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At a later period that portion of the vein which lay medial to the otic vesicle and the 7th, 8th, 9th, 10th, and 11th cerebral nerves has disappeared and has been replaced by a new channel, which is placed lateral to the otic vesicle and the 7th, 8th, 9th, 10th, and 11th cerebral nerves. The new channel extends from the semilunar ganglion of the trigeminal nerve to the upper end of the extra-cranial part of the anterior cardinal vein, that is, to the upper end of the internal jugular vein. This secondary vessel follows the course of the facial nerve and in part of its extent it is extra-cranial.
In the human embryo the stage in which the primitive stem vein lies to the medial side of the otic vesicle and the 7th, 8th, 9th, 10th, and 11th nerves does not seem to occur. At all events in a 3 mm. embryo in the Edinburgh University Collection, and in a 4 mm. embryo in the Collection of the Carnegie Institution of Washington (Streeter), the second stage, in which the posterior part of the cranial portion of the primitive vein lies lateral to the otic vesicle and the 7th, 8th, 9th, 10th, and 11th cerebral nerves, is already present and there is no indication of a vein medial to the otic vesicle.
In the human embryo, therefore, the primitive venous stem in the head region, on each side, consists of an anterior portion medial to the semilunar ganglion of the trigeminal nerve, and a posterior portion lateral to the otic vesicle and the 7th, 8th, 9th, 10th, and 11th cerebral nerves; for this stem Streeter has suggested the convenient term "primary head vein" (Fig. 834).
The most anterior tributaries of the primary head vein are derived from the region of the otic vesicle and remnants of them become converted into the ophthalmic vein, but in addition to the anterior tributaries there are numerous dorsal or upper tributaries which become arranged in three main groups: an anterior plexus associated with the regions of the fore-brain and the
mid-brain; a middle plexus associated with the cerebellar region of the hind-brain; and a posterior plexus associated with the region of the medulla oblongata (Fig. 834).
The vessels of each plexus tend to run together as they approach the stem of the primary head vein and so three stems are formed, the anterior, middle, and posterior; they were described by Mall in 1904 (Fig. 834). This condition persists until the embryo attains a length of about 18 mm. when an anastomosis forms, above the otic vesicle, between the stems from the middle and posterior plexuses (Fig. 835), and at the same time that part of the primary head vein which lay lateral to the otic vesicle and the 7th, 8th, 9th, 10th, and 11th cerebral nerves disappears.
By the time the embryo has become 21 mm. long the anastomosis mentioned has become very important, and a separation has occurred between the lower and the upper portions of the anterior stem tributary; therefore, at that period, the blood from the eye region flows backwards to the anterior end of the primary head vein, then upwards along what was the lower part of the middle stem tributary, next backwards along the anastomosis above the otic region to the posterior stem tributary, down which it passes to the upper part of the extra-cranial portion of the anterior cardinal vein which has now become the internal jugular vein (Fig. 835). At this time the blood from the anterior and middle plexuses reaches the supra-otic anastomosis through the upper or dorsal part of the middle stem tributary (Fig. 835).
In the meantime the subdural and subarachnoid spaces have been forming, and with the formation of those spaces the main parts of the venous plexuses are carried away from the brain, with the membrane which will be transformed into the dura mater, but in part the plexuses still retain their connections with the piamater on the brain surface, and they afterwards establish new connections with the veins which appear on the surfaces of the developing cerebral hemispheres. In the meantime on each side the upper or dorsal tributaries of the anterior and the middle plexus anastomose together (Fig. 835).
When the cerebral hemispheres increase in size the dura-matral tissue is compressed between them, and between the cerebral hemispheres above and the mid- and hind-brain below, in the form of folds (Figs. 836 A and B). As the folds are formed the conjoined anterior and middle plexuses of one side are carried into relation with those of the opposite side in the median plane of the head; there the vessels of opposite sides unite together and are finally resolved into the superior and inferior sagittal sinuses and the straight sinus (Figs. 837 A and B), and at the same
Chorioid plexus of lateral ventricle
Chorioid plexus of third ventricle
FIG. 837 A.-DIAGRAM OF A TRANSVERSE SECTION OF THE BRAIN SHOWING
time some of the smaller vessels of the plexuses which retain their connection with the piamater are transformed into the internal cerebral veins and the great cerebral vein; and from some of the lower or ventral tributaries, on each side, is produced the inferior cerebral vein of the embryo which probably bethe vena
basalis of the adult (Figs. 837 B, 838). Whilst the changes last mentioned are taking place the growth of the hemispheres forces the upper part of the middle stem tributary on each side backwards and then downwards until it becomes the hori zontal part of the transverse sinus (Fig. 838), whilst the anastomosis above the otic region and the posterior stem tributary are converted into the sigmoid portion of the transverse sinus (Fig. 838). By the time this stage is attained the anterior portion of the primary head vein which lies to the medial side of the semilunar gang. lion has become the cavernous sinus, and the lower or ventral part of the middle
FIG. 837 B.-DIAGRAM OF A TRANSVERSE SECTION OF THE BRAIN AFTER
stem tributary has been converted into the superior petrosal sinus (Fig. 838). The inferior petrosal sinus appears to be an independently formed anastomosis which connects the posterior end of the cavernous sinus with the upper end of the internal jugular vein across the medial side of the otic region (Fig. 838)..
