Dissected so as to show the main points characteristic of Aves, and the arrangement of the principal muscles of flight.

Pigeon, Columba livia.

Plate II. Pigeon, Columba livia.

The characters distinctive of Birds shown in this figure are the following: feathers; epidermic scales to the feet; musculature of the wing; characters of the brain; oesophageal crop (absent in some Birds); large size of duodenal loop; pancreas and the number of ducts to this gland; the two coeca; extremely short large intestine as compared with the length of the small intestine; heart resting in a deep notch of the liver; single aorta crossing the right bronchus; mode of division of the innominate artery; deep indentation of the lungs by the ribs; tubular structure of the lungs; and trilobed kidney adapted to the pelvic fossae.

There is no diaphragm as in all lower Vertebrata: the cloaca, as in Crocodilia, Chelonia and Amphibia, receives separately the rectum, urinary and generative ducts: the testes are permanently retained within the abdomen as in a few Mammals and all lower Vertebrata: the bladder is absent as in all Lacertilia and Ophidia.

In some Mammals a portion of the stomach is purely receptive, but no Mammal developes an oesophageal crop; and it is as rare for a Mammal to possess two coeca as it is for a Bird to possess one. The only Mammals - the Prototheria - which have a cloaca, have also a sinus urogenitalis.

a. Right cerebral hemisphere. Its surface is smooth, contrasting with that of the transversely laminated cerebellum seen behind in the median line.

b. The crop, which is bilocular in the Columbidae. A window has been made in its right wall to show its division into two compartments.

c. Right lobe of liver.

d. Heart. The ventricular portion is more acutely conical in most Birds than in Mammals, and the auricles are smaller in relation to the ventricles.

e. Loop of duodenum in which are contained the longitudinally arranged lobes of the pancreas. Into this loop of intestine three ducts open from the pancreas and two from the liver, which has no gall-bladder in this species. Two of the pancreatic ducts open near the middle of. the distal segment of the duodenum close to each other and to one of the gall-ducts; the third pancreatic duct opens near the distal end of the loop, and the second gall-duct near its proximal end. e\. Terminal segment of small intestine ending in the large intestine at f. Two long coils of small intestine have been removed between this terminal segment and the distal end of the duodenum. f. Large intestine, two coeca marking its commencement. In the small size of the coeca the Columbidae contrast with the majority of Gallinaceae. g. Terminal dilatation of the large intestine receiving the vas deferens and ureter dorsally on each side. In this cloacal arrangement Birds resemble Reptiles and Amphibia; in all Mammals there is a sinus urogenitalis developed, into which these ducts open.

In the absence of a urinary bladder Birds resemble Snakes and many Lizards.

h. Testis.

i. Kidney divided into three lobes, which are conformed to the sinuosities of the pelvic bones.

j. Vas deferens, dilating before its termination in the cloaca.

k. Ureter.

l. Teres major muscle, the subscapularis and great part of the scapula having been removed.

m. Right jugular vein receiving the veins from the oesophagus, and by virtue of these vessels, as also of a branch of anastomosis with the left jugular, attaining, as is usual in Birds, a larger size than that vessel.

n. Right jugular vein in thorax.

o. Vena cava inferior, entering the auricle to the right of and posteriorly to the entrance of the vena cava superior of the right side.

p. Lung, showing on its exterior surface indentations corresponding with the ribs.

q. Right bronchus entering the lung. Between the bronchus and the vena cava inferior we see a portion of the glandular proventriculus, and immediately above the bronchus and below the arch of the aorta, which has been displaced a little upwards, the junction with the jugular of the fragment of vein left to represent the subclavian trunk.

r. Right innominate artery, which is seen to break up into three main divisions, the common carotid, the axillary and the pectoral arteries. y. Portion of inner tuberosity of humerus which overhangs the pneumatic foramen of the bone. z. Gizzard.

s. Great pectoral muscle, the main depressor of the humerus and wing. Its main tendon is seen turned back at x; two other tendons which it gives, one to the long extensor, the other to the short extensor of the alar membrane, are not shown in this figure.

t. Second pectoral, the main elevator of the humerus.

u. Coracobrachialis inferior, a muscle arising from the inferior and outer three-fifths of the distal part of the coracoid, and inserted into the internal and proximal lip of the cup-shaped pneumatic cavity of the humerus. The opposite lip of this cavity receives the tendon of the teres major l; and from the triangular space between the muscular bellies of these two muscles, the subscapularis muscle, together with the upper portion of the scapula, and a small muscle, the serratus anticus, which passes between the fibres of the subscapularis to be inserted into the inferior edge of the scapula, have been removed.

v. Coracobrachialis superior, a bicipital muscle with a very extensive origin; arising, superiorly, from the inner surface of the vertebral end of the clavicle; inferiorly, from a facet on the lateral aspect of the upper surface of the sternal rostrum; and between these two points of origin from the upper and inner surface of the ligament connecting the coracoid, clavicles, and sternal rostrum. Its tendon, which is joined by that of the subscapularis, is inserted proximally and anteriorly to that of the preceding muscle u.

w. One head of the extensor plicae alaris anterioris longus, arising from' the upper end of the clavicle in continuity externally with a head of the extensor brevis. These muscular bellies appear to be divarications of the deltoid.

wI. Muscle in connection with the long alar extensor tendons. Its fibres have in the natural condition of the parts much the same direction as those of the muscle w and of the deltoid; but its origin is mainly from the fascia which covers the biceps in front, and being interposed between that muscle and the tendon of the great pectoral, it is continued up into the tendinous expansion by which the posterior layer of the tendon of the great pectoral connects itself more or less intimately with the coracoid head of the biceps and obtains an insertion into that bone. The muscle wr is inserted mainly into the inner of the two tendons at its distal extremity. This tendon is prolonged down to be inserted into the radial process of the carpo-metacarpal bone which carries the pollex. It is more or less intimately connected with the two other long extensor tendons from the muscle w and from the great pectoral, which are here drawn as one; as also with the extensor brevis which is not shown in this figure.

x. Tendon of great pectoral muscle turned back. The posterior portion of this tendon receives at its lower edge the tendon of a cutaneous muscle which is figured as attached to its outer angle, and higher up it receives the main tendon of origin of muscle w1, and is ultimately prolonged either separately or in connection with the tendon of the biceps up to the coracoid.

x1. Biceps. Its tendon is seen running upwards to be inserted into the internal anterior process of the upper end of the coracoid; it has a small insertion into the humerus also, which is not shown here.