerve-bundles.

THE NASAL CAVITY

The nasal cavity (Cavum nasi) is a longitudinal passage which extends through the upper part of the face. It is divided into right and left halves by a median septum nasi. The lateral walls are formed by the maxilla, premaxilla, and the perpendicular part of the palate bones. Attached to them are the turbinal bones, which subdivide each nasal fossa into three meatuses (Meatus nasi). This wall is crossed obliquely by the canal and groove for the naso-lacrimal duct, and its posterior part is perforated by the sphenopalatine foramen. The superior wall or roof is formed by the frontal and nasal bones. It is concave from side to side, and nearly straight longitudinally, except in the posterior part, where it curves downward. It presents a median elevation, the nasal crest. The inferior wall or floor is formed by the palatine processes of the premaxillæ and maxillæ, and the horizontal parts of the palate bones. It is wider but considerably shorter than the roof. It is concave transversely, and nearly horizontal from before backward, except in the posterior third, where there is a slight declivity. The anterior part presents a median groove for the cartilaginous septum, and a furrow for the organ of Jacobson on either side. On either side of the palatine processes of the premaxillæ is the palatine cleft.

The septum nasi is incomplete in the macerated skull. It is formed by the perpendicular plate of the ethmoid behind, and the vomer below. In the fresh state it is completed by a plate of cartilage.

The superior meatus (Meatus nasi superior) is a narrow passage between the roof and the superior turbinal bone. It ends at the cribriform plate of the ethmoid. The middle meatus (Meatus nasi medius) is the space between the two turbinal bones. In its posterior part is the very narrow opening into the maxillary sinus. The inferior meatus (Meatus nasi inferior) is the channel along the floor which is overhung by the inferior turbinal bone. It is much the largest and is the direct path between the anterior and posterior nares.

The external aperture is bounded by the nasal bones and the premaxillæ.

The posterior extremity or fundus is separated from the cranial cavity by the cribriform plate of the ethmoid, and is largely occupied by the lateral masses of that bone.

THE PARANASAL SINUSES

Connected directly or indirectly with the nasal cavity, of which they are diverticula, are four pairs of air-sinuses (Sinus paranasales), viz., maxillary, frontal, sphenopalatine, and ethmoidal.

The maxillary sinus (Sinus maxillaris) is the largest. Its external wall is formed by the maxilla, the lacrimal, and the malar. It is bounded internally by the maxilla, the inferior turbinal, and the lateral mass of the ethmoid bone. It extends backward to a transverse plane in front of the root of the supraorbital process, and its anterior limit is indicated approximately by a line drawn from the anterior end of the facial crest to the infraorbital foramen. Its upper boundary corresponds to a line drawn backward from the supraorbital foramen parallel to the facial crest. The floor is formed by the alveolar part of the maxilla; it is very irregular and is crossed by bony plates running in various directions. The last three cheek teeth project up into the cavity to an extent which varies with age; they are covered by a thin plate of bone. The cavity is divided into anterior and posterior parts by an oblique septum. The outer margin of the septum is commonly about one and a half to two inches (ca. 3.5 to 5 cm.) from the anterior end of the facial crest; from here it is directed inward, backward, and upward. The upper part of the septum (formed by the posterior end of the inferior turbinal bone) is very delicate and usually cribriform.

The position of the septum is quite variable. It is not rare to find it further forward, and in some cases it is further back than is stated above. In the recent state, i. e., when covered by the mucous membrane on both surfaces, it is nearly always complete, but in very exceptional cases there is an opening of variable size in the upper part.

The anterior compartment, often called the inferior maxillary sinus, is partially divided by the infraorbital canal into an external maxillary part and an internal smaller turbinal part. The latter communicates with the middle meatus by a very narrow slit situated at its highest part. The posterior compartment, often called the superior maxillary sinus, is also crossed by the infraorbital canal, internal to which it opens freely into the sphenopalatine sinus. It communicates dorsally with the frontal sinus through the large oval fronto-maxillary opening, situated at the level of the osseous lacrimal canal and the corresponding part of the inner wall of the orbit; the orifice is commonly about one and a half to two inches (ca. 4 to 5 cm.) long and an inch or more (2 to 3 cm.) wide. Just in front of this, and covered by a thin plate, is the narrow naso-maxillary fissure (Aditus naso-maxillaris), by which the sinus opens into the posterior part of the middle meatus.

