Book - Buchanan's Manual of Anatomy including Embryology 13

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Frazer JE. Buchanan's Manual of Anatomy, including Embryology. (1937) 6th Edition. Bailliere, Tindall And Cox, London.

Buchanan's Manual of Anatomy: I. Terminology and Relative Positions | II. General Embryology | III. Osteology | IV. Bones of Trunk | V. Bones of Head | VI. Bones of Upper Limb | VII. Bones of Lower Limb | VIII. Joints | IX. The Upper Limb | X. Lower Limb | XI. The Abdomen | XII. The Thorax | XIII. Development of Vascular Systems | XIV. The Head and Neck | XV. The Nervous System | XVI. The Eye | XVII. The Ear | Glossary
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Chapter XIII Development of Vascular Systems

Principal Arteries and Veins

It was shown at the beginning of this book (pp. 51, 91) that the vase system, in its earliest stages, came into existence as the result of the establishn of connections between networks of channels formed on the yolk-sac, in body-stalk, and in the embryonic body; an extension from the body-stall

Fig. 670. — Scheme of Earliest Circulatory System. (Founded on Eternc

The lower figure is a diagrammatic section to show position of aortae and umbil veins (UV). Am, amnion; N, neural groove; coe, intra-embryonic coelom

the chorion (where similar channels are possibly formed also) puts tl anastomosing systems into relation with the growing villi.

The primitive circulation would seem to be possible through channels est lished in this way at an early stage in embryonic formation; such might represented schematically as in Fig. 670. In this figure it can be seen t

ns (vitelline) pass up the wall of the yolk-sac to reach the posterior part of ! embryonic rim, where they join a vein coming from the body-stalk and ming forward to enter the primitive heart-tube. This (C) is a very short )e, doubled in origin, which runs back towards the bucco-pharyngeal area (bp), ing off here two primitive aortcs, which pass back on each side of the area the future membrane, and continue their course caudally to reach the bodylk. As they pass back in the embryo they give off vitelline branches downrds on to the wall of the yolk-sac; later, as the somites form, they will be found give intersegmental branches running dorsally between the somites. When

y reach the chorion they are distributed throughout it and to its villi. The veins which return the blood in this early circulation must come into stence, of course, with the * arteries,’ and can be said in general terms to

Fig. 671. — Schemes founded on Conditions in an Embryo of About Twelve Somites, in which the Intra- and Extra-embryonic Ccelomic Cavities are Continuous.

se from the common vascular network as enlargements of its ‘ peripheral ’ t, the ‘ central ’ vessels becoming arterial. Thus veins are found extending the heart along the embryonic rim, which receive the blood returning through body-stalk; these veins (uv), are the umbilical veins, which, passing forward ng the margins of the embryonic plate, reach the anterior end of the heart,e in this margin. The vein is shown in black in the figure, where only the sels of the left side are seen; actually there are two primitive aortae and two bilical veins.

The conditions illustrated in the last figure are those present in the embryo, which there are as yet no somites and no body cavity continuous with the ernal coelom. The result of somite formation, with the reversal of the erior end of the embryonic plate, is shown in the next figure (Fig. 671), where the heart is now reversed, and the arterial end points forward, while paired aortae run upwards beside the bucco-pharyngeal membrane and then ti back; intersegmental branches are appearing now. It can be seen now, moreo\ that well-defined vitelline veins are reaching the venous end of the heart (n posterior) by passing up the front of the yolk-sac; the earlier posterior ve have disappeared. The splitting of the embryonic rim by the ccelomic extens leaves the vitelline arteries on the splanchnic wall, but puts the umbilical ve in the somatic wall, close to the continuity of this wall with the amnion; thi: shown in the section in Fig. 671.

The paired aortae lie oh the roof of the primitive pharynx, and receive in t situation ‘ aortic arches ’ from the ventrally placed arterial trunk and vessi but behind the pharynx they fuse into a single vessel, which divides again ii two as it approaches the hinder end; these two umbilical arteries pass into • body-stalk.

Development of Principal Arteries

The arterial end of the heart opens into a dilated arterial sinus, from which right and left arteries run into the pharyngeal arches, passing through th to join the right and left dorsal aortee which are lying on the roof of the primit pharynx. All told, there are six of these aortic arches (or pharyngeal artei arches) on each side, but they are never present at any one moment in th totality. They appear from before backward, like the mesodermal arches which they lie. The first is possibly the direct descendant of the primitive ao: of its side, passing dorsally round the bucco-pharyngeal membrane; in any c; it is found very early, before reversal is nearly completed. The second aor arch comes a little later. The third, when it appears, seems to take on the din supply of blood to the dorsal aorta at its anterior end, where this vessel is givi branches to the growing neural (brain) tube, and the first and second arcl break up rapidly and disappear, except for their upper and lower ends for soi little time. The fourth arch appears at about the same time as the third, i fifth is very small and short-lived, like the rudimentary mesodermal arch in whi it lies, and the sixth appears behind this. All these arterial arches, then, appt as vessels running ventro-dorsally within their corresponding pharyngeal arch and conveying blood from the ventrally-placed arterial sinus to the dor aortae for distribution.

The presence of the arterial sinus makes the details of further developmt slightly different in the human embryo from those in lower forms, but the differences are only slight, and the main evolution of the adult pattern frc that of the aortic arches is in line with the generalized vertebrate evolution Such a generalized conception of the system of aortic arches is that (Fig. 6; given by Rathke many years ago, on which the special variations found amo vertebrates can be worked out. The plan shows the arterial arrangemei flattened out, so that, from the arterial stem, right and left ventral aortce r forward (below the pharynx) and give off at intervals the six aortic arches reach the d or sal aortas (above the pharynx). Small branches pass from i sixth arches into the lungs, evidently the future pulmonary arteries, while f anterior end of each dorsal aorta is carried (beyond the scheme) into the cran cavity as a cerebral artery. It may be added here that eight intersegmen: branches arise from each dorsal aorta, the lowest coming off about oppos or just below the fourth arch on each side; seven of these arteries are cervic but the first is cranial, accompanying the hypoglossal.

