Part 64

The =buccinator vein= (V. buccinatoria) extends backward from the external maxillary along the lower border of the depressor labii inferioris and buccinator under cover of the masseter, passes between the ramus of the mandible and the tuber maxillare and is continued as the internal maxillary vein. It has a large fusiform dilatation and is valveless. It receives a large common labial vein from the labial plexus, and is usually connected with the masseteric vein.

[Illustration:

FIG. 463.—DEEPER VESSELS AND NERVES OF HEAD OF HORSE.

The parotid gland, most of the masseter muscle, and a portion of the ramus of the mandible are removed. _a_, Remnant of masseter muscle; _b_, internal pterygoid muscle; _c_, stylo-mandibularis; _d_, _d′_, levator labii superioris proprius (portion removed); _e_, periorbita; _1_, masseteric artery; _1′_, parotid branch; _2_, trunk for anterior (_3_) and posterior (_4_) auricular arteries; _5_, _5′_, superficial temporal artery; _5″_, transverse facial artery; _6_, inferior alveolar (or dental) artery; _6′_, mental continuation of _6_; _7_, buccinator artery; _8_, infraorbital artery; _9_, _11_, jugular vein; _10_, external maxillary vein; _12_, inferior cerebral vein; _13_, facial vein; _14_, angular vein of eye; _15_, dorsal nasal vein; _16_, lateral nasal vein; _17_, superior labial vein; _18_, _19_, inferior labial veins; _20_, labial plexus; _21_, _22_, emergent veins of plexus; _23_, vena reflexa; _24_, trunk of sphenopalatine and infraorbital veins; _25_, palatine vein; _26_, great auricular vein; _27_, _28_, superficial temporal vein; _29_, transverse facial vein; _30_, buccinator vein; _31_, dorsal lingual vein; _32_, inferior alveolar or dental vein; _33_, pterygoid vein; _34_, deep temporal vein; _35_, superior cerebral vein; _36_, external nasal nerve; _37_, anterior nasal nerve; _38_, superior labial nerve; _39_, masseteric nerve; _39′_, end branches of _39_; _40_, buccinator nerve; _41_, pterygoid nerve; _41a_, inferior alveolar or dental nerve; _41b_, mylo-hyoid nerve; _42_, posterior auricular nerve; _43_, auricular branch of vagus; _44_, internal auricular nerve; _45_, auriculo-palpebral nerve (cut); _46_, digastric nerve. (After Leisering’s Atlas.) ]

The =lingual vein= (V. lingualis) is not a satellite of the artery. It is formed at the side of the lingual process of the hyoid bone by the confluence of several veins which come from the substance of the tongue. One or two considerable vessels run partly in the substance of the hyo-glossus, and another in the genio-hyoideus. The vein is at first covered by the mylo-hyoideus, perforates that muscle, runs back along the omo-hyoideus in relation to the submaxillary lymph glands, and joins the external maxillary near the posterior border of the jaw. Near its termination it receives the =sublingual vein=, or the latter may open into the external maxillary directly.

8. The =thyroid vein= (V. thyreoidea) is a large vessel which joins the jugular near the external maxillary vein. It receives anterior thyroid, laryngeal, and pharyngeal radicles, and sometimes a posterior thyroid vein.

9. =Muscular=, =tracheal=, and =œsophageal= veins.

10. The =cephalic vein= (Vena cephalica) enters the jugular near its termination. It will be described with the veins of the thoracic limb.

11. The =inferior cervical vein= (V. cervicalis ascendens) accompanies the artery. It may open into the brachial vein.

THE SINUSES OF THE DURA MATER

These (Sinus duræ matris) are blood-spaces between the meningeal and periosteal layers of the dura mater and are lined with endothelium. In many places the lumen is crossed by fibrous strands. They receive the veins of the brain, communicate with the meningeal and diploic veins, and with veins outside of the cranium; their connections with the latter are by means of small =emissary veins= (Emissaria). They convey the blood directly or indirectly to the jugular veins. Some are paired, others unpaired. They may be divided into dorsal and basilar systems. The =dorsal system= comprises the following:

The =superior longitudinal= or =sagittal sinus= (S. sagittalis superior) is situated in the upper border of the falx cerebri along the internal sagittal crest. It begins at the crista galli and ends at the tentorium osseum by dividing into two transverse sinuses. It receives the superior cerebral veins. Along each side are small pouches (Lacunæ laterales) into which the veins open. The lumen of the sinus is traversed by fibrous bands and is partially divided by a longitudinal septum.