The foregoing statements refer to the arrangement in the average adult animal. In the foal the cavity (with the exception of its turbinal part) is largely occupied by the developing teeth. In horses five to six years of age the maxillary part of the sinus is still filled up to a large degree by the embedded parts of the teeth. As the teeth are extruded to compensate the wear, more and more of the cavity becomes free, until in old age only the short roots project up in the floor, covered by a layer of bone. Other facts in this connection will be given in the description of the teeth. In exceptional cases the posterior part of the inferior turbinal is smaller than usual and leaves a considerable interval, through which the maxillary sinus communicates with the nasal cavity.

The frontal sinus (Sinus concho-frontalis) consists of frontal and turbinal parts. The frontal part is bounded chiefly by the two plates of the frontal bone, but its floor is formed in part by the lateral mass of the ethmoid. It extends forward to a plane through the anterior margins of the orbits, backward to one through the temporal condyles, and outward into the root of the supraorbital process. It is separated from the sinus of the opposite side by a complete septum. It is partially subdivided by a number of bony plates. The turbinal part is situated in the posterior part of the superior turbinal bone, roofed in by the nasal and lacrimal bones. It extends forward to a transverse plane about half-way between the anterior margin of the orbit and the end of the facial crest. Behind it is in free communication with the frontal part over the lateral mass of the ethmoid. It is separated from the nasal cavity by the thin turbinal plate. The frontal and maxillary sinuses communicate through the large opening described above.

The sphenopalatine sinus (Sinus sphenopalatinus) consists of two parts which communicate under the lateral mass of the ethmoid. The sphenoidal (posterior) part is excavated in the body of the presphenoid. The palatine (anterior) part is between the two plates of the perpendicular part of the palate bone, under the lateral mass of the ethmoid; it communicates freely with the maxillary sinus. The septum between the right and left sinuses is not usually median in the sphenoidal part.

In about one-third of the cases (according to Paulli) the sphenoidal and palatine parts are separated by a transverse septum, and the sphenoidal part then communicates only with the lower ethmoidal meatuses.

The term ethmoidal sinus is often applied to the cavity of the largest ethmoturbinal. It communicates with the maxillary sinus.

The Bones of the Thoracic Limb the scapula

The scapula is a flat bone, situated on the anterior part of the lateral wall of the thorax, and extending obliquely from the vertebral end of the seventh or eighth rib to the sternal end of the first rib. It is curved slightly and slopes outward in adaptation to the form of the thoracic wall. It is triangular in outline, and has two surfaces, three borders, and three angles.

The external surface or dorsum (Facies lateralis s. dorsalis) is divided into two fossæ by the spine (Spina scapulæ), which extends from the vertebral border to the neck of the bone, where it gradually subsides. The free edge of the spine is thick, rough, and in great part subcutaneous. A little above its middle is a variable prominence, the tubercle of the spine (Tuber spinæ), to which the trapezius muscle is attached. The supraspinous fossa (Fossa supraspinata) is situated in front of the spine, and the infraspinous fossa (Fossa infraspinata) behind it. The former is much the smaller of the two; it is smooth and is occupied by the supraspinatus muscle. The infraspinous fossa lodges the infraspinatus muscle; it is wide and smooth in its upper part, narrower below, where it is marked by several rough lines for the attachment of the teres minor muscle; near the neck is the nutrient foramen, and a little lower is a vascular groove.

The costal surface or venter (Facies costalis) is hollowed in its length by the subscapular fossa (Fossa subscapularis); this occupies nearly the whole of the lower part of the surface, but is pointed above and separates two rough triangular areas (Facies serrata), to which the serratus magnus is attached. In the lower third there is a vascular furrow with several branches.

The anterior or cervical border (Margo cranialis) is convex and rough above, concave and smooth below.

The posterior or dorsal border (Margo caudalis) is slightly concave. It is thick and rough in its upper third, thin in its middle, and thickens again below.

