ascending ramus of the mandible, and a smooth joint-like surface, fitting into the upper concavity of the inverted angle, answering to the persistent inner articular part of the condyle in birds. The fourth hamal arch is close to the occiput in the Ruminant, and retrogrades as the neck is lengthened out by vertebræ interposed between head and chest. It retains, in Cetaceans,

almost the typical position exemplified in Fishes.

The common ossification of articular ends of bones from centres distinct from that of the shaft is a mammalian developmental characteristic. The ultimate confluence of the epiphyses' (vol. ii. p. 297) with the diaphysis' indicates maturity of growth: but in this relation there are differences in the same skeleton and in different species. In Man the epiphyses of the limb-bones toward which the arteriæ nutritiæ' run (p. 619) first coalesce with the shaft; those at the distal end of the humerus and proximal ends of the two antibrachials, e. g., at puberty, those at the opposite ends of the same bones at the twentieth year. The proximal epiphysis of the femur coalesces about the eighteenth year, the distal one at the twentieth; the proximal epiphysis of the tibia joins the shaft about the twenty-fifth year, the distal epiphysis five years earlier. The epiphyses of the vertebral bodies coalesce about the twenty-first year in Bimana, but they continue distinct for a much longer proportional period of life in Cetacea. Epiphyses and short bones of limbs, those of the carpus and tarsus, e. g., continue cartilaginous some time after the shafts of the long bones are ossified, as shown in fig. 586. This figure also exemplifies the early manifestation of ordinal characters; the inner digit of the pelvic limb, in the foetal Monkey (Cercopithecus

a

586

b

sabæus) already shows by its relative shortness and divergence from the others that it is destined to oppose them, and to terminate the member by a prehensile hand: while, from the earliest manifestation of the digits of the same limb in the human embryo, the hallux' by its proportions and parallelism with the other toes indicates the destination of the answerable. part to become a plantigrade foot, perfected to sustain and move the body of an erect Biped.

From fetal lower limb; nat. size. a, Monkey, b, Man. CCLXXVII",

§ 409. Membrana pupillaris.—The differences in degree of indi

1 ccxcv". p. 727.

A

587

B

vidual development attained at birth parallel, in Mammals, those in Birds expressed by the terms altrices and præcoces (vol. ii. p. 265). The hoofed quadruped enters the world with the use of all its senses; in a few hours can follow the dam, and keep pace with her if she sees cause for flight: the feline is born blind and helpless; some days elapse ere the commissure of the eyelids is unsealed. Corresponding steps in the human organ of vision are completed before birth. At the fifth month of foetal life the pupillary aperture is very wide, and is occupied by a rich layer of looped capillaries supported by a production of the membrane of the aqueous humour, fig. 587, A. As the iris is developed the pupil contracts and the vessels of the pupillary membrane diminish in size and number; so that at the eighth month only a few vessels are seen crossing the transparent membrane, as at B. Shortly before birth, or for a week after, a mere shred of the membrane may be detected, as in C and D, and these are soon absorbed.

D

§ 410. Fatal circulation.-The early stages in the development of the vascular system closely correspond, in Mammals, with those in Birds (vol. ii. p. 263, fig. 136): the steps in the establishment of the aortic arch, with their relations to conditions of primary branches characteristic of species, and to rare anomalies, have been explained at pp. 534-537; here, therefore, there only remain a few words to be said of the foetal characters of the circulating system.

The blood of the foetus, after passing through the ramifications of the allantoic arteries, fig. 588, u', u, in the placenta, returns by the allantoic vein, u. This, on entering the abdomen, passes above and superficial to the duodenum, within the peritoneal fold called suspensory ligament' of the liver, to the great fissure of that organ, where it carries part of its blood directly, by the 'ductus venosus,' d, to the post-caval, v, and part is distributed by the branches of the portal vein, L, through the substance of the liver, and is then conveyed by the hepatic veins, l, into the general current of the returning blood. Thus, the right auricle