[Illustration:

FIG. 464.—MEDIAN SECTION OF HEAD OF HORSE, UPPER PART WITH SEPTUM NASI REMOVED.

_a_, Lateral mass of ethmoid bone; _b_, superior turbinal; _c_, inferior turbinal; _d_, _d′_, turbinal folds; _e_, frontal sinus; _f_, falx cerebri; _g_, tentorium cerebelli; _h_, medial surface of hemisphere; _i_, cerebellum; _k_, occipital bone; _k′_, occipital condyle; _k″_, paramastoid process; _l_, external auditory meatus; _m_, temporal condyle; _n_, parieto-temporal canal; _1_, branches of ethmoidal artery; _2_, _2′_, branches of sphenopalatine artery; _3_, _3′_, branches of sphenopalatine vein; _4_, branches of ethmoidal nerve; _5_, _5′_, branches of sphenopalatine nerve; _6_, artery of corpus callosum; _7_, superior sagittal sinus; _8_, straight sinus; _9_, vena magna cerebri; _10_, inferior sagittal sinus; _11_, _11′_, transverse sinuses; _12_, superior petrosal sinus; _13_, superior occipital sinus; _14_, superior cerebral vein; _15_, corpus callosum; _16_, fornix. (After Ellenberger, in Leisering’s Atlas.) ]

The =transverse sinuses= (S. transversi), right and left, pass outward in the transverse grooves of the parietal bones, enter the parieto-temporal canals, and are continued by the superior cerebral veins to the superficial temporal veins. The two sinuses are connected by the sinus communicans.

The =inferior longitudinal= or =sagittal sinus= (S. sagittalis inferior) runs backward on the upper surface of the corpus callosum along the concave edge of the falx cerebri and joins the great cerebral vein (of Galen) to form the straight sinus.

The =straight sinus= (S. rectus) passes upward and backward between the cerebral hemispheres and in the tentorium cerebelli and joins the superior sagittal sinus. The point of meeting is the confluence of the sinuses (Confluens sinuum).

The =occipital sinuses= (S. occipitales) lie on either side of the vermis cerebelli. They empty anteriorly into the sinus communicans and communicate behind with the spinal veins.

The =superior petrosal sinuses= (S. petrosi superiores) pass in the tentorium cerebelli to end in the transverse sinuses.

The =basilar system= consists of the following:

The =cavernous sinuses= (S. cavernosi) lie in the inner grooves of the root of the temporal wings of the sphenoid bone at either side of the sella turcica. The two are connected by a wide traverse branch (Sinus intercavernosus) behind and below the posterior part of the pituitary body. Each is continuous in front with the ophthalmic vein and below with the inferior petrosal sinus. The third, sixth, and the ophthalmic and maxillary divisions of the fifth nerve lie along the outer wall of the sinus. The internal carotid artery traverses the sinus and is connected with its fellow by a transverse branch which lies in the corresponding part of the sinus. An oval opening in the floor communicates with the inferior petrosal sinus and transmits the internal carotid artery.

The sinus is not subdivided by strands of fibrous tissue as in man, but a few delicate bands attach the artery to its wall.

The =inferior petrosal sinuses= (S. petrosi inferiores)[172] lie along the borders of the basilar part of the occipital bone, inclosed in the thick dura which closes the foramen lacerum. The anterior part extends about half an inch (ca. 12 mm.) under the temporal wing of the sphenoid. Here communications exist with veins in the pterygo-palatine fossa. The posterior end is bulbous and lies in the condyloid fossa; it communicates with the condyloid vein issuing from the hypoglossal foramen and is drained by the inferior cerebral vein; it also receives an emissary vein from the parieto-temporal canal. The roof of the sinus is perforated by an oval opening at the carotid notch which opens into the cavernous sinus and transmits the internal carotid artery; the latter forms the first bend of its =S=-shaped curve in the petrosal and the second in the cavernous sinus.

The =basilar plexus= (Plexus basilaris) is a venous plexus situated on the upper surface of the basilar part of the occipital bone. It is connected through the hypoglossal foramen with the inferior petrosal sinus and communicates behind with the spinal veins.

THE VEINS OF THE CRANIUM

The =veins of the brain= (Venæ cerebri) do not in general accompany the cerebral arteries. They have very thin walls, no muscular coat, and no valves. They are arranged in two sets, superficial or cortical and deep or central. The superficial veins are more numerous and larger than the arteries. They lie on the surface of the brain in the pia mater and the subarachnoid space.

