Part 8

The roots of the cranial nerves are shown on the left side and are designated by number. _I_, Anterior cranial fossa; _II_, middle cranial fossa; _III_, posterior cranial fossa; _a_, ethmoidal fossa; _b_, ethmoidal foramen; _c_, foramen for nasal branch of ophthalmic artery; _d_, orbital wing of sphenoid bone; _e_, optic fossa; _f_, sella turcica; _g_, spheno-occipital crest; _h_, _h′_, dotted line indicating contour of pituitary body; _h″_, slight elevation representing dorsum sellæ; _i_, _k_, grooves for nerves and cavernous sinus; _l_, depression for pyriform lobe of cerebrum; _m_, groove for middle meningeal artery; _n_, depression for pons; _o_, foramen lacerum anterius; _p_, foramen lacerum posterius; _q_, incisura carotica; _q′_, incisura ovalis; _q″_, incisura spinosa; _r_, depression for medulla oblongata; _s_, hypoglossal foramen; _t_, internal auditory meatus; _u_, foramen magnum; _v_, frontal sinus; _w_, zygomatic process of temporal bone; _x_, section of petrous temporal; _y_, section of occipital bone; _z_, crista galli; 1, 1′, 1″, dotted lines indicating position of olfactory tracts and peduncle. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

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.

[Illustration:

FIG. 36.—MEDIAN SECTION OF SKULL OF HORSE WITHOUT THE MANDIBLE.

The septum nasi is removed, but the mucous membrane on the turbinal bones is retained. _a_, _a′_, Superior turbinal bone, dotted line indicating limit between anterior coiled part and posterior uncoiled part; _b_, _b′_, superior turbinal folds, inclosing bars of cartilage; _c_, fold of mucous membrane formed by union of _b_ and _b′_; _d_, _d′_, anterior coiled and posterior uncoiled part of inferior turbinal, dotted line indicating septum between them; _e_, _f_, inferior turbinal folds, former (alar fold) inclosing bar of cartilage; _g_, _h_, _i_, superior, middle, inferior meatus; _o_, _o′_, frontal sinus; partial septum between _o_ and _o′_; _q_, nasal part of frontal sinus (nasal sinus); _r_, lateral mass of ethmoid bone; _s_, sphenoidal sinus; _t_, cranial cavity; _u_, opening made in superior turbinal bone at point where drainage of frontal sinus may be obtained. (After Ellenberger, in Leisering’s Atlas.) ]

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.

[Illustration:

FIG. 37.—CROSS-SECTION OF NASAL REGION OF SKULL OF HORSE; THE SECTION PASSES THROUGH THE ANTERIOR END OF THE FACIAL CREST, AND BETWEEN THE THIRD AND FOURTH CHEEK TEETH.

_a_, Superior, _b_, inferior turbinal bone; _c_, _d_, cavities of _a_ and _b_; _e_, common meatus; _f_, _g_, _h_, superior, middle, inferior meatus; _i_, _k_, passages to cavities of turbinal bones; _l_, naso-lacrimal duct; _m_, infraorbital canal; _n_, anterior end of maxillary sinus; _o_, septal cartilage. (After Ellenberger, in Leisering’s Atlas.) ]

[Illustration:

FIG. 38.—CROSS-SECTION OF NASAL REGION OF SKULL OF HORSE; THE SECTION IS CUT ABOUT HALFWAY BETWEEN THE ORBIT AND THE ANTERIOR END OF THE FACIAL CREST, AND PASSES BETWEEN THE FIFTH AND SIXTH CHEEK TEETH.

_a_, Superior, _b_, inferior turbinal bone; _c_, _d_, cavities of _a_ and _b_; _e_, common meatus; _f_, superior, _g_, middle, _h_, inferior meatus; _i_, placed over ridge in maxillary sinus; _k_, communication between outer and inner (turbinal) part of maxillary sinus; _l_, naso-maxillary opening; _m_, naso-lacrimal canal; _n_, infraorbital canal. (After Ellenberger, in Leisering’s Atlas.) ]

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.

[Illustration:

FIG. 39.—SKULL OF HORSE, LATERAL VIEW WITHOUT MANDIBLE. THE SINUSES ARE OPENED UP.

_a_, Posterior part, _b_ and _c_, anterior part of frontal sinus; _d_, roof of superior meatus; _e_, lateral mass of ethmoid bone; _f_, _f′_, naso-lacrimal duct, exposed in its posterior part; _g_, _h_, posterior and anterior compartments of maxillary sinus (also designated as superior and inferior maxillary sinuses); _i_, septum between _g_ and _h_; _k_, lower limit of upper thin and partly membranous portion of septum; _l_, infraorbital canal; _m_, turbinal part of maxillary sinus; _n_, bullous prominence of inferior turbinal; _o_, orbit; _p_, infraorbital foramen; _q_, continuation of infraorbital canal to premaxilla; _r_, limit of maxillary sinus. (After Ellenberger, in Leisering’s Atlas.) ]

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.

[Illustration:

FIG. 40.—SKULL OF HORSE, DORSAL VIEW, WITH SINUSES EXPOSED BY REMOVAL OF THE OUTER PLATE OF BONE.

_1_, Frontal bone; _2_, nasal bone; _3_, lacrimal bone; _4_, maxilla; _a_, posterior part of frontal sinus; _a′_, middle part of frontal sinus; _b_, anterior (turbinal) part of frontal sinus; _c_, lateral mass of ethmoid bone; _d_, roof of superior meatus; _e_, fronto-maxillary opening; _f_, naso-maxillary opening below plate which forms the anterior margin of _e_; _g_, _h_, posterior and anterior compartments of maxillary sinus—often called the superior and inferior maxillary sinuses; _i_, septum between _g_ and _h_; _k_, orbit; _l_, point at which superior turbinal bone may be perforated to obtain drainage into nasal cavity. (After Ellenberger, in Leisering’s Atlas.) ]

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.

[Illustration:

FIG. 41.—LEFT SCAPULA OF HORSE, EXTERNAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

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.

[Illustration:

FIG. 42.—LEFT SCAPULA OF HORSE, COSTAL SURFACE.—(After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

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.

[Illustration:

FIG. 43.—DISTAL END OF LEFT SCAPULA OF HORSE, END VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

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.

[Illustration:

FIG. 44.—PROXIMAL END OF LEFT HUMERUS OF HORSE, END VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

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.

[Illustration:

FIG. 45.—LEFT HUMERUS OF HORSE, INTERNAL SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

[Illustration:

FIG. 46.—LEFT HUMERUS OF HORSE, EXTERNAL SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

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.

[Illustration:

FIG. 47.—LEFT HUMERUS OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]