The alligator and its allies

by A. M. Reese

Caudal to the first hæmorrhoidal arises the second hæmorrhoidal, he²; also unpaired, leading to the cloaca.

Posterior to the second hæmorrhoidal, the aorta continues into the tail as the large caudal artery, ca.

The Anterior Arteries.

The origin of the great arterial trunks—the pulmonary, aortic arches, primary carotid, and right subclavian has already been given and the distribution of the pulmonary arteries and aortic arches has been described, so that it now remains to describe the distribution of the right subclavian, [Fig. 62], sc.d., and the primary carotid, capr.

The right subclavian, sc.d., since it has an independent origin from the heart, instead of arising as a branch of the primary carotid, will be described first. After leaving the heart it passes cephalad and laterad and gives off the following branches in order, beginning at the heart: an œsophageal artery, oe, a small, caudally directed vessel carrying blood to the posterior region of the œsophagus. Close to the œsophageal arises another small, caudally directed vessel, the pleural artery, plu, extending to the pleura and possibly to the pericardium. From the same region as the preceding two arteries, but extending cephalad along the trachea and œsophagus, arises the much larger branch of the right subclavian, the right collateralis colli, cc, whose course and distribution will be described later.

Close to the distal side of the collateralis colli arises the very small thyroid artery, th, leading to the oval thyroid gland that lies against the ventral surface of the trachea a short distance anterior to the heart.

A short distance distal to the thyroid artery the subclavian gives off a fairly large artery, the internal mammary, im¹ (shown too large in the figure), that passes to the inner surface of the ribs near the sternum and lies parallel to the vein of the same name, described above.

A short distance distal to the internal mammary arises an artery of about the same diameter, the vertebral, v¹; it passes dorsad and caudad to the region of the thoracic vertebræ.

After giving off the five vessels just described, the subclavian artery passes into the shoulder where it divides into three main branches: (a) the subscapular, sc¹, going to the skin and muscles of the shoulder; (b) the thoracic, t¹, carrying blood to the posterior muscles of the shoulder and to the posterior region of the upper arm; (c) the brachial, br¹, which is really the continuation of the subclavian and is the chief artery of the anterior appendage.

After sending several branches to the upper arm the brachial divides, in the region of the elbow, into two main vessels, the radial, ra¹, and ulnar, ul¹, arteries, [Fig. 62], A. The radial artery, in the carpal region, divides in a complicated way into five main vessels that extend into the digits. The ulnar artery gives off several branches to the forearm, but apparently does not connect directly with the branches to the digits.