to the right auricle of the heart, into which cavity is also pouring the blood that has circulated in the head and neck and arms, and has been brought to the auricle by the superior vena cava. It might be naturally expected that the two streams of blood would be mingled in the right auricle, but such is not the case, or only to a slight extent. The blood from the superior vena cava,—the less pure fluid of the two-passes almost exclusively into the right ventricle, through the auriculo-ventricular opening, just as it does in the adult; while the blood of the inferior vena cava is directed by a fold of the lining membrane of the heart, called the Eustachian valve, through the foramen ovale into the left auricle, whence it passes into the left ventricle, and out of this into the aorta, and thence to all the body, but chiefly to the head and neck. The blood of the superior vena cava, which, as before said, passes into the right ventricle, is sent out thence in small amount through the pulmonary artery to the lungs, and thence to the left auricle, as in the adult. The greater part, however, by far, does not go to the lungs, but instead, passes through a canal, the ductus arteriosus, leading from the pulmonary artery into the aorta just below the origin of the three great vessels which supply the upper parts of the body; and there meeting that part of the blood of the inferior vena cava which has not gone into these large vessels, it is distributed with it to the trunk and lower parts,-a portion passing out by way of the two umbilical arteries to the placenta. From the placenta it is returned by the umbilical vein to the under surface of the liver, from which the description started.

Changes after Birth.-After birth the foramen ovale closes and so do the ductus arteriosus and ductus venosus, as well as the umbilical vessels; so that the two streams of blood which arrive at the right auricle by the superior and inferior vena cava respectively, thenceforth mingle in this cavity of the heart, and passing into the right ventricle, go by way of the pulmonary artery to the lungs, and through these, after purification, to the left auricle and ventricle, to be distributed over the body. (See Chapter on Circulation.)

The Nervous System.

The Cranial and Spinal Nerves.-The cranial nerves are derived from a continuous band, called the neural band. They are formed before the neural canal is complete. The neural band is made up

of two laminæ going from the dorsal edges of the neural groove to the external epiblast. It becomes separated from the epiblast, and then forms a crest attached to the upper surface of the brain. The posterior roots of the spinal nerves arise as outgrowths of median processes of cells from the dorsal side of the spinal cord, which become attached laterally to the spinal cord as their original point of attachment disappears. The anterior roots probably arise from the ventral part of the cord as a number of strands for each nerve. They appear later than the posterior roots. The rudiment of the posterior root is differentiated into a proximal round nerve connected to the cord, a ganglionic portion and a distal portion. To the last the anterior nerve-root becomes attached.

The Spinal Cord.-The spinal cord consists at first of the un

Fig. 482.-Diagram of development of spinal cord: cc, central canal; af, anterior fissure; Pf, posterior fissure; g, grey matter; w, white matter. For further explanation see

text.

differentiated epiblast of the walls of the neural canal, the cavity of which is large, with almost parallel sides. The walls are at first composed of elongated irregular nucleated columnar cells, arranged in a radiate manner. The cavity then becomes narrow in the middle and of an hour-glass shape (fig. 482). When the spinal nerves make their first appearance, about the fourth day in the chick, the epiblastic walls become differentiated into three parts: (a) the epithelium lining the central canal; (b) the grey matter; (c) the external white matter. The last is derived from the outer most part of the epiblastic walls by the conversion of the cells into longitudinal nerve-fibres. The fibres being without any myelin sheath, are for a time grey in appearance. The white matter corresponds in position to the anterior and posterior nerve-roots, and are the anterior and posterior white columns. It is at first a very thin layer, but increases in thickness until it covers the whole cord. The grey matter too arises from the cells by their being prolonged into fibres. The change in the central cells is sufficiently obvious. The anterior and posterior cornua of grey matter and the anterior grey commissure then

appear. The anterior fissure is formed on the fifth day by the growth downwards of the anterior cornua of grey matter towards the middle line. The posterior fissure is formed later. The whole cord now becomes circular. The posterior grey commissure is then formed.