Disregarding the gill-bearing vertebrates, we may come at once to 1 Amniota, where the differences in the various phyla are classifiable easily on t scheme. In the first place it is a general rule that the third aortic arch is devot to supplying the brain, while the fourth becomes the main stem for the sup], of the body. Thus the third arch plus the dorsal aorta in front of it becon; the internal carotid, and the fourth arch becomes the ‘ arch 5 of the syster aorta, from which facts it can be assumed that the dorsal aorta between 1 third and fourth arches loses its function, is stretched out, and disappea

e matter of the two fourth arches, these persist in the reptiles ; the right one 5 the systemic arch in birds, and the left one in mammals. he human conditions are thus shown (Fig. 672) on the scheme as mammalian, first two arches disappear, leaving the ventral aorta opposite them as nal carotid; the third, with the dorsal aorta in front of it, forms the nal carotid; the dorsal aorta behind it, between it and the fourth arch, •pears; the ventral aorta between third and fourth becomes the common id. The original symmetry is disturbed behind this. The fourth left arch s part of the systemic arch, but not the whole of it; the dorsal part of this is formed by dorsal aorta, and the ventral end is the beginning of the ventral

this is represented on the right side by the innominate artery. The right i arch becomes part of the right subclavian; the whole of the left subclavian he terminal piece of the right subclavian are of intersegmental value. Behind

Fig. 672. — Scheme of the Aortic Arches and their Destination.

e the fifth arches disappear; the left sixth remains as the ductus arteriosus; right sixth disappears except for its ventral end, from which the right lonary artery arises. The common arterial trunk is divided by a septum a dorsal part continuous with the sixth arches, and a ventral part for the ral aortae, and this septum extends towards the heart in a spiral manner, nee the changing relations of pulmonary artery and ascending aorta; this vessel represents the common trunk and its systemic subdivision, binally, right dorsal aorta disappears behind the right fourth arch, so that the rest ie thoracic aorta comes from the fused vessels.

In the human embryo there is no ventral aorta giving origin, as in the scheme, 11 the arterial arches, but if we look at the arterial sinus as having the value hortened ventral aortae conjoined, this difficulty disappears. In any case difference is not of much importance, big. 673 gives, in the uppei row, is showing the changes in the human embryo as seen from the left. The external carotid showing here may be a new formation, but might be some dr< out persisting remnant of the ventral supply to the early anterior arches, the common carotid is either part of the third arch or a drawn-out portion o sinus; it is a very difficult question to decide. The lower figure shows

Fig. 673. — Plans to show Changes in Human Embryo: Upper Row Left, Lower from Right. Aortic arches and their remnants numbered in Roman figures.

conditions on the right side; the carotid developments are as on the left, but fourth arch is part of the subclavian, and the dorsal aorta disappears behind

In estimating and following the changes which occur in the art< arches in the neck it must be remembered that the head grows forw leaving structures behind it which were originally ventral to it; th what is really meant when the heart, for example, is said to ‘ desce

Fig. 674. — Plans to show how the Embryonic Relations between Ner and Arteries are exhibited in the Adult Condition.

The growth of the head and elongation of the neck straighten out carotids, while the connection of the fourth arch with the trunk keeps it 1 the heart.

The relationship between the arterial stems (in the pharyngeal arc] and the nerves of the arches is of interest from the point of view of the nor relations in the adult, and also in cases where the adult conditions are unusu

The nerves of the first four arches lie near the grooves in front of them, and thus in front of the arteries which lie more or less in the middle of the mesomal masses. In the sixth arch, however, possibly because its artery runs k to it and is formed relatively late, the artery lies in front of the nerve. In

ases the nerve, having crossed the dorsal aorta on its lateral side, then turns ards to gain the visceral surface; thus they all cross obliquely the lines of ir corresponding arteries, but whereas the first four cross in front of their sels, the last crosses behind it. It follows that, when the neck is elongated the vessels drawn out, they are drawn over the nerves of the arches behind m. Thus the internal carotid (third arch) has the superior laryngeal (fourth arch ve) deep to it, while its own nerve, the glosso-pharyngeal, is superficial to it rsal aorta). The fourth aortic arch (arch of aorta and right subclavian) has the irrent laryngeal deep to it, but this nerve on the left turns first round the mentum arteriosum because it crosses behind that artery in the sixth arch.

The fifth arch, with its artery and nerve, is a tiny and transient formation, only brought into description to complete the various systems. Its artery may be as in Fig. 673, or may arise from the arterial sinus and end in one of the neighbouring arches, or even in the dorsal vessel, or may even arise from the proximal part of the sixth artery.

It disappears early and completely.

The right subclavian artery, as shown in the scheme, has its first part formed n the fourth right aortic arch, and its terminal piece from the seventh cervical srsegmental artery. Between these two there is a portion of the right dorsal ta; there is considerable doubt about the limits of these various parts.

There are two well-known varieties of the right subclavian artery of developntal interest. In one the artery arises from the descending thoracic aorta, the other it arises from the left end of the arch of the aorta; in both cases it ses behind the oesophagus to gain the right side. The first of these varieties ms to be an example of persistence of the right dorsal aorta ; the same explanation iometimes given for the second variety, but with much less probability, and would seem to be more probably an example of anastomosis between the two sal aortw. In both cases the fourth aortic arch has evidently disappeared the right side, and as a result the right recurrent nerve turns round the next h in front— i.e., it runs directly downwards to the lower border of the larynx , sing deep to the internal carotid.

Intersegmental Branches : Vertebral Artery. —The first intersegmental artery en by each dorsal aorta accompanies the hypoglossal nerve. After this come en cervical intersegmental vessels. These pass back between the somites, ich they supply, and give branches also to the neural formations lying ernal to these. Since the cranial end of the dorsal aorta is about to be stretched

in the internal carotid, and the next part of it is about to disappear, it is dent that, if these intersegmental arteries are to continue to supply the uctures mentioned, they must be provided with another artery of origin, e provision is made very early in their history in the form of a longitudinal istomosis connecting them together some little distance from their origins.

An anastomosis of such a sort is a normal occurrence among the intersegmental vessels in the trunk. Usually it remains very small or disappears, but is seen occasionally in the adult, especially in the thoracic region. In the ordinary way, however, it is only in the cervical region that it enlarges and becomes functionally important.

This longitudinal anastomosis ends caudally in the seventh cervical intermental artery. Thus, when the origins from the dorsal aorta fail, the series vessels obtains its blood from this seventh artery. The longitudinally running 'tebral artery is thus made up of these bits of interarterial anastomosis, the ginal intersegmental vessels persisting as its branches. The portion above the as, however, which has a different relation to the issuing nerve, is of another [ue, being the enlarged intersegmental spinal or neural branch.

The origin of a vertebral artery precedes that of the corresponding subcla 1 artery, so that the subclavian artery is originally a lateral branch of the verte artery. As the development proceeds, however, the subclavian artery incre in size, and greatly exceeds the vertebral artery, the latter vessel being : regarded as a branch of the subclavian.

Origin of Left Vertebral Artery from Aortic Arch. —This, the commo additional branch arising from the arch, is probably an example of the sistence (see Fig. 675) of the sixth intersegmental origin from the dorsal ac with the portion of this aorta remaining as far as the level of the fourth ac arch.

The thoracic and abdominal intersegmental arteries have been refe to already in connection with the aorta.

Fig. 675. — Development of Cervical, Intersegmental, and Vertebra Arteries, according to Scheme.

