CHANGES IN THE CIRCULATION AT BIRTH

The changes which occur in the organs of circulation and respiration at birth, and which lead to the establishment of their permanent condition, are more immediately determined by the inflation of the lungs with air in the first respiration, the accompanying rapid dilatation of the pulmonary blood-vessels with a greater quantity of blood, and the interruption to the passage of blood through the placental circulation. These changes are speedily followed by shrinking and obliteration of the ductus arteriosus, and of the hypogastric arteries from the iliac trunk to the place of their issue from the body by the umbilical cord; by the cessation of the passage of blood through the foramen ovale, and somewhat later by the closure of that foramen, and by the obliteration of the umbilical vein as far as its entrance into the liver, and of the ductus venosus behind that organ.

The process of obliteration of the arteries appears to depend at first mainly on the contraction of their coats, but this is very soon followed by a considerable thickening of their substance, reducing rapidly their internal passage to a narrow tube, and leading in a short time to final closure, even although the vessel may not present externally any considerable diminution of its diameter. It commences at birth, and is perceptible after a few respirations have occurred. It makes rapid progress in the first and second days, and by the third or fourth day the passage through the umbilical arteries is usually completely interrupted. The ductus arteriosus is rarely found open after the eighth or tenth day, and by three weeks it has in almost all instances become completely impervious.

The process of closure in the veins is slower; but they remain empty of blood and collapsed, and by the sixth or seventh day are generally closed.

Although blood ceases at once to pass through the foramen ovale from the moment of birth, or as soon as the left auricle becomes filled with the blood returning from the lungs, and the pressure within the two auricles tends to be more equalised during their diastole, yet the actual closure of the foramen is more tardy than any of the other changes now referred to. It is gradually effected by the union of the forepart of the valve of the fossa ovalis with the margin of the limbus of Vieussens on the left side; but the crescentic margin is generally perceptible in the left auricle as a free border beyond the place of union, and not unfrequently the union remains incomplete, so that a probe may be passed through the reduced aperture. In many cases a wider aperture remains for more or less of the first year of infancy, and in certain instances there is such a failure of the union of the valve as to allow of the continued passage of venous blood, especially when the circulation is disturbed by over-exertion, from the right to the left auricle, as occurs in the malformation attending the morbus cœruleus.

THE LYMPHATIC SYSTEM.

The development of the lymphatic system has been studied in the chick by Budge. Here there exist a network of lymphatics agreeing in their general distribution with the bloodvessels in the vascular area. This is the first lymphatic circulation; a second one is formed later, corresponding with the allantoic circulation. The lymphatics of both systems communicate with the cœlom, but only those of the second circulation communicate with veins. The lymphatics lie in the vascular area above and close to the blood-vessels, but are separated near the embryo by a layer of mesoblast continuous with the splanchnopleure.

The method of development of lymphatic vessels and lymphatic glands, is dealt with in the chapter on Histology.

RECENT LITERATURE,

Blaschek, A., Untersuchung über Herz, Pericard, Endocard und Pericardialhöhle, Mittheil. aus dem embryol. Institut der Universität Wien, 1885.

Born, G., Entwicklung des Säugethier-herzens, Archiv f. mikr. Anat., Bd. 33, 1889.

Budge, A., Untersuchungen über die Entwickelung des Lymphsystems beim Hühnerembryo, Arch. f. Anat. u. Physiol., Anat. Abtheil., 1887.

Fürstig, J., Untersuchungen über die Entwickelung der primitiven Aorten mit besonderer Berücksichtigung der Beziehungen derselben zu den Anlagen des Herzens, Schriften herausgegeben von der Naturf. Gesellsch. bei der Univers. Dorpat, I., 1884.

Greenfield, W. S., Case of malformation of the heart, &c., Journal of Anat. and Physiol., 1890. Hochstetter, F., Beiträge zur Entwickelungsgeschichte des Venensystems der Amnioten, Morpholog. Jahrbuch, Bd. xiii., 1888.

Mackay, J. Yale, The development of the branchial arterial arches in birds, with special reference to the origin of the subclavians and carotids, Philosophical Transactions, B, 1888.

Marius, J., Quelques notes sur le développement du cœur chez le poulet, Arch. de biol., 1889. Mayer, P., Ueber die Entwickelung des Herzens und der grossen Gefässstämme bei den Selachiern, Mitth. d. zool. Station zu Neapel, Bd. vii., 1887.

Müller, Erik, Studien über den Ursprung der Gefässmuskulatur, Archiv. f. Anat. u. Physiol. Anat. Abth., 1888.