588

of the heart, h', receives not only the blood which has circulated through the body of the foetus, but also that which has passed through the placenta, consequently a mixture of venous and arterial blood; the blood in the precaval, v, being entirely venous, that in the post-caval, v, being mixed. A part of this blood so accumulated in the right auricle passes into the left auricle, h, by the foramen ovale,' f, in the septum auricularum, and it is chiefly the blood from the postcaval which takes that course. The rest of the blood entering the right auricle passes into the right ventricle, H', and thence into the pulmonary artery: but very little blood is sent to the collapsed lungs, for a passage of communication continues from the pulmonary artery into the descending aorta by retention of part of the third primitive arch, fig. 420, forming the ductus arteriosus," fig. 588, D; thus the greater mass of the blood, which in the adult would have proceeded to the lungs, is in the foetus immediately transmitted to the aorta, A. after its origin from the left ventricle, delivers almost all the blood expelled by the contraction of that cavity into the carotid and subclavian arteries, while the ductus. arteriosus passing between the trunk of the pulmonary artery and the descending aorta directs the blood which passes through the right ventricle to the lower regions of the body. In this manner the upper regions are supplied with the most arterialised part of the blood from the left side of the heart and aorta, while the purely venous blood is propelled from the right ventricle through the pulmonary artery and ductus arteriosus into the descending aorta, and consequently into the lower part of the body, and by the allantoic arteries to the placenta. The circulation in the foetal Mammal thus offers a close and interesting analogy to that in adult Crocodilian Reptiles (vol. i. p. 512).

This,

The foramen ovale in the septum of the auricles, the ductus arteriosus passing from the pulmonary artery to the aorta, the ductus venosus leading from the allantoic vein to the post-caval, and the allantoic (umbilical) vein and arteries, are the structural peculiarities of the mammalian foetal circulating organs. These

passages are all closed up, and the allantoic vessels obliterated at the navel, after pulmonic respiration is established at birth.

589

§ 411. Definition of Male and Female Organs.-In the Mammalian as in other vertebrate embryos the urogenital parts, before showing distinction of sex, appear in a seemingly more complex or multiplex condition than when perfected at a later stage. As in fig. 589, we recognise the basis of the true or persistent kidneys, a, with their duct, e; the antecedent deciduous kidney, or Wolffian' body, b, is here on the wane, with its excretory duct, f; the beginning of the essential genital gland is marked c, the adrenals, d, and the tubes called ducts of Müller,' g.

In

In the male Mammalian embryo the duct, f, becomes connected by a white granular process with a similar one from the gland, c, on the inner side of the Wolffian body: by the union of these offshoots is formed the epididymis, and the gland c can then be determined as the testis: the ducts, g, shrink and are metamorphosed into the protometra. the female there are not such converging growths between the duct f and the gland c: the duct f shrinks with the Wolffian body, and is reduced to the remnant recognisable in the adult as a 'canal of Malpighi.' But the tube, g, rises above the Wolffian body, expands as at e, fig. 590, and afterwards opens at f. As the Wolffian body atrophies, the duct, f, fig. 589, begins to be tortuous in the male, and becomes spermduct'; while g shrinks: but, in the female, g widens, and becomes, as in fig. 591, oviduct, c, and uterine horn, b: but the distinction is late in the Ruminant embryo.

In the human embryo at

590

Urogenital organs, foetal Sheep. cccxx1".

three months, the lower or distal portions of c, c, fig. 592, have dilated and become fused into the uterus, a, which still shows

the indent of its primitive division. The remnant of the Wolffian body, e, e, is long recognisable as the 'parovarium.' The ovaries, d, d, are smooth and elongate; the round ligaments, b, b, are relatively large.

In certain malformations more or less of the primitive condi

Female organs, fœtal Deer. cccxxi".

tions of the genital organs are maphrodite' states of the parts. with which the ducts, f, in fig.

Female organs, Human foetus of three months. anu"

retained, and give rise to her-
In fig. 593, a, a, are the testes
589, had effected their union,
and become '
vasa de-
ferentia,' fig. 593, d, d:
b is a combination of epi-
didymis with the abdo-
minal ends of the 'mul-
lerian ducts,'g, figs. 589,
593, here continuing
closed and having be-
come adherent to the
mass including a rem-
nant of the Wolffian body.
Development of the mul-
lerian ducts has, how-
ever, proceeded to a de-
finition of the oviduct or
fallopian tube, fig. 593,
q, and of the uterine
horn, f, f, with the body
of the uterus and vagina,
e: it is normally reduced
to protometra,' in fig.
525, c.

Genital organs of Hermaphrodite Goat, the male parts
predominating. CCXXXIX".

§ 412. Descent of testes.-In all Mammals, save the true testiconda, a preparatory structure is established for either periodical or permanent withdrawal of the testes from out the abdomen.