The intracranial prolongation of the dorsal aorta on each side is an exan of the enlargement of a neural branch arising from the beginning of this ari and running dorsally into the paraxial tissue round the brain; the dorsal a< itself lies on the upper in-turned ends of the visceral mesoderm of the arc below the layer of paraxial mesoderm. This terminal neural branch en the paraxial layer beside Rathke’s pouch, and, in the adult, pierces the d mater here. Before reaching this, the internal carotid lies on the roof of tubo-tympanic recess, covered by the otic capsule; much later, this cap: extends its ossification partly round it, enclosing it in the carotid canal, bi portion of the artery, still unenclosed, lies in front of this and crosses (as it in the foramen lacerum) the anterior margin of the recess, the auditory tube,

The ramifications of the cerebral arteries are formed in accorda with the growth of the parts of the brain, which is surrounded by a vasci network from a relatively early stage. These vessels are dealt witl part in the section on the central nervous system.

Arteries of Limbs. — The arteries of the upper limb have been already considered, the lower limb, the accepted views on their development are in large part lptions based on comparative anatomical observations, direct observation » human embryo being an undertaking of great difficulty.

Lower Limb. — At the beginning of the second month the main artery accoms the sciatic nerve, passes deep to the rudiment of the popliteus, and runs

en the primordia of the leg bones to the foot. This ‘ axial ’ vessel is cted with a small plexus on the extensor aspect of the limb, from which at ir stage the femoral artery will form. As the femoral channel enlarges, axis ’ vessel sends a secondary branch down superficial to the popliteus, hen degenerates above the level of its connection with the femoral; this dary branch divides to form the tibial arteries, anastomosing with the lal interosseous trunk, which has already given off the anterior tibial, and becomes a small branch of this. The inferior gluteal artery, the popliteal, •art of its middle genicular branch, are remains of the original ‘ axis ' supply,

Fig. 676. — The Venous Trunks of the Septum Transversum of the Human Embryo (His).

X.X. Upper separated portions of Umbilical Veins R.U.V. Right Umbilical Vein L.U.V. Left Umbilical Vein

V.P. Venous Loops round Gut V.V. Vitelline Veins

the peroneal and perhaps part of the arterial structures in the sole are ed from its interosseous prolongation; the two femoral arteries and the snous branch of the descending genicular artery, when present, are remains e femoral extensor plexus.

Development of the Principal Veins (p. 5 1 ) he primitive veins form two groups. One group returns the blood from the sac and the placenta; and the other group returns the blood from the head neck, anterior limbs, body-wall, mesonephric bodies, and posterior limbs, first group comprises: (1) the vitelline veins, in connection with which the il vein is developed; and (2) the umbilical veins. The second group consists ) the anterior cardinal or primitive jugular veins; (2) the posterior cardinal and (3) the subcardinal veins; they drain into the veins (or ducts) of 2r on each side. The veins of each group are arranged in pairs, right and

J.V. Primitive Jugular Vein

.V. Cardinal Vein V.C. Right Duct of Cuvier /.C. Left Duct of Cuvier

Sinus Venosus. — This is the venous space made by the confluence of veins of the body; it discharges directly into the common atrium the bloo receives from the veins. It is placed in the septum transversum (pp. 46 and and consists of two ‘ horns,' each of which is made by the junction of the umbili vitelline, and Cuvierian veins of its own side. As described in the developrr of the heart, the right horn is taken up into the right atrium, the left horn becon the coronary sinus. The vitelline veins reach the sinus venosus by rum in the visceral wall to the septum transversum, the others get to the septum

the body-wall. 1

1. Vitelline Veins and Portal Veins— These veins are developed early, they return the blood from the yolk-sac. They enter the body of the emt along the vitelline duct, and finally open into the sinus venosus after traver the septum transversum. Within the body they ascend parallel with each ot at first in front of, and subsequently on either side of, the duodenal portio the primitive intestinal tube. In the latter region on the caudal side of

Fig. 677. Schemes to show Formation of Portal Vein from Doi

Connecting Loop between the Right and Left Vitelline Veins.

hepatic bud they become connected by three transverse anastomotic ves two of which lie across the ventral aspect of the gut, and one being placed 0 dorsal aspect. The first or lowest anastomotic vessel lies on the ventral as of the gut; the second or middle vessel is dorsal to the gut; and the tnir highest, like the first or lowest, is ventral to the gut. This is shown m the scheme in Fig. 677.

By means of these three anastomotic vessels two venous rings—lowe caudal, and upper or cephalic—are formed around the duodenal portion oi primitive intestinal tube, these rings constituting the sinus annularis. t their formation the two divisions of the liver-bud are breaking up into ne cylinders, and these are giving off secondary cylinders. Owing to these ne] developments the vitelline veins cease to communicate directly with tne . venosus. The portions of the vitelline veins above the upper duodenal ve ring (shown in Fig. 676 to proceed from its sides) become surrounded hepatic cylinders, and invaded by the secondary cylinders. In this ma

portions of the vitelline veins are freely subdivided into blood-channels,

1 are known as sinusoids (Minot). These sinusoids form a network which >ies the meshes of the network formed by the branches of the hepatic cylinders, veins which convey blood from the upper duodenal ring to the hepatic oids are now known as the vence advehentes, and they become the right and \ivisions of the portal vein. The veins which carry the blood from the tic sinusoids to the sinus venosus are known as the vence revehentes, and they ne the hepatic veins.

Trunk of the Portal Vein. — The portions of the two vitelline veins which id in front of the primitive duodenum lie close together and parallel with other. These portions fuse for a short distance, and form a single venous , which opens into the first, or lowest, ventral anastomotic vessel, or, in ■ words, into the lower part of the lower duodenal venous ring. This short receives the veins of the primitive intestinal tube, and it forms the root e portal vein. The primitive portal vein, therefore, receives its blood from tie yolk-sac, and (2) the primitive intestinal tube within the abdomen. The r ventral anastomosis and the right vein immediately above it quickly disappear (Fig. 677).

is the yolk-sac atrophies the portions of the vitelline veins between it and commencement of the portal vein also atrophy, and the tributaries of the il vein gradually assume their condition in adult life. The vitelline vein, ever, does not disappear for a considerable time, but remains as a free cord ,4) passing out of the umbilicus up to the entrance of the umbilical loop into belly. #

The following parts of the sinus annularis, or double duodenal ring, undergo phy (see Fig. 677):

1. Right half of lower ventral anastomotic vessel.

2. Right half of lower duodenal ring.

3. Left half of upper duodenal ring.

The following parts of the sinus annularis persist:

1. Left half of lower ventral anastomotic vessel.

2. Left half of lower duodenal ring.

3. Middle or dorsal anastomotic vessel.

4. Right upper half of duodenal ring.

5. Upper ventral anastomotic vessel.

These persistent portions, with the exception of the upper ventral anastomotic el, form the greater part of the trunk of the portal vein, the upper ventral stomotic vessel represents a part of the left division of the portal vein.