Rabl, C., Ueber die Bildung des Herzens der Amphibien, Morphol. Jahrbuch, 1886; Ueber die Bildung der Herzanlage, Wiener Medicin. Presse, 1886.

Röse, C., Beiträge zur Entwickelungsgeschichte des Herzens, Dissert. Heidelberg, 1888, and Morphol. Jahrb. xv.

Rückert, J., Ueber den Ursprung des Herzendothels, Anat. Anzeiger, 1887; Ueber die Entstehung der endothelialen Anlagen des Herzens und der ersten Gefässstämme bei Selachier-Embryonen, Biolog. Centralb., Bd. viii., 1888.

Schimkewitsch, W., Ueber die Identität der Herzbildung bei den Wirbel- und wirbellosen Thieren, Zoolog. Anz., 1885.

Schwink, F., Ueber die Entwickelung der Herzendothels der Amphibien, Anat. Anzeiger, 1890. Türstig, J., Mittheilungen über die Entwickelung der primitiven Aorten nach Untersuchungen an Hühnerembryonen, Dissert. Dorpat., 1886.

Van Bemmelen, J. F., Die Visceraltaschen und Aortenbogen bei Reptilien und Vögeln, Zool. Anzeiger, 1886.

Zimmermann, W., Ueber ein zwischen Aorten- u. Pulmonalbogen gelegenen Kiemenarterienbogen beim Kaninchen, Anat. Anzeiger, 1889.

DEVELOPMENT OF THE

SEROUS CAVITIES AND OF THE MUSCLES AND SKELETON.

The serous cavities-peritoneum, pleuræ, pericardium-are derived from the original split or cleavage of the mesoblast, which constitutes the cœlom or general body cavity (pleuro-peritoneal cavity of older authors). This cleft is formed in the head as well as in the trunk, and when the heart is formed as a double tube, each half is enclosed within a portion of that cavity, which later on, when the body walls bend round and meet to enclose the fore-gut, comes, like the heart itself, to occupy a position on the ventral aspect of the alimentary tube. The part of the cœlom which thus contains the heart is not for some time entirely distinct, but communicates dorsally by two comparatively narrow channels with the anterior part of the general body cavity, here separated into lateral halves, which ultimately become the pleuræ,1 by the alimentary canal. Subsequently these communications become obliterated, and the heart-coelom separated as a distinct cavity (pericardial cavity). Below, where the great veins enter the heart, they pass into a mass of mesoblastic tissue, which is connected with the anterior body wall (where it receives the umbilical and vitelline veins), and also with the lateral wall (where it receives the ducts of Cuvier), and which, as the heart bends, so that the venous end passes behind the ventricle and bulb, is carried along with the veins up behind that organ, and thus forms an obliquely placed thick septum, at first incomplete, but subsequently becoming entirely closed, which separates the heart within the pericardial part of the body cavity in front and above from the stomach and alimentary canal within the peritoneal part of that cavity behind and below. The thick septum, besides containing the saccus reuniens and the portions of the great veins (vitelline, umbilical, ducts of Cuvier) which open into that cavity, also contains the rudiments of a part of the diaphragm and the mesoblastic part of the liver, into which the hypoblastic part grows from the adjacent duodenum; it has been termed by His the transverse septum (see figs. 177, 178). As development proceeds, the septum becomes gradually differentiated into its several parts. The great veins become still further shifted behind the heart, and the saccus reuniens becomes incorporated with that organ. The liver, which is at first contained entirely within the septum, becomes split off from its upper layer, which now forms the thin portion of the diaphragm, while the cavity of the peritoneum extends from either side, and separates them from one another, except along the attachment of the broad ligaments.

The diaphragm is completed by a growth of mesoblast which occurs on each side, and cuts off the anterio-dorsal portions of the body-cavity into which the lungs are invaginated (recessus pulmonales) from the posterior or peritoneal part.

The serous membranes are formed by differentiation of the lining mesoblast of the cœlom.

The formation of the omenta, and the changes which the mesenteric folds of peritoneum undergo, have been already mentioned in connection with the development of the abdominal viscera.

Development of the muscles.-The muscles of the trunk are formed from the protovertebræ. These are at first, as previously described, separate masses of mesoblast, the cells of which have at the periphery of the mass a tendency to a radial disposition (fig. 189, A), whilst toward the centre they are loosely arranged,

1 The manner in which the pleuræ are invaginated by the growing lungs has already been alluded to (p. 110).

and may even leave a more or less distinct space unoccupied by cells (protovertebral cavity). This cavity has occasionally been noticed (Lockwood, Bonnet) to be continuous laterally with the mesoblastic (coelomic) cleavage (fig. 189, B; see also

A

B

Fig. 189.-Two SECTIONS OF A SHEEP
EMBRYO. (Bonnet.)