The superior or vertebral border (Margo dorsalis s. basis) carries the scapular cartilage (Cartilago scapulæ). In the young subject the edge of the bone is thick, and is pitted by impressions into which the cartilage fits. The cartilage is the unossified part of the fœtal scapula. Its lower edge fits the depressions and elevations of the bone. It thins out toward the free edge, which is convex and lies alongside of the upper parts of the vertebral spines. In front it continues the line of the scapula, but behind it forms a rounded projection. The lower part of the cartilage undergoes more or less ossification, so that the vertebral border of the bone in old subjects is thin, irregular, and porous.

The anterior or cervical angle (Angulus cranialis) is at the junction of the anterior and vertebral borders and lies opposite to the second thoracic spine. It is relatively thin and is about a right angle.

The posterior or dorsal angle (Angulus caudalis) is thick and rough; its position can be determined readily in the living animal.

The inferior or articular angle (Angulus glenoidalis) is joined to the body of the bone by the neck of the scapula (Collum scapulæ). It is enlarged, especially in the sagittal direction. It bears the glenoid cavity (Cavitas glenoidalis) for articulation with the head of the humerus. The cavity is oval in outline, and its margin is cut into in front by the glenoid notch (Incisura glenoidalis), and is rounded off externally; just above its postero-external part is a tubercle to which a tendon of the teres minor is attached. The bicipital tuberosity or tuber scapulæ is the large rough prominence in front, to which the tendon of origin of the biceps brachii is attached; projecting from its inner side is the small coracoid process (Processus coracoideus), from which the coraco-brachialis muscle arises.

Development.—The scapula has four centers of ossification, viz., one each for the body of the bone, the bicipital and coracoid processes, the anterior part of the glenoid cavity, and the tuber spinæ. The last ossifies after birth and fuses with the spine about the third year. The bicipital tuberosity and coracoid fuse with the body of the bone about the end of the first year.

In old subjects the spongy substance disappears at the middle part of the fossæ, so that the bone consists here of a thin layer of compact substance. Considerable ossification of the cartilage is usual, the borders become much rougher, the muscular lines are more pronounced, and a medullary cavity may appear in the neck. Much variation occurs in dimensions and slope. The average ratio between the length and breadth (scapular index) is about 1 ∶ 0.5, but in many cases the base is relatively wider. The inclination on a horizontal plane varies from 50 to 65 degrees. Exceptionally the coracoid process reaches a length of an inch or more (2½ to 3 cm.), and the chief nutrient foramen may be on the posterior border or in the subscapular fossa.

THE HUMERUS

The humerus is a long bone which extends from the shoulder above, where it articulates with the scapula, to the elbow below and behind, where it articulates with the radius and ulna. It is directed obliquely downward and backward, forming an angle of about 55 degrees with a horizontal plane. It may be divided into a shaft and two extremities.

The shaft or body (Corpus humeri) is irregularly cylindrical and has a twisted appearance. It may be regarded as having four surfaces. The external surface is smooth and is spirally curved, forming the musculo-spiral groove (Sulcus musculi brachialis), which contains the brachialis muscle; the groove is continuous with the posterior surface above and winds around toward the front below. The internal surface is nearly straight in its length, rounded from side to side, and blends with the anterior and posterior surfaces. Just above its middle is the internal or teres tubercle (Tuberositas teres), to which the tendon of the latissimus dorsi and teres major muscles is attached. The nutrient foramen is in the lower third of this surface. The anterior surface is triangular, wide and smooth above, narrow and roughened below. It is separated from the external surface by a distinct border, which bears on its upper part the deltoid tuberosity (Tuberositas deltoidea). From the latter a rough line curves upward and backward to the outer surface of the neck, and gives origin to the external head of the triceps muscle. Below the tuberosity the border inclines forward, becomes less salient, and ends at the coronoid fossa. The posterior surface is rounded from side to side and smooth.