The portal vein has originally a spiral relation to the duodenal portion of primitive intestinal tube—that is to say, it winds round the left side and sal aspect of the duodenum, and then appears on its right side.

Divisions of the Portal Vein.— These are connected with the upper duodenal ous ring. As previously stated, the veins which convey the blood from this to the hepatic sinusoids are known as the vence advehentes, right and left.

right division of the portal vein is formed by the right vena advehens, which ings from the right half of the upper duodenal venous ring, dhe left division firmed by (1) the upper ventral anastomotic vessel, and (2) the left vena ehens

Umbilical Veins.— The two umbilical veins return the blood from the placenta the sinus venosus. They are of small size during the period of the vitelline mlation, but become enlarged as the placenta gradually forms, the two ns unite and form a single trunk within the umbilical cord At the umbi icus 5 trunk enters the body of the embryo, and immediately divides into wo bilical veins, right and left, which traverse the septum transversum and open n the sinus venosus. As they traverse the septum transversum ey are se to the developing liver.

The left umbilical vein enlarges fairly rapidly, that on the right s atrophying more slowly. Just before the 5 mm. stage the left vein effect capillary junction with the left vitelline vein on the caudal aspect of the sept transversum— i.e., on the caudal or visceral surface of the small liver. 1 connection between the umbilical and vitelline veins of the left side enlar very rapidly, thus making the vessel on the visceral aspect of the liver wt is usually referred to as the ‘ left umbilical vein ’; actually, of course, it is a vite umbilical anastomosis, and the real umbilical vein passes up still beside the lb but dwindles rapidly, and cannot be certainly traced after a fairly short inter When the anastomotic vessel collapses, after birth, it makes the ligament teres of the liver.

In this region part of this fibrous cord lies in the abdominal wall ; part below the liver; the first of these parts is the true umbilical vein, w the second is the vitello-umbilical anastomosis.

Fig. 678. — Scheme to show Early Circulation in Liver, and Direct of Ductus Venosus.

The right umbilical vein also seems to develop some similar anastom with the vitelline system, but owing to the atrophy of the vein the anastom does not become evident.

The anastomosis of the left umbilical vein with the left vitelline vein oc at the level (in this last vessel) of the upper junctional loop with its fellow, as sh in Fig. 677. As already stated, this upper loop becomes embedded in the lb and forms a part of the left portal vein, whence the fact that the ligamen; teres, buried between two lobes of the liver, runs to join the left division of portal vein.

There is possibly some connection between the entrance of the umbi blood at this level and the disappearance of the original vitelline (A in Fig. 677) between this and the next anastomotic loop, the vol of blood from the larger vein prohibiting entrance of vitelline blood it below.

Ductus Venosus. —Blood from the left umbilical vein enters the live; j increasing amount as the placenta grows, and before long there becomes evil a dilatation into a large vessel of the vascular spaces in the liver lying bet\i the point of entrance of the blood and that of its discharge into the infc

, cava. This vessel is termed the ductus venosus (or ductus Arantii). It nds from the left portal vein, which has been formed from the uppermost line loop and receives (Fig. 678) the vitello-umbilical anastomotic vessel; ,sses upwards and to the right, reaching the terminal part of the right hepatic nage into the inferior vena cava, which becomes dilated to form its terminal

After birth, when the placental circulation has ceased, the ductus venosus becomes a fibrous cord, the ligamentum venosum.

minute portion of the lumen he left umbilical vein remains ious within the ligamentum > of the liver. This pervious ion communicates at the liver . the left division of the portal , and at the umbilicus it is lected with the epigastric veins

The abdominal wall. It thus is a channel of communication yeen the left division of the al vein and the systemic veins tie anterior abdominal wall. In pervious portion the blood can towards the umbilicus. This stomosis between the portal and emic circulations accounts for enlargement of the veins of the irior abdominal wall in cases of

al obstruction within the liver. 2. The cardinal system of veins iprises several vessels on each

, which ultimately drain their d into the sinus venosus through right and left veins {or ducts) of ier. A general idea of their ribution can be gained from . 679A. A large vessel, the primijugular or anterior cardinal (or cardinal), drains the cranial part the body, beginning in associa1 with the venous drainage of brain, and passing back through cervical region, where it re/es intersegmental veins, includthe subclavian (S). The posterior dinal runs caudo-cranially, being med in association primarily with mesonephros (W) and receiving srsegmental veins. The posterior 1 anterior cardinal veins join to m the ‘duct of Cuvier,’ situated the septum transversum, cranial the liver ('LL and thus

Fig. 679. — Scheme of Main Venous Drainages of Embryo.

SV, sinus venosus; DC, duct of Cuvier; PJ, anterior cardinal or primitive jugular; C, posterior cardinal; UV and VV, umbilical and vitelline veins.

(L), and thus running ctly into the sinus venosus.

rhe anterior cardinal or primitive jugular veins are two m number—right left—and return the blood from the head, neck, and fore-limbs. Each vein >ists of two parts—intracranial and extracranial. The intracranial part gives , directly and indirectly, to the intracranial sinuses, whilst the extracranial ‘ becomes the internal jugular vein. The intracranial part is known as the principal or primary head vein. This vein at its anterior end is on the inner of the trigeminal ganglion; passing backwards from this point, it lies just al the outer part of the tubo-tympanic recess lateral to the facial ganglion and f nerve, and reaches behind this the outer side of the glosso-pharyngeal and va

Its different relations with the several cranial nerve-roots indicate it has been produced from two venous trunks, connected between the rc these trunks are represented in lower vertebrates by the vena capitis late and vena capitis medialis.

At its anterior end the principal head vein receives on each side the anti cerebral veins, a plexiform set of vessels draining the mid-brain, the back ol fore-brain, and the optic outgrowth. The anterior cerebral veins anastomose with their opposite fellows dorsally between the two small cerebral vesic thus forming the rudiment of the superior sagittal sinus, which is elongated the vesicles grow backwards (Fig. 680).

Fig. 679A. — To show Main Trunk Veins in Young Embryo.

Between the trigeminal and facial ganglia the principal vein receive middle cerebral vein or plexus, draining the front limb of the pontine flex and anastomosing with the anterior veins.

A plexiform posterior cerebral vein opens into the principal head vein betw the facial and glosso-pharyngeal ganglia; it drains the back of the poni flexure and the myelencephalon. These details are shown in Fig. 680, f scheme.

As the cerebral vesicles grow backwards, the anterior plexus becomes n closely connected with the middle set, and its blood is returned by this set

asing amount. Ultimately the superficial middle cerebral vein, the sphenotal sinus, and the ophthalmic veins, are the only vessels left of the original isive anterior venous plexus. The cavernous sinus is the persisting anterior of the principal head vein (second figure).