A, shows the cavity within the protovertebræ. In B, the protovertebra on the left side of the section is united with the lateral mesoblast; on the right side its cavity also is continuous with the coelomic cleft in that mesoblast. am, amnion; n.c, neural canal; p. v, protovertebra; ao, aorta; p.p, pleuro-peritoneal space (coelom).

fig. 139, p. 117), and it is probably the morphological equivalent of the coelom in this part of the mesoblast. Whether there be originally a cavity or not in it, the protovertebra presently becomes filled up with cells and then forms a fairly compact mass of cells which are mostly irregularly arranged, but externally (next to the cutaneous epiblast) become regularly disposed into an epithelium-like plate of columnar cells. This is known as the muscle plate, and when the inner part of the protovertebra becomes broken up as a distinct mass and joins with the neighbouring protovertebræ to form the membranous vertebral column (see below), the muscle plates still remain distinct in them therefore the original

which immediately lines the interna. surface of the columnar layer and forms an inner muscle-plate (fig. 190). It is uncertain whether the cells of this inner muscleplate are derived from part of the columnar layer which has folded over, or whether they spring from other cells of the protovertebra. Soon after their appearance as a distinct layer of the muscle-plate they begin to elongate in the sagittal (antero

posterior) direction (fig. 191, i.m.), and it may presently be observed that they are becoming developed into longitudinal groups or segments of muscle-fibres which stretch between the original intervals between the protovertebræ. The destination of the outer layer of the muscle-plate has not been traced with certainty. Balfour

which has been derived from the inner parts of the protovertebræ, partly interrupted by the ganglion rudiments, gl. The original intervals between the protovertebræ here are still indicated by vessels, v. i.cl, cleft in the deeper protovertebral tissue (according to Ebner this is the remains of the original protovertebral cavity).

described it as also eventually becoming transformed into muscle-fibres (in elasmobranchs), but others have failed to confirm this opinion, and it is by some believed that it may assist in the formation of the cutis vera.

Although the muscle-plates are originally mainly concerned with the formaticr of the muscles which move the central skeletal axis, it is probable that all the skeletal muscles both of the trunk and limbs are eventually derived from them (see below, p. 163).

Formation of the muscles of the head and evidences of head segmentation.— Although perhaps no part of the cranium actually represents a vertebra, there is nevertheless abundant evidence of an original segmentation of the head corresponding with the mesoblastic somites of the trunk. Such segmentation is shown by the existence of the visceral arches, which in the typical and least modified vertebrates (e.g., elasmobranchs) are at least nine in number, by the successive separation of the part of the body cavity which extends into the head into separate portions, or head cavities, one corresponding to each visceral arch, the parietes of which develop into muscles, and which therefore correspond with the muscle. plates of the proto vertebræ of the trunk (this is in fact the typical mode of formation of mesoblastic somites, r. page 26), and lastly by the mode of development of the cranial nerves and their relations to the visceral and branchial arches, which correspond in a general way with the relations of the ventral branches of the spinal nerves to the ribs.

The formation and destination of the head cavities have been investigated of late years (chiefly in elasmobranchs, but also in reptiles and birds) by Balfour, Milnes Marshall, and van Wijhe, and the result of these investigations tends to show that, in all, nine portions of the original head cavity become separated off on either side, their formation proceeding from behind forwards. Each somite cavity becomes subsequently divided into a dorsal part corresponding with the protovertebræ of the trunk and a ventral part, corresponding with the pleuroperitoneal cavity, and lying in the middle of the corresponding visceral arch (1). Both parts give rise by differentiation of their parietes to muscles; the visceral arch portions to the muscles of the jaw and hyoidean and branchial apparatus ; the dorsal portions, some (first. second, and third) to the muscles which move the eyes, some (seventh, eighth, and ninth) to the muscles which connect the head with the shoulder girdle, whilst some, viz., the fourth, fifth, and sixth, are said to disappear. The first head cavity forms the eye muscles, which are supplied by the third nerve; the second, the muscle supplied by the fourth (superior oblique); and the third, the muscle supplied by the sixth (external rectus). In higher vertebrates, the formation of the head cavities and their subsequent destination have not been as yet clearly followed out, although indications of their existence are not wanting.

According to v. Wijhe the intermediate part of the typical somite cavity represents a segmental (ariniferous) organ, but this intermediate part is not seen in the head, although it begins to appear in the immediately succeeding somites.

VOL. I.

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