The proximal extremity consists of the head, neck, two tuberosities, and the bicipital groove. The head (Caput humeri) presents an almost circular convex articular surface, which is about twice as extensive as the glenoid cavity of the scapula, with which it articulates. In front of the head is a fossa, in which are several foramina. The neck (Collum humeri) is well defined behind, but is practically absent elsewhere. The external tuberosity (Tuberculum majus) is placed antero-externally, and consists of two parts; the anterior part forms the outer boundary of the bicipital groove and gives attachment to the external branch of the supraspinatus muscle; the posterior part gives attachment to the short insertion of the infraspinatus, while its outer surface is coated with cartilage, over which the chief tendon of the same muscle passes to be inserted into a triangular facet on the outer aspect of the anterior part. The internal tuberosity (Tuberculum minus) is less salient, and consists of anterior and posterior parts; the anterior part forms the inner boundary of the bicipital groove, and furnishes insertion to the inner branch of the supraspinatus above, and the posterior deep pectoral muscle below; the posterior part gives attachment to the subscapularis muscle. The bicipital or intertubercular groove (Sulcus intertubercularis) is situated in front; it is bounded by the anterior parts of the tuberosities, and is subdivided by an intermediate ridge. The groove is covered in the fresh state by cartilage, and lodges the tendon of origin of the biceps brachii muscle. Just below the intermediate ridge is a small fossa in which several foramina open.

The distal extremity has an oblique surface for articulation with the radius and ulna, which consists of two condyles of very unequal size, separated by a ridge. The internal condyle (Condylus medialis) is much the larger, and is crossed by a sagittal groove, on the anterior part of which there is usually a synovial fossa. Posteriorly the groove extends upward considerably above the rest of the articular surface and reaches the olecranon fossa, and this part articulates with the semilunar notch of the ulna. The external condyle (Condylus lateralis) is much smaller and is placed somewhat lower and further back, giving the extremity an oblique appearance; it is marked by a wide shallow groove. The coronoid fossa (Fossa coronoidea) is situated in front above the groove on the internal condyle; it furnishes origin to part of the extensor carpi, and external to it is a rough depression from which the anterior or common extensor of the digit arises. Behind and above the condyles are two thick ridges, the epicondyles. The internal or flexor epicondyle (Epicondylus medialis s. flexorius) is the more salient; it furnishes origin to flexor muscles of the carpus and digit, and presents internally a tubercle for the attachment of the internal lateral ligament of the elbow joint. The external or extensor epicondyle (Epicondylus lateralis s. extensorius) bears externally the external supracondyloid crest (Crista condyloidea lateralis), which forms here the outer boundary of the musculo-spiral groove, and gives origin to the extensor carpi. Below this is a rough excavation in which the external lateral ligament is attached. The lower border of the epicondyle gives attachment to the flexor carpi externus. Between the epicondyles is the deep olecranon fossa (Fossa olecrani).

Development.—The humerus ossifies from six centers, viz., three primary centers for the shaft and epiphyses, and three secondary centers for the external tuberosity, the deltoid tuberosity, and the internal condyle. The proximal end fuses with the shaft at about three and one-half years, the distal at about one and a half years of age.

THE RADIUS

The radius is much the larger of the two bones of the forearm in the horse. It extends in a vertical direction from the elbow, where it articulates with the humerus, to the carpus below. It is gently curved, the convexity being anterior. It consists of a shaft and two extremities.

The shaft (Corpus radii) is curved in its length, somewhat flattened from before backward, and expanded at its ends. It presents for description two surfaces and two borders. The anterior surface (Facies dorsalis) is smooth, slightly convex in its length, and rounded from side to side. The posterior surface (Facies volaris) is correspondingly concave in its length and is flattened in the transverse direction. At its upper part there is a smooth shallow groove, which concurs with the ulna in the formation of the interosseous space of the forearm; the nutrient foramen is in the lower part of this groove. Below this there is in the young subject a narrow, rough, triangular area to which the ulna is attached by an interosseous ligament; in the adult the two bones are fused here. A variable rough elevation below the middle of the surface and close to the internal border gives attachment to the superior check ligament. The internal border (Margo medialis) is slightly concave in its length and is largely subcutaneous; at its proximal end there is a smooth area on which the tendon of insertion of the brachialis muscle lies, and a small rough area just below gives attachment to that muscle and the long internal lateral ligament of the elbow joint. The external border (Margo lateralis) is more strongly curved, but presents no special features.