The middle cerebral vein, however, has in the meantime formed a secondary ’ding channel with the posterior cerebral vein, this channel lying above the capsule and beside the pontine flexure within the developing cranium, the blood which has already been seen to be derived from the anterior 1 into the middle set is carried back through this new channel, and the lal principal vein in its intermediate part gets smaller and disappears, the 1 from its anterior end (cavernous sinus) now passing up the lower portion e original trunk of the middle cerebral vein to reach the new channel, through h it passes to the hinder part of the posterior vein, along which it runs down -enter the hinder part of its old vessel, and thus leaves the cranium with r agus.

680.— Semi-schematic Figures to show Changes in Venous Drainage of Brain (founded on Streeter’s Figures).

Continuing their growth backwards (third figure), the cerebral vesicles carry elongating longitudinal sinus with them, so that it ultimately comes to open

th figure) into the posterior set of veins, from the lower part of which the al sinus is made. The middle cerebral plexus is covered over by the growing des, but is represented in the great cerebral veins and their tributaries, the ar veins being also remnants of their connections with the anterior venous The straight sinus is a new connection (arising on the dorsal side of the -brain flexure when this closes) between the middle and posterior sets of s. The inferior petrosal sinus represents the secondary re-establishment direct connection between the original and persisting anterior and posterior 1 of the primary head vein, but the new channel is within the cranium, and is a reappearance of the old vein. .

die extracranial part of the anterior cardinal vein, after the obliteration of vena capitis lateralis, commences at the jugular foramen, whence it extends lad, receiving in its course the cervical inter segmental veins. In the vicinity te sinus venosus it meets the posterior cardinal vein of its own side, with which lites. The venous trunk thus formed is called the duct of Cuvier, of which e are two—right and left. These two ducts pass transversely, one on either side, to the sinus venosus, into which they open. At this stage the sinus venc also receives the vitelline and umbilical veins of each side. The ducts of Cu are in the septum transversum (see pp. 46 and 52).

As the heart descends from the region of the fore-gut, the ducts of Cu become vertical, and are in line with the anterior cardinal veins. When sinus venosus becomes merged into the right atrium, and the heart undergo^ slight rotation from right to left, the left duct of Cuvier is placed dorsal to left atrium before opening into the right atrium.

Each anterior cardinal vein is joined, near its caudal extremity, by corresponding subclavian vein.

Close to the junction the subclavian vein receives the external jugular V This vein is a secondary formation, and is probably derived from a poste auricular vein, being subsequently reinforced by a pre-auricular vein.

Fig. 681. — The Venous Plexus lying between the Two Primitive.Jugui

Just Beginning To Form The Left Innominate Connection (19 Mm.)

A transverse anastomotic vessel is now formed, called the transverse jug vein. It is developed (Fig. 681) in the ventral plexus connecting the primitive jugulars. It extends from the junction of the left anterior cardinal left subclavian veins to the right anterior cardinal vein at a point a little b< the place where it receives the right subclavian vein. This transverse jug vein, which extends obliquely from left to right, with a downward inclmal gives rise to the left innominate vein. The venous blood from (i) the left of the head and neck and (2) the left fore-limb now passes through the innominate vein into the permanent superior vena cava. The right innomi vein is formed by the short portion of the right anterior cardinal vein w intervenes between the place where it receives the right subclavian vein and place where the transverse jugular vein joins it. The portion of the right ante cardinal vein, which lies immediately below the place where the transverse jug vein joins it, forms the upper or extra-pericardial part of the permanent sup( vena cava —that is to say, the part above the point where the vena azygos o] into it. The lower or intrapericardial part of the superior vena cava is develc

the right duct of Cuvier. The permanent superior vena cava therefore isents (i) the lower part of the right anterior cardinal vein, and (2) the

duct of Cuvier. 'he portion of the left anterior cardinal vein immediately below the left unity of the transverse jugular vein forms the upper part of the left superior costal vein, thus accounting for the ending of that vein in the left innominate which, as stated, is formed by the transverse jugular vein.

'he left duct of Cuvier undergoes partial obliteration. Its terminal part, l with the left lateral cornu of the sinus venosus, gives rise to the coronary 5. The portion next the terminal part also persists in the form of a very ite vein, called the oblique vein of left atrium, which lies over the posterior ct of the left auricle. The obliterated portion is represented by the vestigial of left atrium, which is a small triangular fold of the serous pericardium in t of the root of the left lung.

Fig. 682. — Sectional Plans to illustrate Venous Modifications in Cardinal Systems in the Middle Region of the Abdomen.

Posterior Cardinal Veins. — These are two in number — right and left — and return the blood from the mesonephroi, body-wall, and hind-limbs. They >n either side of the aorta dorsal to the mesonephroi. The caudal end of vein receives the inferior gluteal vein, which is the primitive vein of the limb. A little above this point it is joined at a later period by the external vein, which has now been developed.

die cephalic end of each posterior cardinal vein joins the corresponding nor cardinal vein, and the trunk so formed is the duct of Cuvier.

Amongst other tributaries, the posterior cardinal veins receive intersegmental s, such as the lumbar and intercostal veins, and veins from the intermediate mass.

The posterior cardinal veins lie on the dorsal side of the mesonephroi, cardinal veins run longitudinally on the inner and ventral sides of the mesonep within which the cardinals and subcardinals are connected by anastomt veins; they run into the cardinals at the two ends of the bodies. A third sy: of veins develops on the dorsal side of the cardinal on each side, forming a pi round the groups of sympathetic ganglion cells as these grow down from posterior root ganglia. As this ‘ periganglionic ’ system enlarges with the gn and extension of the nervous masses, it forms an extensive plexus, joi laterally with the inter segmental veins , ventrally with the cardinal, and wit) subcardinal internal to this. In many animal forms this venous system constil a definite supracardinal vein on each side, with retro-aortic anastomoses, but a development is not found in the human subject, and retro-aortic conned are few and far between; hence it seems convenient to use the term ganglionic as representing the human condition or modification of the supracarc formations.

Progressing with the ganglionic and nervous growth, the perigangli plexus extends forwards and inwards on each side as a juxta-aortic plexi veins, and from this a pre-aortic plexus is quickly formed. The pre-ai extension is seen in the thorax, and is very marked in the abdomen belov level of the superior mesenteric artery; between these two parts the growt the suprarenal and its association with the sympathetic and the formatio the diaphragmatic crura seem to interfere with the formation of the pre-ai plexus. The part of the left renal vein that lies in front of the aorta is made 1 this plexus below the superior mesenteric artery, putting the two subcard] in connection owing to the extensive anastomoses between these and the gny supracardinal vessels. Caudally, the aorta divides into two umbilical arte and the supracardinal plexus is carried down on each side dorsal to these arte extensions from this plexus here on the sacrum appear to be the origin of left common iliac vein in this part.