When it first appears, the spinal cord occupies the whole length

Fig. 483.-Early stages in development of human brain (magnified). 1, 2, 3, are from an embryo about seven weeks old; 4, about three months' old. m, middle cerebral vesicle (mesencephalon); e, cerebellum; o, medulla oblongata; i, thalamencephalon; h, hemispheres; infundibulum; Fig. 3 shows the several curves which occur in the course of development; Fig. 4 is a lateral view, showing the great enlargement of the cerebral hemispheres which have covered in the thalami, leaving the optic lobes, m, uncovered. (Kölliker.)

N.B.-In fig. 2 the line i terminates in the right hemisphere; it ought to be continued into the thalamencephalon.

of the medullary canal, but as development proceeds, the spinal column grows more rapidly than the contained cord, so that the latter appears as if drawn up till, at birth, it is opposite the third lumbar vertebra, and in the adult opposite the first lumbar. In the same way the increasing obliquity of the spinal nerves in the neural canal, as we approach the lumbar region, and the "cauda equina" at the lower end of the cord, are accounted for.

Brain. We have seen that the front portion of the medullary canal is almost from the first widened out and divided

into three vesicles. From the anterior vesicle (thalamencephalon) the two primary optic vesicles are budded off laterally: their further history will be traced in the next section. Somewhat later, from the same vesicle the rudiments of the hemispheres appear in the form of two outgrowths at a higher level, which grow upwards and backwards. These form the prosencephalon.

In the walls of the posterior (third) cerebral vesicle, a thickening appears (rudimentary cerebellum) which becomes separated from the rest of the vesicle by a deep inflection.

At this time there are two chief curvatures of the brain (fig. 483, 3). (1.) A sharp bend of the whole cerebral mass downwards round the end of the notochord, by which the anterior vesicle, which was the highest of the three, is bent downwards, and the middle one comes to occupy the highest position. (2.) A sharp bend, with the convexity forwards, which runs in from behind beneath the rudimentary cerebellum separating it from the medulla.

Thus, five fundamental parts of the foetal brain may be distinguished, which, together with the parts developed from them, may be presented in the following tabular view :—

Table of Parts Developed from Fundamental Parts of Brain.

The cerebral hemispheres grow rapidly upwards and backwards, while from their inferior surface the olfactory bulbs are budded off, and the prosencephalon, from which they spring,

remains to form the third ventricle and optic thalami. The middle cerebral vesicle (mesencephalon) for some time is the most prominent part of the foetal brain, and in Fishes, Amphibia, and Reptiles, it remains uncovered through life as the optic lobes. But in Birds the growth of the cerebral hemispheres thrusts the optic lobes down laterally, and in Mammalia completely overlaps

them.

In the lower Mammalia the backward growth of the hemispheres ceases as it were, but in the higher groups, such as the monkeys and man, they grow still further back, until they completely cover in the cerebellum, so that on looking down on the brain from above, the cerebellum is quite concealed from view. The surface of the hemispheres is at first quite smooth, but as early as the third month the great Sylvian fissure begins to be formed (fig. 483, 4).

The next to appear is the parieto-occipital or perpendicular fissure; these

Fig. 484.-Side view of fatal brain at six months, showing commencement of formation of the principal fissures and convolutions. F. frontal lobe; P, parietal; 0, occipital; T, temporal; a a a, commencing frontal convolutions; s, Sylvian fissure; s', its anterior division; c, within it the central lobe or island of Reil; r, fissure of Rolando; p, perpendicular fissure. (R. Wagner.)

two great fissures, unlike the rest of the sulci, are formed by a curving round of the whole cerebral mass.

In the sixth month the fissure of Rolando appears: from this time till the end of foetal life the brain grows rapidly in size, and the convolutions appear in quick succession; first the great primary ones are sketched out, then the secondary, and lastly the tertiary ones in the sides of the fissures. The commissures of the brain (anterior, middle, and posterior), and the corpus callosum, are developed by the growth of fibres across the middle line.

The Hippocampus major is formed by the folding in of the grey matter from the exterior into the lateral ventricles. The essential points in the structure and arrangement of the various parts of the brain, are diagrammatically shown in the two accompanying figures (figs. 483, 484).