The =superior cerebral veins= drain the upper and outer part of the cerebral cortex. They receive veins from the medial surfaces of the hemispheres and open into the superior sagittal sinus. Their terminal parts are bulbous and their openings are directed obliquely forward, _i. e._, contrary to the direction of the bloodstream in the sinus.

The =inferior cerebral veins= lie on the inferior and external aspect of the hemispheres. They open into the inferior system of sinuses. The =middle cerebral vein= runs in the lateral fissure (of Sylvius) and opens into the cavernous sinus.

The =deep cerebral veins= issue from the central or ganglionic parts of the brain at the transverse fissure. They converge to form the =great cerebral vein= (of Galen) (V. magna cerebri). This passes upward and backward behind the splenium of the corpus callosum and is continued as the straight sinus to join the sagittal sinus.

The =superior cerebellar veins= ramify on the upper surface of the cerebellum. They open into the superior system of sinuses and the great cerebral vein. The =inferior cerebellar veins= are larger and go chiefly to the basilar plexus. The veins of the medulla and pons end in the inferior system of sinuses.

The =meningeal veins= (Venæ meningeæ) arise in capillary plexuses in the superficial and deep faces of the dura mater. Some end in the sinuses of the dura, others accompany the meningeal arteries.

The =diploic veins= (Venæ diploicæ) are anastomosing channels in the spongy substance of the cranial bones. Their walls are thin, consisting in many places only of the endothelium, and they have no valves. Some open inward into venous sinuses, others into extracranial veins.

SPINAL VEINS

Two =longitudinal spinal veins= or =sinuses= (Sinus columnæ vertebralis) extend along the floor of the vertebral canal, one on either side of the superior common ligament. They are continuous in front with the basilar plexus. They lie in the grooves on the bodies of the vertebræ and are connected by a series of transverse anastomoses which pass between the central part of the bodies of the vertebræ and the superior common ligament or in channels in the bone. They receive veins from the spinal cord, the dura mater, and the bodies of the vertebræ (Venæ basis vertebræ). Through the intervertebral foramina efferent vessels connect with the vertebral, intercostal, lumbar, and lateral sacral veins.

VEINS OF THE THORACIC LIMB

The =brachial vein= (V. brachialis) is a satellite of the extrathoracic part of the brachial artery. It arises at the inner side of the distal end of the shaft of the humerus and passes upward in the arm behind the artery under cover of a layer of fascia and the posterior superficial pectoral muscle. At the shoulder it is ventral to the artery, crosses the anterior border of the first rib, and concurs with its fellow and the two jugulars in the formation of the anterior vena cava. The roots of the vein are somewhat variable, but most often four radial veins in addition to a large oblique branch from the cephalic unite in its formation. Its tributaries correspond in general to the branches of the artery, but a few differences are worthy of notice. The =thoracico-dorsal vein= joins the brachial directly or opens into the deep brachial. The =external thoracic= or =“spur” vein= (V. thoracica externa) is a large vessel which arises in the ventral wall of the abdomen, passes forward (embedded more or less in the panniculus) along the outer border of the posterior deep pectoral muscle, and joins the brachial vein near the first rib. It often communicates with the subscapular vein.

[Illustration:

FIG. 465.—SPINAL VESSELS OF HORSE.

The vertebral canal has been opened by sawing off the arches. The nerve-roots are cut on one side and the spinal cord turned over to right. _1_, Ventral or middle spinal artery; _2_, reinforcing branches from vertebral, intercostal, or lumbar arteries (according to region); _3_, longitudinal vertebral sinuses; _a_, ventral surface of spinal cord; _b_, dura mater (cut); _c_, nerve-roots; _d_, superior common ligament. (After Ellenberger, in Leisering’s Atlas.) ]

The =cephalic vein= (V. cephalica) arises at the inner side of the carpus as the continuation of the internal superficial metacarpal vein. It runs upward on the deep fascia of the forearm at first in the furrow between the flexor carpi internus and the radius. Toward the middle of the forearm it inclines gradually forward on the inner surface of the radius, accompanied by a cutaneous branch of the median nerve, and arrives at the insertion of the biceps. Here it detaches a large branch (Ramus communicans) which passes upward and backward over the inner insertion of the biceps, the posterior radial artery, and the median nerve, and joins the brachial vein. The vein to this point is often termed the internal subcutaneous vein of the forearm (V. cephalica antebrachii). It is continued (as the V. cephalica humeri) in the furrow between the mastoido-humeralis and the anterior superficial pectoral with a branch of the inferior cervical artery, crosses the deep face of the cervical panniculus, and opens into the terminal part of the jugular or the brachial vein. It receives an =accessory cephalic vein= (V. cephalica accessoria), which arises from the carpal network, runs upward along the inner border of the extensor carpi, turns along the lower border of the brachialis, passes under the superficial tendon of the biceps and joins the cephalic.