The proximal extremity or head (Capitulum radii) is flattened from before backward and wide transversely. It presents an articular surface (Fovea capituli) which corresponds to that on the distal end of the humerus; it is crossed by a central sagittal ridge, which has a synovial fossa on its posterior part, and ends in front at a prominent lip, the coronoid process (Processus coronoideus). Just below the posterior border there are two concave facets for articulation with the ulna, and between these and the interosseous space is a quadrilateral rough area at which the two bones are united by an interosseous ligament. At the inner side of the anterior surface is the bicipital tuberosity (Tuberositas radii), into which the biceps tendon is inserted. The internal tuberosity is continuous with the preceding eminence, and furnishes attachment to the short part of the internal lateral ligament. The external tuberosity is more salient; it gives attachment to the external lateral ligament and to the anterior and lateral extensor muscles of the digit.

The distal extremity is also compressed from before backward. It presents the carpal articular surface (Facies articularis carpea) which consists of three parts. The inner facet is the largest, is quadrilateral, concavo-convex from before backward, and articulates with the radial carpal bone (or scaphoid); the middle one is somewhat similar in form but smaller, and articulates with the intermediate carpal bone (or semilunar); the outer facet is smaller, is convex, and articulates below with the ulnar carpal (or cuneiform) and behind with the accessory carpal (or pisiform). The anterior surface presents three grooves, separated by ridges. The middle one is vertical and gives passage to the tendon of the extensor carpi radialis; the outer one is similar and contains the tendon of the anterior extensor of the digit; the inner one is small and oblique and lodges the tendon of the extensor carpi obliquus. The posterior aspect is crossed by a rough ridge, below which are three depressions. On either side is a tuberosity (Tuberculum ligamenti) to which the lateral ligament is attached. The outer one is marked by a small vertical groove for the passage of the lateral extensor tendon.

Development.—The radius ossifies from four centers, viz., one each for the shaft, the two extremities, and the outer part of the distal end; the last is morphologically the distal end of the ulna which has fused with the radius, and the line of fusion is often indicated by a distinct groove on the carpal articular surface. The proximal extremity unites with the shaft at about one and a half years, the distal end at about three and a half years.

THE ULNA

The ulna of the horse is a reduced long bone situated behind the radius, with which it is partially fused in the adult.

The shaft (Corpus ulnæ) is three-sided and tapers to a point below. The anterior surface (Facies dorsalis) is applied to the posterior surface of the radius, and below the interosseous space the two bones are fused in the adult. The surface which enters into the formation of the space is smooth and usually presents a small nutrient foramen, directed upward. Above the space it is rough and is attached to the radius by an interosseous ligament which is usually permanent. The internal surface (Facies medialis) is smooth and slightly concave. The external surface (Facies lateralis) is flattened. The internal and external borders are thin and sharp, except at the interosseous space. The posterior border is slightly concave in its length and is rounded. The lower end is pointed and is usually a little below the middle of the radius. It is commonly continued by a fibrous cord to the distal external tuberosity of the radius, but this band may be replaced in part or entirely by bone.

The proximal extremity is the major part of the bone. It projects upward and somewhat backward behind the lower end of the humerus, and forms a lever arm for the extensor muscles of the elbow. The internal surface is concave and smooth. The external surface is convex and is roughened above. The anterior border bears on its middle a pointed projection, the processus anconæus or “beak,” which overhangs the semilunar notch or sigmoid cavity (Incisura semilunaris). The latter is triangular in outline, concave from above downward, and articulates with the humerus; in the middle of its lower part is an extensive synovial fossa. Just below the notch are two convex facets which articulate with those on the posterior aspect of the proximal end of the radius. The posterior border is nearly straight, and is thick and rounded. The free end or summit is a rough tuberosity, the olecranon, which gives attachment to the triceps brachii and other muscles.

The primitive distal extremity has, as previously stated, fused with the radius.

Development.—The ulna ossifies from three centers, of which one is for the main part of the bone, one for the olecranon, and one for the distal end. The cartilaginous embryonic ulna extends the entire length of the forearm. The lower part of the shaft is usually reduced to a small fibrous band or may disappear entirely; in some cases a variable remnant of it ossifies. The distal extremity fuses early with the radius. The olecranon unites with the rest of the bone at three to three and a half years. A medullary canal appears to occur constantly in the adult—contrary to the statements of some authors.

