415

sheet of pellucid membrane, to which are adherent fine reticulate fibres, mostly affecting a longitudinal direction. It is also frequently perforated with small holes, fig. 415, a, a, from which circumstance it is called fenestrate.' This homogeneous membrane has the property of rolling itself up in the form of a scroll, somewhat like the elastic laminæ of the cornea. It is strengthened in many parts by longitudinal anastomosing fibres of elastic tissue; and together with the epithelial deposit forms the inner coat' of the artery. The middle coat' consists of a fibrous tissue, circularly disposed, in layers more numerous as may be the size of the artery and thickness of the coat, fenestrate tissue intervening; of a reddish-yellow, clearer when fresh than yellow elastic tissue: it consists of bundles of slender fusiform filaments, commonly nucleate, with fine elastic fibres traversing them in a reticulate manner. Acetic acid dissolves the chief substance of the filament, and demonstrates the long staff-shaped nucleus, fig. 416, a, and the cell-wall.' This muscular tissue' predominates in the smaller arteries; of which, when treated by soluble reagents,

Fenestrate membrane.

IF

Fusiform nucleate filaments, or 'muscular fibre-cells. 1. Natural. 2. Treated with acetic acid. CCVIII".

the coats present the appearance shown in the portion of a splenic arteriole, fig. 417, where c is the outer coat with the sheath of areolar tissue, e the elastic inner coat, and d the dissolved middle

or muscular coat. The external coat consists of an inner stratum of elastic fibres, and an outer one of the same, blended with a large proportion of closely-fitted bundles of white fibres, identical with those of the areolar tissue of the arterial sheath. By virtue of the above-described structures arteries possess not only elasticity, but an allied power of slow and long-enduring contraction, excitable by stimulus of touch, cold, and electricity during life; and lost after death.

In the Mammalian class the aorta, fig. 418, A, bends over the

418

a

left, not over the right bronchial tube. The chief primary branches of the arch are given off, not immediately after, but at a little distance from, its origin; and there is less constancy in the order of their origin than in birds: the phrenic arteries, the coeliac axis, and the superior mesenteric artery are branches of the abdominal aorta, which terminates, save in Mutilata, by dividing beyond the kidneys into the iliac arteries, from which usually spring both the femoral, a, and ischiadic branches: the caudal or sacromedian artery, which, in Mutilata and long-tailed quadrupeds, assumes the character of the continued trunk of the aorta, never distributes arteries to the kidneys, rarely to the legs, as it does in birds. After the arteries to the heart (coronaries') the aortic arch sends off those to the head ('carotids') and to the pectoral limbs (brachials'). I use, with Barclay, the latter term in preference to those by which Anthropotomy designates for surgical purposes parts of the same artery, as where it passes beneath a clavicle (as subclavian'), or sinks into the arm-pit (as axillary') before reaching the arm. The principal varieties in the origins of the large primary branches of the aortic arch, characteristic of Mammalian genera or families, are given in the order of their complexity in fig. 419.

V

Central organs of circulation in Man. CCLXVII.

In Tapirine, Equine, Bovine, and most ruminant Ungulates,

the aorta sends off a large common trunk, a, b (the anterior aorta' of Veterinarians), which divides into two brachiocarotids, each subdividing after a longer or shorter course into the brachial d or d', and the carotid e ore of its respective side: the vertebral artery, v, is given off by the brachial. The arch of the aorta, diminished after dismissing b, is the posterior aorta' of Hippotomy; and, indeed, in this variety the trunk of the arterial system appears to bifurcate shortly after its origin. In the Rhinoceros the anterior aorta' sends off the two internal thoracics, the two brachials, and a common trunk subdividing into the two carotids.' In Auchenia, fig. 419, B, the left brachial, d',

A, Ox. B, Lama. c, Giraffe. D, Lion. E, Otter. F, Gibbon. G, Hedgehog. H, Man. I. Dugong.

comes off close to, but distinct from, the innominate trunk, b; which, after dismissing the right brachial, d, sends onward a long common bi-carotid trunk, dividing into c, c'. A similar arrangement obtains in the Giraffe, ib. c; but the bi-carotid is still longer before its division, and the left internal thoracic, v', has a distinct origin from the aorta, a, beyond that of the left brachial, d'. In Suide a longish innominata gives off the right brachial and both carotids, almost at the same terminal point: the left brachial rises close to the innominata. In the Elephant I found a short innominata giving off the right brachial and both carotids, the