[Illustration:

FIG. 466.—DISSECTION OF PECTORAL REGION AND ANTERIOR PART OF ABDOMINAL WALL OF HORSE.

_1_, Jugular vein; _2_, loose connective tissue of axillary space; _3_, ascending branch of inferior cervical artery; _4_, descending branch of same; _5_, cephalic vein; _6_, median nerve; _7_, ulnar nerve; _8_, brachial artery; _9_, brachial vein; _10_, external thoracic or “spur” vein; _11_, anterior abdominal artery and vein; _12_, branches of sixth cervical nerve; _13_, cutaneous branch of axillary nerve; _14_, cutaneous branch of musculo-cutaneous nerve; _15_, cervical panniculus; _16_, sterno-cephalicus; _17_, scalenus; _18_, mastoido-humeralis; _19_, anterior superficial pectoral; _20_, posterior superficial pectoral; _21_, anterior deep pectoral; _22_, posterior deep pectoral; _23_, abdominal panniculus; _24_, obliquus abdominis externus; _25_, rectus abdominis; _26_, coraco-brachialis; _27_, tensor fasciæ antibrachii; _a_, prescapular lymph glands; _b_, prepectoral lymph glands; _c_, axillary lymph glands; _d_, cubital lymph glands; _c.x._, xiphoid cartilage. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The deep veins of the forearm are variable. Commonly two =posterior radial veins= (Venæ medianoradiales) accompany the artery of like name, one in front and one behind. A third vein arises by radicles emerging from the proximal part of the deep flexor; it joins the posterior satellite of the posterior radial artery or forms one of the roots of the brachial vein. The =common interosseous vein= joins the posterior satellite. The =anterior radial vein= (V. collateralis radialis) is a satellite of the artery. The =ulnar vein= is usually double at its proximal part, and communicates with the deep brachial vein.

There are three chief =metacarpal veins=. The =internal metacarpal vein= (V. metacarpea volaris superficialis medialis) arises from the volar venous arch above the fetlock. It is the largest vein of the region and lies in front of the large metacarpal artery. It separates from the artery at the proximal end of the metacarpus, passes upward on the inner part of the posterior surface of the carpus, under cover of the superficial layer of the posterior annular ligament, and is continued as the cephalic vein. It communicates at its proximal part with the origin of the posterior radial veins by a short but relatively large branch. The =external metacarpal vein= (V. metacarpea volaris superficialis lateralis) arises from the venous arch above the fetlock and passes upward behind the external border of the suspensory ligament in front of the external branch of the median nerve and accompanied by a small artery. At the proximal end of the metacarpus it is connected with the deep metacarpal vein by two transverse anastomoses which pass across the suspensory ligament. It then passes upward as a satellite of the external volar metacarpal artery and concurs in the origin of the ulnar and posterior radial veins. The =deep metacarpal vein= (V. metacarpea volaris profunda medialis) arises from the venous arch, passes forward between the two branches of the suspensory ligament, and ascends on the posterior surface of the large metacarpal bone. At the proximal end of the latter it communicates with the other metacarpal veins, ascends with the small metacarpal artery, and concurs in forming the radial veins.

The =venous arch= (Arcus venosus volaris), from which the metacarpal veins arise, is situated above the sesamoids of the fetlock between the suspensory ligament and the deep flexor tendon. It is formed by the junction of the two digital veins.

The =digital veins=, internal and external (V. digitalis medialis, lateralis), drain the venous plexuses of the foot. They arise at the upper edge of the lateral cartilages and ascend in front of the corresponding arteries.

It is convenient to recognize the following venous plexuses of the foot, which, however, communicate very freely:

1. The =coronary plexus= encircles the upper part of the foot. It is attached to the terminal part of the extensor tendon, the lateral cartilages, and the bulbs of the plantar cushion.

2. The =dorsal= (or laminal) =plexus= covers the dorsal or wall surface of the third phalanx in the deep layer of the matrix of the wall of the hoof. It forms the circumflex vein of the third phalanx or vein of the distal border of the third phalanx, which corresponds to the artery of like name.

3. The =volar plexus= is in the deep layer of the matrix of the sole of the hoof and on the deep surface of the lateral cartilages. It communicates around the inferior border of the third phalanx with the dorsal plexus and through the lateral cartilages with the coronary plexus.

