Comparative anatomical studies on the thyroid and thymic arteries. III. Guinea pig (Cavia cobaya).

The thyroid and thymic arteries were investigated in 30 male and 30 female Hartley guinea pigs. The superior thyroid artery was the most common (73% of males, 55% of females). It arose from the external carotid artery and gave rise to the superior and inferior laryngeal arteries in addition to glandular rami. In the remaining instances, the independence of the superior laryngeal resulted in the presence of middle and inferior thyroid arteries. The middle thyroid artery, arising from the external carotid or distal half of the common carotid artery, passed the thyroid gland cranially or penetrated it to terminate in the inferior laryngeal artery. The inferior thyroid artery, arising from the distal or proximal half of the common carotid, reached the gland caudally also to end in the inferior laryngeal. A thyroid ima artery was rare. There were many 'independent thyroid arteries'. Some of them arose from the lingual and the ascending pharyngeal arteries, suggesting that they were the original thyroid arteries. Thus the thyroid arteries were much more abundant and variable in the guinea pig than in man. The arteries supplying the 'superficial cervical thymus', which is characteristic of this animal, were the superior and inferior superficial cervical thymic arteries. The latter artery, arising from the superficial cervical, appeared in about 30% of the specimens. The former was always present and arose from the lingual (75% of males, 67% of females); superior laryngeal, and superior thyroid or common carotid arteries. The cranial extension of the thymus and the superior superficial cervical thymic artery had complex relationships with the hypoglossal nerve.