3

left brachial having a distinct origin, but more remote than in the Hog and Giraffe.' A like condition prevails in the order Carnivora, ib. D. In the Otter a longer interval divides the origin of the left brachial, d, from the innominata, b; which, after sending off the left carotid, c', is continued as a brachio-carotid trunk a short way before dividing into the right carotid, c, and right brachial, d.2 In the Quadrumana, from the Aye-aye up to and including Hylobates and Pithecus, the innominata, ib. F, b, gives off, first the right brachial, d, and then a short bi-carotid trunk. In the Hedgehogs, Moles, and Bats, there are usually two symmetrical brachio-cephalics, G, b, b'. Cuvier ascribes a like condition to Delphinus; but in Phocana the otter-type, E, is repeated, only with relatively smaller brachials and larger carotids. Hyperoödon and Whales, the Seals, Beavers, Rats and most claviculate Rodents, the Ornithorhynchus and Chimpanzees partake, with Man, of the mode of origin shown in H: the innominata b being the common trunk of the right carotid e and brachial d. The same pattern obtains essentially in Sirenia, but with wider intervals between b, c, and d', and with a distinct origin of the left internal thoracic artery, v'.

These varieties, pretty constant in the groups they characterise, are to be distinguished from the anomalies which are exceptional in species. Both, and especially the latter, are explicable by reference to modified or arrested stages of development; and an embryonal phase, exemplified in fig. 420, affords a ground-plan on which most Mammalian arrangements of the aortic arch and branches can be laid down, or from which they can be picked out.

In the rare mammalian anomaly of a double aorta bending, one over the right, the other over the left bronchus, before uniting to form the descending trunk, the second of the three pairs of similar vessels by which the blood passes from the heart to the dorsal vessel in the embryo is retained, and such persistent aorta answer to the vessels A, A, D, fig. 420 (in Saurians). When a single aorta is found bending over the right bronchus, the primitive vascular arch A' is retained, and A D is obliterated, as in Birds: this arrangement is a rare anomaly, the rule in mammals being to retain the left of the mid-pair of primitive vascular arches, A, D, with complete obliteration of the right arch A'.5 In the variety A, fig. 419,

cxcv. p. 61. Cuvier seems to have found the right as well as left brachial rising separately, and between them the carotids by a common trunk, XII. tom. vi. p. 112. CXCV. p. 16. cxcn". p. 5.

2

3

Ib. p. 15.

I have failed to find in any embryo of bird or mammal more than three pairs of primitive vascular arches, conveying the blood, in that form, from the heart to the dorsal aorta. In the exceptional minority of Vertebrates, in which branchia are deve

420

characteristic of the Horse and Ox, the common trunk of the foremost pair of vascular arches, fig. 420, a, is retained and lengthened, the arches being modified into brachials, fig. 419, d ď, and carotids, e c', and the communications with the succeeding arches obliterated in most of the other varieties the communication of the left of the second pair with that of the first pair of primitive arches, as at fig. 420, D, persists and becomes the distinct origin of the left brachial, a*, the intermediate part of the first left arch being obliterated as far as the artery to the head, or the trunk transmitting such. But this way of explanation has its limits. Most of the varieties in fig. 419 bear relation to the breadth of the chest, with which that of the heart and aortic arch, in a measure, coincides. Thus, in the non-claviculate narrow-chested Ungulates the

a

varieties A, B, C, are met with, that Primitive vascular arches, as retained in of A prevailing: in non-claviculate,

Saurians.

but broader-chested Unguiculates, with flexile and rotatory fore limbs, the separate origin of the left brachial is more constant and remote from the innominata: the same is better marked in the broader-chested Swimmers (Lutra, Phocana, E), and in the claviculate Quadrumana, F: in many Insectivora G, an analogous but other arrangement prevails. In the broad-chested species illustrating the variety H, the head and pectoral limbs are supplied by three primary trunks: in the still broader and flatterchested Sirenia, I, the heart itself is able to expand laterally, even to a partial severance of the ventricles, the aortic arch shows its widest span, the intervals between the innominata, b, the left carotid, c', and the left brachial, d', are longer, and the left internal thoracic artery has likewise an independent origin.

I have not met with an instance of a double aorta, or of a single one arching over the right bronchus, or of the origin of the right brachial from the termination of the arch, in any mammal below Man: but such rare anomalies may, perhaps, be found when as many individuals of the brute have been anatomised as those of the human kind.

loped from the primitive arches, four or more of these may exist. But the notion of the human embryo having gills and gill-slits tickles the fancy; and so the term 'branchial' may long continue to be misapplied to the hæmal vascular arches and blastemal folds of the fœtal mammal, bird and reptile.