THE CARPUS

The carpus of the horse consists of seven or eight bones (Ossa carpi) arranged in two rows, proximal or antibrachial, and distal or metacarpal. The (abbreviated) names and relative positions of the bones of the left carpus as seen from in front are indicated below.

The Radial Carpal Bone

The radial carpal bone (Os carpi radiale, scaphoid) is the largest bone of the upper row; it is somewhat compressed laterally, and is clearly six-sided. The superior or proximal surface is convex in front, concave behind, and articulates with the inner facet on the distal end of the radius. The inferior or distal surface is also convex in front and concave behind; it articulates with the second and third carpal bones. The external surface bears upper and lower facets on its anterior part for articulation with the intermediate; between and behind these it is excavated and rough. The anterior or dorsal surface is rough and slightly convex. The internal surface and the posterior or volar surface are rough and tuberculate.

The Intermediate Carpal Bone

The intermediate carpal bone (Os carpi intermedium, semilunar, lunar) is somewhat wedge-shaped, wider in front than behind. The superior or proximal surface is saddle-shaped, and articulates with the middle facet on the distal end of the radius. The inferior or distal surface is smaller, convex in front, concave behind, and articulates with the third and fourth carpal bones. The internal surface has upper and lower facets for articulation with the radial carpal, and between these it is excavated and rough. The external surface is similar to the preceding and articulates with the ulnar carpal. The anterior or dorsal surface is rough and slightly convex. The posterior or volar surface bears a tuberosity on its lower part.

The Ulnar Carpal Bone

The ulnar carpal bone (Os carpi ulnare, cuneiform) is the smallest and most irregular bone of the upper row. The superior or proximal surface is concave, and fits the lower part of the outer facet on the distal end of the radius. The inferior or distal surface is oblique and undulating for articulation with the fourth carpal bone. The internal surface has upper and lower facets for articulation with the intermediate. The anterior or dorsal and external surfaces are continuous, convex, and rough. The posterior or volar surface is oblique, and bears a concave facet for articulation with the accessory carpal bone; below this is a tubercle.

The Accessory Carpal Bone

The accessory carpal bone (Os carpi accessorium, pisiform) is situated behind the ulnar carpal bone and the outer part of the distal end of the ulna. It is discoid and presents for description two surfaces and a circumference. The internal surface is concave and forms the outer wall of the carpal groove. The external surface is convex and rough; a smooth groove for the outer tendon of the flexor carpi externus crosses its anterior part obliquely downward and slightly forward. The anterior border bears two facets; the upper one is concave and articulates with the back of the outer facet on the distal end of the radius; the lower one is convex and articulates with the ulnar carpal bone. The remainder of the circumference is rounded and rough.

The accessory does not directly bear weight, and may be regarded as a sesamoid bone interposed in the course of the tendons of the middle and external flexors of the carpus, which it enables to act at a mechanical advantage. The posterior border furnishes attachment to the transverse carpal ligament, which completes the carpal canal for the flexors of the digit.

The first carpal bone (Os carpale primum, trapezium) is a small inconstant bone, commonly about the size and shape of a pea, which is situated in the lower part of the internal lateral ligament behind the second carpal bone.

This bone appears to be absent on both sides in about half of the cases; in a good many subjects it is present on one side only. In size it varies from a minute nodule to a discoid mass 10 to 12 mm. in length. In exceptional cases it articulates with both the second carpal and the second (inner) metacarpal bone, in other cases with the former only, but in the majority of specimens no articular facet is present.

The Second Carpal Bone

The second carpal bone (Os carpale secundum, trapezoid) is the smallest constant bone of the lower row, and is irregularly hemispherical in shape. The superior or proximal surface is a convex facet which is continued upon the posterior or volar surface and articulates with the posterior part of the radial carpal. The external surface faces obliquely outward and forward, and bears three facets for articulation with the third carpal bone. The anterior or dorsal and the internal surface bear a tuberosity to which the lateral ligament is attached. The inferior or distal surface is articular and consists of a large flattened facet for the inner (second) metacarpal bone, and a small one for the large (third) metacarpal bone. Some specimens have a small facet on the lower part of the posterior surface which articulates with the first carpal bone.