The sectional plans shown in Fig. 682 will be of use probably in enabling reader to follow the complicated changes going on in these different sets of ves In I are shown the two mesonephric formations in section, beside the mei mesentery. On the left an intersegmental vein (in) is seen opening into posterior cardinal (C), which is situated on the dorsal and lateral side of mesonephros, and is joined by a plexus within that body with the subcarc (SC) in the medial part of the body near the base of the mesentery. On the r side is shown a stage a very little later, in which a plexus of minute veins is surrounding the sympathetic neuroblasts which have descended from the ne crest; this ‘ periganglionic ' plexus is connected with the intersegmental \ and also (although this is not shown) actually with the posterior cardinal, plexus, following the extension of the neuroblasts, shows a rapid ventral extern as seen in II, towards the ventral aspect of the aorta, over which (III) it pa and joins with the plexus of the other side. It is to be noted that this exten is correlated with a marked anastomosis with the subcardinal, as seen in III, in the same figure is shown the evident tendency of the intersegmental vei: transfer its drainage to the plexus—that is, in general terminology, to trar it to the supracardinal vessels. The connection with the subcardinals ii marked that the transaortic plexus appears very soon to be intersubcardi as is suggested in III, and becomes intersubcardinal in actuality before (IV), as the original anastomosis with the plexus begins to atrophy or break This break is favoured by the establishment of longitudinal anastomoses betv the successive ‘ plexus ' units, so that their blood is now carried cranially longitudinally running and definite supracardinal vessel, which only communic with the subcardinals here and there. In the meantime the intersegme veins have come to drain altogether via the ‘ plexus ’ into this longitud vessel, and the posterior cardinal (in this abdominal region) disappears (c) f the scene. These changes are shown in IV. The last figure (V) is a schem illustrate the age-changes described above. The intersegmental veins in f younger stages (below) are joined by a plexus with the subcardinal (SC), but become older (higher) they establish a longitudinal drainage of their own lose their connections with the subcardinal, except, for example, where the renal vein is made.

The changes just described are of the nature of general changes; their modificai and extensions, as shown in the development of the inferior vena cava azygos systems, remain for description.

humming up these matters from the viewpoint of the respective cardinal it may be said that the posterior cardinal is formed on each side in association the mesonephros, which it drains, and also receives intersegmental veins, he mesonephros degenerates the cardinal vein gets smaller, and disappears pletely in the abdomen. Its intersegmental tributaries have previously . transferred to the supracardinal (periganglionic) system. Changes in the ax will be dealt with later.

die subcardinal is an accessory channel in the inner part of the mesonephros, extent corresponds with this body, and it joins the main cardinal at its smities. It is connected with this vein by a venous plexus throughout its se, and also secondarily with the supracardinal system and its derivatives; of these last is the left renal vein in front of the aorta, which in this way mes practically an intersubcardinal connection.

rhe supracardinal system is not developed so completely in man as in most le other mammals; its early state constitutes a ‘ periganglionic ’ system, but term, it must be understood, is only a descriptive word applicable to the human yo, and indicates its supracardinal arrangements. The veins of this system lorsal to those described above, with which they are connected by free .tomosis. They take over secondarily the intersegmental drainage, allowing posterior cardinals to disappear, and they develop a longitudinal drainageel of their own, so that their contained blood is not (or is only in part) carried the subcardinal; at the upper end, however, this longitudinal vessel opens the subcardinal, as will be seen when treating of the thoracic vessels, rhe compound systems of cardinal veins, as shortly described above, are metrically placed on the right and left sides, in the abdominal and (future) acic parts of the embryo. The development of the suprarenal glands, of liver, and of the diaphragmatic structures altogether breaks the continuity le systems, which can now be said to have thoracic and abdominal developts only indirectly connected. The abdominal development is concerned he formation of the inferior vena cava and its associated vessels, while the

os and left superior intercostal systems are produced from the thoracic dopments; these vessels, therefore, can be considered at once.

Development of the Inferior Vena Cava.

The inferior vena cava, as regards its development, consists of two divisions >wer or postrenal, and upper or prerenal.

These descriptive names are given to the two parts because it is at the level of the left renal vein, which has been seen to be practically intersubcardinal, that the developmental values of the parts of the vein .change ; the ‘ renal' term, therefore, applies to the venous level, and not necessarily to the kidney itself.

Below the level of the left renal vein, the inferior vena cava is formed from the it longitudinal vessel of the supracardinal system, which receives the intermental (lower lumbar) veins of the right side; a similar formation is found ■he left side. The posterior cardinals disappear when the intersegmental veins their terminations, and the subcardinals also atrophy, as the mesonephros ins to move down, only retaining the drainage of the gonad at this level; s the supracardinal alone is left to carry on the drainage of the body-wall, Further changes usually occur on the left side, where the longitudinal vessel supplanted by a deeper longitudinal channel connecting the intersegmental



veins on the transverse processes, deep to the psoas. Thus the more superb vessel disappears; but it occasionally persists as a left inferior vena cava, extend up to the left renal vein (see Fig. 684).

Sometimes the retro-aortic anastomosis, which is a feature of the suf cardinal system in other forms, may make a partial appearance in the hun individual, when one or more lower left lumbar veins will pass behind aorta to empty into the inferior vena cava.

Above the level of the left renal vein the subcardinals on each side lie in fr of the suprarenal glands, round the outer sides of which the posterior cardii course cranially. Before long the right subcardinal establishes a commumcat

Fig. 683.— Cardinal and Subcardinal Veins, etc.

(Frederick T. Lewis, in the American Journal of Anatomy.)

with the veins (hepatic) emerging from the dorsal aspect of the liver; this C( munication is situated in the right-hand part of the common dorsal mesentf This junction, enlarging steadily, provides a new and direct channel by which blood in the right subcardinal can reach the heart, and the left renal, ahead) position, affords a means by which that from the left subcardinal can ' advantage also of the new channel. Thus the prerenal portion of the inte. vena cava is of subcardinal origin from the entrance of the left renal vein to relation with the suprarenal gland, where it receives the suprarenal vein.. Ab this it is formed by the hepatic anastomosis and the common hepatic vein, wi

The supracardinal system-which empties itself fundamentally (w. possible) into the subcardinal—forms the lower part of the main vein, but as s



ssible— i.e., where the subcardinal persists—opens into it. Thus the cony of the great vein is effected.

le kidneys, growing cranially, lie among the veins of the supracardinal n, and drain into them. Thus the right renal vein is altogether supra ml in value, and joins the longitudinal supracardinal vessel as this reaches bcardinal ending, thus a very little below the level of the left vein. The snal vein, at its renal end, is of the same value as the whole of the right vein,

s transaortic portion is not represented on the right side; it passes through

Fig. 684. —Schemes to show 1 Formation of Inferior Vena Cava.

ardinals (Sub) are plain; supracardinals (Sup) black; posterior cardinals are not shown, as they are not concerned in the formation.