The deep vein of the third phalanx accompanies the terminal part of the digital artery. It drains the =intraosseous plexus=.

THE POSTERIOR VENA CAVA (Figs. 270, 450)

The =posterior vena cava= (V. cava caudalis) returns almost all of the blood from the abdomen, pelvis, and pelvic limbs. It is formed by the confluence of the right and left common iliac veins at the fifth lumbar vertebra, above the terminal part of the aorta and chiefly to the right of the median plane.

The mode of origin is variable. In some cases there is a common trunk formed by the union of the two internal iliac veins so that the arrangement resembles the termination of the aorta. In other cases the internal iliac vein does not exist.

It passes forward on the ventral face of the psoas minor to the right of the abdominal aorta. At the last thoracic vertebra it separates from the aorta and runs forward between the right crus of the diaphragm and the pancreas till it reaches the liver. Here it inclines downward along the inner border of the right lobe and the parietal surface of the liver, largely embedded in the gland substance, and passes through the foramen venæ cavæ of the diaphragm. It then runs forward and somewhat downward between the mediastinal lobe and the main mass of the right lung at the upper margin of a special fold of the right pleura, accompanied by the right phrenic nerve, and opens into the posterior part of the right atrium. It receives the following tributaries:

1. The =lumbar veins= (Vv. lumbales) correspond to the arteries. Five pairs usually empty into the vena cava. Sometimes the corresponding veins of opposite sides unite to form a common trunk. The first communicate with the vena azygos.

2. The =spermatic veins= (Vv. spermaticæ internæ) (Fig. 450) accompany the arteries of like name. In the spermatic cord they form the pampiniform plexus about the artery and nerves. Their termination is variable. The right one commonly joins the vena cava near the renal vein, often by a common trunk with the left one. Frequently the left vein joins the left renal, and sometimes the right one ends similarly.

2_a_. The =utero-ovarian veins= are much larger than the preceding and are satellites of the arteries in the broad ligaments. The ovarian branch is plexiform near the ovary. The uterine branches form a rich plexus in the wall of the uterus. The trunk is very short.

3. The =renal veins= (Vv. renales), satellites of the arteries, are of large caliber and thin-walled. The right vein passes inward and backward on the ventral face of the kidney between the artery in front and the ureter behind. It joins the vena cava above the right adrenal. The left vein is somewhat longer. It passes inward at first like the right one, then bends around the posterior end of the adrenal, crosses the origin of the renal artery and opens into the vena cava a little further back than the right one. They receive veins from the adrenals, but some adrenal veins open directly into the vena cava.

4. The =hepatic veins= (Vv. hepaticæ) return the blood from the liver, and open into the vena cava as it lies in the groove in the liver. Three or four large vessels open into the vena cava just before it leaves the liver, and numerous small ones discharge into its embedded part.

5. The =phrenic veins= (Vv. phrenicæ), two or three in number, return the blood from the diaphragm. They are very large in comparison with the arteries, and join the vena cava as it lies in the caval opening.

In some cases there is a small middle sacral vein (V. sacralis media), a satellite of the artery. It opens into the angle of junction of the common iliac veins or into one of the latter.

THE PORTAL VEIN

The =portal vein= (V. portæ) is a large trunk which returns the blood carried to the viscera by the gastric, splenic, and mesenteric arteries. Its peripheral tributaries correspond closely with the branches of the arteries, but the terminal trunks do not. The vein is formed behind the pancreas and below the posterior vena cava by the confluence of the =anterior= and =posterior mesenteric= and =splenic veins=. It passes forward, traverses the posterior part of the pancreas very obliquely, inclines ventrally and a little to the right, and reaches the portal fissure of the liver. Here it divides into three branches which enter the liver and ramify in the substance of the gland like an artery, terminating in the lobular capillaries. From the lobules the blood passes into the hepatic veins and through these to the posterior vena cava. Thus the blood which is distributed to the stomach, nearly the entire intestinal tract, the pancreas, and the spleen, passes through two sets of capillaries prior to its return to the heart, viz., the capillaries of these viscera and of the liver.

1. The =anterior mesenteric vein= (V. mesenterica cranialis) is the largest of the portal radicles. It is situated to the right of the artery of like name, and its tributaries correspond in general to the branches of the artery. Usually a single colic vein corresponds to the two arteries of the right portions of the colon.

2. The =posterior mesenteric vein= (V. mesenterica caudalis) is the smallest of the radicles of the portal. It accompanies the artery in the colic mesentery and its rectal branches anastomose with those of the internal pudic vein.