The extra-cranial parts of the anterior cardinal veins become connected together, in the upper or cephalic part of the thoracic region, by a transverse anastomosis which becomes the greater part of the left innominate vein. Á short distance cranialwards to this transverse connexion, the primitive vein of the upper limb ultimately opens into the anterior cardinal The portion of the anterior cardinal vein of the right side, which lies between the entrance of the limb vein and the transverse anastomosis, becomes the right innominate vein (Fig. 839), and the corresponding part on the left side forms the commencement of the left innominate vein.
The part of the extra-cranial portion of each anterior cardinal vein which lies cephalwards of the entrance of the limb vein forms the internal jugular vein; and the part of the right anterior cardinal vein which lies caudalwards of the transverse anastomosis becomes the upper
or cephalic part of the superior vena cava, whilst the corresponding portion of the left vein is converted into the upper part of the left superior intercostal vein (Fig. 839).
The external jugular vein is a new formation which receives for a time the cephalic vein of the upper extremity; but the cephalic vein, which is a secondary vessel, is eventually transposed to the axillary vein, which is a part of the primitive upper limb vein.
The Posterior Cardinal Veins, the Subcardinal Veins, and the Inferior Vena Cava.The posterior cardinal veins appear later than the anterior cardinal veins and they terminate cranialwards in the ducts of Cuvier. They lie dorsal to the Wolffian bodies and they become connected with each other, dorsal to the descending aorta, by numerous transverse anastomoses.
The subcardinal veins appear later than the posterior cardinals. They lie along the ventral borders of the Wolffian bodies and they are connected not only by dorso-ventral anastomoses with the posterior cardinal veins, but also by transverse anastomoses with one another. The majority of both sets of anastomosing vessels ultimately disappear, but two remain; one which joins the right subcardinal to the right posterior cardinal, at the level of the right renal vein; and one which connects the subcardinal veins together, across the ventral surface of the aorta, at the same level.
After a time an anastomosis is formed between the right subcardinal vein and the cephalic end of the right vena revehens, dorsal to the liver, by the caudal outgrowth of an anastomosing Superior sagittal sinus
Transverse sinus, horizontal part =upper part of middle stem
Inferior sagittal sinus
Remnant of anterior stem
Sigmoid part of transverse sinus
Terminal part of sigmoid portion of transverse sinus = posterior stem
Superior petrosal sinus = lower part of middle stem
FIG. 838.-DIAGRAM OF THE VENOUS SINUSES. (Only one transverse sinus is shown).
offset from the right vena revehens. As soon as the anastomosis is completed the blood from the caudal portion of the body and from the lower limbs is short-circuited to the heart, and extensive changes occur in the primitive posterior cardinal veins, into which in the meantime the veins from the lower extremities have opened.
The Posterior Cardinal Veins. The right posterior cardinal vein, cephalwards of the right renal vein, becomes the vena azygos. Between the right renal vein and the entrance of the lower limb vein it forms the caudal or lower part of the inferior vena cava and the right common iliac vein; the remainder of the right posterior cardinal vein becomes the right hypogastric vein. Two of the transverse anastomoses between the posterior cardinal veins form the transverse parts of the hemiazygos and accessory hemiazygos veins; others become converted into those parts of the left lumbar veins which lie dorsal to the aorta, and one, which lies opposite the fifth lumbar somite, becomes the greater part of the left common iliac vein.
Portions of the left posterior cardinal vein remain as the left hypogastric vein, the hemiazygos and the accessory hemiazygos veins and the lower part of the left superior intercostal vein.
The tributaries of the right posterior cardinal vein become the right intercostal, subcostal, and the right lumbar veins. The right renal vein is also one of its tributaries. The tributaries of the left posterior cardinal become the left intercostal and subcostal veins, and they form those parts of the left lumbar veins which lie to the left of the vertebral column, and the corresponding part of the left renal vein (Fig. 839).
The Subcardinal Veins.-The only important parts of the subcardinal system which remain in the adult are a portion of the right subcardinal vein, one of its anastomoses with the right posterior cardinal vein, and a transverse anastomosis between the subcardinal veins. The last
forms the part of the left renal vein which crosses the front of the abdominal part of the aort and the first two form that part of the inferior vena cava which extends from the liver to the entrance of the renal veins.
The Inferior Vena Cava.-It follows, from what has been said, that the inferior vena cava a composite vessel derived from five sources: (1) the cephalic end of the right vitelline vein.