In A the supracardinal system is establishing its longitudinal vessel on ?ach side, and so communicates with subcardinal. Subcardinals pass aeyond suprarenals (dotted ovals) and there is no connection with the aepatic veins (H). They are joined by a transaortic (dotted) junction, and receive veins (G, G) from the gonads.

In B the supracardinal no longer communicates with subcardinal, except it R; here the renal vein is placed on each side. Subcardinals accordingly iegenerate below entrance of gonad veins. Right subcardinal has now diected a junction with hepatic veins behind liver.

In C any upper continuation of subcardinal above suprarenal is cut off 3 y diaphragm. Final values of parts of I.V.C. are apparent. The ascending supracardinal (X) on left is replaced by the deeper (Y). The junctional part )f left common iliac is an intersupracardinal (periganglionic) formation.

tion of the subcardinal, where this remnant exists to receive the left supraand left spermatic or ovarian veins.

he right and left veins of the gonad drain from the beginning into the 'rdinals, and thus, in the adult, reach the renal level because there is no ordinal remaining below this.

he left suprarenal vein is probably a remnant of the left subcardinal, correcting with the part of the vena cava formed by the subcardinal above the level; the current of blood in it, however, is reversed.

he posterior cardinal vein, having lost its intersegmental branches, disappears the abdomen by the middle of the second month, having taken no part in 'rrnation of the inferior vena cava.



Thoracic Cardinal Formations.

The prolongation of the three cardinal systems into the thorax from abdomen is interrupted first by the rapid enlargement of the suprarenal gla This particularly affects the subcardinals, but these glands still, for a little i discharge some of their blood by small veins into the thoracic parts of the cardinals; soon, however, the extension of the diaphragm, over and be them, cuts them off finally from this way of discharge and leaves them onl) abdominal subcardinals for drainage.

The terminal piece of the posterior cardinal remains on both sides, persisting portion is the part above the entrance of the subcardinal, exten

Fig. 685.— Schemes of Cardinal Systems in the Thorax.

On each side upper part of posterior cardinal (PC) persists. Longituc supracardinal (Sup), connected at first with subcardinal (Sub), only ret ultimately its terminal opening into this; thus this extreme terminal f of subcardinal persists, the rest of subcardinal and of postcardinal appearing below this point. On left side the original symmetry is fur broken up, and retro-aortic junctions cross to right supracardinal.

A is early condition, B the final state. VC, duct of Cuvier; PJ, primi jugular or anterior cardinal.

from this to the duct of Cuvier. The extreme terminal bit of the subcard also persists, joining the posterior cardinal; immediately below this tern section the longitudinal supracardinal vessel joins the subcardinal, and the of the subcardinal disappears. This junction of the two veins corresponds r or less with the entrance of the sixth thoracic intersegmental vein into the of the supracardinal longitudinal vein. These details will be followed r easily, perhaps, with the aid of the diagrams in Fig. 685.

On the right the azygos vein is formed, below the sixth intercostal level, 1 supracardinal elements; at this level a very short ring of subcardinal come and above this it is persistent posterior cardinal.

On the left the conditions are at first symmetrical and similar, but as the innominate vein forms and the intersupracardinal junctions appear behind



, the system is broken in variable ways. The termination of the posterior tal is in the left superior intercostal vein, but the terminal piece of this is true jugular (anterior cardinal). The lower hemiazygos is always supralal.

Development of the Lymphatic System.

ie lymphatic system consists of lymphatic vessels and lymphatic glands des. There is also lymphoid or adenoid tissue, as in the thymus body, tioid follicles and villi of the intestinal mucous membrane, lymphatic es of the spleen, palatine tonsils, and pharyngeal tonsil. There are two 5 in the development of the lymphatic system—primary and secondary. irimary stage is concerned in the formation of lymph-sacs, and the secondary consists in the formation of lymphatic vessels and lymph-glands. fmph-Sacs. —There are two pair of lymph-sacs and two single sacs, as vs:

1. Jugular (2).

2. Retro-peritoneal (1).

3. Cisterna chyli (1).

4. Posterior (2).

pinions differ as to the development of these lymph-sacs. The view of nee R. Sabin will be stated first. According to this lady the sacs are oped as sproutings from the endothelial lining of veins, and this constitutes rimary stage in the development of the lymphatic system, lgular Sacs. —The jugular lymph-sacs, right and left, are the first to appear, is situated on the outer side of the lower part of the primitive anterior nal vein, which becomes the internal jugular vein. It is formed from part capillary venous plexus, connected in early life with the anterior cardinal A large part of this plexus disappears, whilst the connection of the inder with the anterior cardinal vein is severed. There thus results a died collection of capillaries, lined with endothelium and in close proximity e outer side of the lower part of the anterior cardinal vein. These capillaries ne dilated, and subsequently join to form the jugular lymph-sac of each side, h is lined with endothelium, and which establishes a fresh connection with ower part of the anterior cardinal vein, where it is joined by the subclavian a valve being formed at the venous junction by a protrusion of part ie lymph-sac at the place of junction. The jugular sac of each side is ected externally with peripheral lymphatic vessels which extend to the , neck, and fore-limb bud of the same side. The caudal end of the left sac mes connected with the thoracic duct, whilst the caudal end of the right becomes connected with the right lymphatic duct. The dorsal part of sac becomes converted into a plexus of lymphatic vessels, from which is of lymph-glands are developed.

tetro-peritoneal Sac. —The retro-peritoneal or pre-aortic lymph-sac is single, is formed from a capillary venous plexus in the root of the mesentery, which ns is connected with the great pre-aortic transverse anastomotic vessel which nds between the two subcardinal veins. The capillary plexus in the root tie mesentery becomes converted into a lymph-sac, without any venous tection, and this constitutes the retro-peritoneal sac, which establishes a munication with the cisterna chyli, and through it with the thoracic duct, ultimately replaced by a plexus of lymphatic vessels, and from this plexus ns of lymphatic glands are developed which lie along the ventral aspect of abdominal aorta. From the retro-peritoneal sac lymphatic vessels pass in the mesentery along the branches of the superior mesenteric artery, and e form another lymphatic plexus, from which the mesenteric lymphatic leal) glands are developed. Subsequently lymphatic (lacteal) vessels enter wall of the small intestine.

’osterior Sacs. —The sciatic lymph-sacs, right and left, are developed from llary venous plexuses in connection with the two primitive iliac veins. On




either side the sac extends from near the caudal end of the cisterna chyli alo the outer side of the primitive iliac vein. It ultimately becomes convert into lymphatic glands.