(2) a caudal outgrowth from the cephalic part of the right vitelline vein; (3) a portion of the right subcardinal vein; (4) an anastomosis between the right subcardinal vein and the right posterior cardinal vein; and (5) a portion of the right posterior cardinal vein.
The Veins of the Extremities. The primitive veins of the extremities are superficial vein which run along the corresponding borders of the two limbs, ie, the ulnar border of the upper and the fibular border of the lower limb.
The primitive vein of the upper extremity, which becomes the basilic vein, the axillary vein. and the subclavian vein in different regions, opens at first into the posterior cardinal vein, after wards into the duct of Cuvier, and finally into the anterior cardinal vein.
The cephalic vein of the upper extremity appears at a later period and, in the first instance, joins the external jugular vein, the primitive termination being occasionally retained in adult life. At a later period its terminal extremity is transferred to the axillary vein.
The deep veins which accompany the arteries are the latest venous formations of the extremities.
The primitive vein in the lower extremity becomes the small saphenous vein, which is continued proximally, as the inferior gluteal vein, to that part of the posterior cardinal vein which later becomes the hypogastric vein. The great saphenous vein and the femoral vein appear later and are continued to the posterior cardinal vein as the external iliac vein. After the external iliac vein joins the posterior cardinal vein the part of the latter vessel which lies caudal to the point of union is called the hypogastric vein.
THE DEVELOPMENT OF THE LYMPH VASCULAR SYSTEM.
Very little is known regarding the origin of the vessels of the lymph vascular system in man, but during recent years numerous investigations have been made with the object of discovering the various phases of the development of the lymph vascular system in other mammals; some of the main points are however still subjects of dispute.
It is generally admitted that the terminal parts of the great lymph vessels, that is, the terminal part of the thoracic duct and the right lymphatic duct, are derived from venous channels, which, for a time, lose their connexion with the larger veins and become modified into terminal lymph sacs which obtained a secondary union with the great veins at the root of the neck; but regarding the origin of the thoracic duct and the cisterna chyli and the peripheral lymph vessels there is, as yet, no agreement. According to Florence Sabin, and those who agree with her, the peripheral lymph vessels are outgrowths from a series of lymph sacs, themselves of venous origin, and from veins. Of the lymph sacs the two cervical, a retro-peritoneal, a cisterna chyli, and two posterior sacs, lying along the inferior gluteal veins, are recognised in human embryos of about 24 mm. length. The origin of these sacs, in the human embryos, has not been proved, but it appears probable that the cervical sacs are derived as in other mammals from venous capillary Vessels. There is no agreement, however, concerning the origin of the other sacs, for whilst, on the one hand, Florence Sabin appears to believe that they have an origin like that of the jugular lymph sacs and that the thoracic duct is developed in the same manner, Huntington, on the other hand, believes that, with the exception of the jugular or cervical lymph sacs, all the other lymph vessels, including the thoracic duct, are developed neither as outgrowths from lymph sacs nor by the modification of venous capillaries, but by the formation of endothelial lined spaces in the mesodermal tissues. The spaces are at first entirely closed but afterwards attain union with one another and with the terminal lymph vessels.
According to the view upheld by Florence Sabin and her supporters, the lymph vessels are outgrowths from the venous system, and are, therefore, lined with endothelium which is genetically the same as that in the veins. According to Huntington this is not the case; for his observations lead him to believe that the endothelium of the lymph vessels is formed, in situ, from the mesodermal cells, and it has, therefore, no direct genetic connexion with the endothelium of the veins, which is derived from the original angioblast. The evidence brought forward by the supporters of the opposite views is interesting and instructive upon many points, but the question must still be regarded as an open one.
The Development of Lymph Glands.-Lymph glands are developed from plexuses of lymph vessels. The transformation is brought about by the aggregation of numerous lymphocytes in the connective tissue strands of the plexuses and the transformation of the lumina of the vessels into the peripheral and central portions of the lymph sinus. The stroma, the capsule, and the proper substance of a lymph gland are therefore formed from the fibro-cellular reticulum of the lymphatic plexus, and the cavity of the lymph sinus is formed from the lumina of lymph capillaries. The rudimentary lymph glands possess blood vascular as well as lymphatic capillary networks, and if the blood vascular network preponderates over the lymph vascular the developing gland has a reddish appearance and is known as a hæmal gland. Such glands are found in man (Schäfer) as well as in other mammals, and it would appear from the recent observations of S. v. Schumaker that hæmal glands are merely rudimentary forms of true lymph glands (Arch. f. mikr. Anat. H. 2., 1912).
MORPHOLOGY OF THE VASCULAR SYSTEM.
In conformity with the general plan of the vertebrate body, the vascular system is essentially segmental in character. This is obvious, even in the adult, in the intercostal and lumbar vessels. It is distinguishable, though less obvious, in the vessels of the head and neck and of the pelvis.
The intersegmental arteries and veins form a series of bilaterally symmetrical vessels, each of which is united to the vessels of adjacent segments by segmental channels, which anastomose