Cisterna Chyli .—The cisterna or receptaculum chyli is a single sac, situat at the caudal end of the thoracic duct. It is developed from, and replaces venous plexus. The thoracic duct connects the cisterna chyli with the 1 jugular lymph-sac, and the cisterna chyli communicates with each poster sac. From each jugular sac a vessel grows caudalwards. On the right si this vessel constitutes the right lymphatic trunk, and on the left side it forms 1 thoracic duct. As the thoracic duct approaches the developing aortic arch divides and gives rise to two thoracic ducts, which embrace that arch and th pass to join separately the cisterna chyli opposite the mesonephroi. Subsequen these two ducts fuse, and one duct is formed.

The thoracic duct is developed from a series of venous capillaries, origina having a connection with veins.

Lymphatic Vessels. —The formation of these vessels, along with that ymph-glands, constitutes the second stage in the development of the lympha

Afferent Lymphatics

Fig. 686.—Diagrammatic Section of Lymphatic Gland (Sharpey, fr


system. According to Sabin, the lymphatic vessels are formed from the en thelial lining of the lymph-sacs, whilst the lymph-glands are. developed ft plexuses of lymphatic capillaries. The development of the thoracic duct, as stat takes place from several groups of venous capillaries. It is the largest lympha vessel, and it connects the cisterna chyli with the left jugular sac.

The lymphatics derived from the jugular lymph-sacs pass to the head, ne and fore-limb buds; those from the posterior lymph sacs extend to the hi] limb buds; and those from the retro-peritoneal (mesenteric) lymph-sac en the mesentery, within which lymph-glands (mesenteric) are developed connection with them. From these mesenteric glands, in succession, m lymphatic vessels pass to the intestinal tube.

The other view of the development of the lymphatic system, held by Hu ington amongst others, is that the connection of the lymphatic vessels with venous system is secondary\ and not primary, as Sabin holds. According this other view the lymphatic vessels spring from lymph-spaces, which are forn in the mesoderm (mesenchyme), and are lined with mesothelium. The endothe cells of the lymphatic vessels are derived from this mesothelium, and not fr



lous endothelium, according to Sabin, and the vessels establish a connection le venous system at a later period.

nph-Glands. —Some of these are formed in connection with the lymphdiilst others are formed in connection with peripheral lymphatic vessels, are three stages in the development of a lymph-gland. The first stage s in the formation of a plexus of lymphatic capillaries pervaded by con;-tissue septa. The second stage consists in the invasion of these septa od-capillaries, surrounded by lymphocytes. The third stage consists in mation of a lymph-sinus from the original plexus of lymphatic capillaries.

3 essential elements of a lymph-gland are thus threefold—namely: (1) a of lymph-capillaries; (2) blood-capillaries, surrounded by lymphocytes, connective-tissue septa; and (3) a lymph-sinus.

h lymph-gland is connected with several lymphatic vessels. 1 hose which he gland are known as afferent vessels, and they open into the peripheral deal part of the lymph-sinus. Those which emerge from the gland are L efferent vessels, and they arise in the central or medullary part of the -sinus. In no case does a lymphatic vessel pass uninterruptedly through >h-gland. The gland is a station in the path of a lymphatic vessel, which 3nts the terminus of that vessel, but not the terminus of the lymph. That after leaving the afferent vessel, flows in succession through the cortical edullary parts of the lymph-sinus, and from the medullary part it flows re efferent vessels. It is, therefore, so far as the lymph is concerned, a f, so to speak, changing carriages at a glandular station. During this s any injurious matter is taken up by the branched cells (phagocytes) of nph-sinus, and the lymph is furnished with a contingent of lymphocytes.

Structure of Lymphatic Vessels.

lymphatic vessel, of large size, consists of three coats-—inner, middle, iter. The internal coat {tunica intima) consists of a longitudinal network stic fibres lined with endothelial cells. The middle coat {tunica media) ts of plain muscular and elastic tissues disposed for the most part transy. The external coat {tunica externa) consists of (1) longitudinal bundles of

tive tissue, and (2) plain muscular and elastic fibres, disposed for the part longitudinally, lined with endothelium.

mphatic vessels are furnished with bloodvessels and nerves. Most ol are also provided with valves, similar to those of the veins. Each valve ts of two semilunar segments, facing one another, which are foldings of the coat, containing connective and elastic tissues. They project slightly ds each other, and their free edges are directed in the course of the lymfih1. The valves are situated at short intervals, and they serve to prevent of lymph. When a lymphatic vessel becomes distended it presents a d appearance, with constrictions between the projections, these constrictions ponding to the attached margins of the valvular segments, svelopment. —According to Sabin, the lymphatic vessels are developed from Ldothelial lining of the primitive lymph-sacs (see Development of Lymphatic m).

Structure of Lymphatic Glands.

lymphatic gland consists of a capsule, which encloses the glandular sube. The capsule is composed of connective tissue, containing elastic fibres.

ie glandular substance presents two parts—superficial or cortical, and al or medullary. Each of these parts is permeated by a supporting frarneof trabeculae derived from the capsule. Ihese trabeculae are composed o active tissue, with a few plain muscular fibres. They subdivide the cortex follicles, between which they form incomplete septa. In the medulla the

culae are arranged in a reticular manner. .. ,

be glandular substance is formed by lymphoid or adenoid tissue, wnic sts of retiform tissue, with lymphocytes in its meshes. In the cortex o



the gland this lymphoid tissue is disposed as lymph-follicles, and in the m it forms lymph-cords. In both parts of the gland there are spaces betwe< glandular substance and the supporting trabecular framework. These s which are for the passage of lymph, constitute the lymph-sinus. This si broken up at all parts of the gland by retiform tissue, the meshes of whi partially lined with branched cells of the nature of phagocytes. The glai substance, except the lymph-sinus, is permeated by blood-capillaries.

Lymphatic glands are furnished with bloodvessels, nerves, and lym vessels. The arterioles pass to the glandular substance, being at first enshe by the supporting trabeculae of connective tissue, and subsequently by the re tissue of the glandular substance. The nerves are destined for the plain mu tissue of the trabeculae and bloodvessels.

The lymphatic vessels are of two kinds—afferent and efferent. The a vessels enter the gland over its surface, whilst the efferent vessels emerge definite part of the gland, where there is a slight depression, called the t The afferent vessels convey lymph to the cortical part of the lymph-sinus, the efferent vessels convey lymph from the medullary part of that sinus, much as the medullary part of the gland extends quite to the surface at the where the depression, known as the hilum, exists, the efferent vessels e from the gland through this hilum, which also gives passage to the arterie veins.

Development. —Lymphatic glands are developed partly from the prii lymph-sacs and partly from peripheral lymphatic vessels (see Development Lymphatic System).