Origin from the median line of the ventral surface of the first two caudal vertebræ. The muscle passes caudoventrad, lying between the levator ani and the caudorectal (j), and is inserted into the ventral side of the urogenital sinus, at the base of the clitoris.
M. urethralis (f).—This consists of fibres surrounding the cranial part of the urogenital sinus and the caudal parts of the vagina and neck of the bladder.
Origin partly on the caudal part of the symphysis of the ischium, partly from the ventral surface of the urogenital sinus, where the fibres are attached to the corpora cavernosa clitoridis. The fibres pass dorsad over the surface of the sinus, and over the surface of the union of the vagina and neck of the bladder, to be inserted into the sides of the vagina and the dorsal surface of the urogenital sinus.
THE CIRCULATORY SYSTEM.
I. THE HEART. COR.
The heart lies in the mediastinum, enclosed in the pericardial sac, and projects rather more toward the left than toward the right. It is an ovoid or pear-shaped organ, with its long axis directed approximately craniocaudad. Its caudal end or apex is, however, directed slightly ventrad and to the left, while the larger cranial end or base faces slightly dorsad as well as craniad.
Laterally and dorsally the heart is largely covered by the lungs. The ventral side and a considerable portion of the lateral surface are, however, not thus covered, so that they lie against the thoracic wall. The heart extends from about the fourth or the fifth to the eighth rib, and its apex touches the diaphragm.
The cavity of the heart is divided by a longitudinal dorsoventral septum into lateral halves—a right and a left side. Each side is again divided by a transverse dorsoventral septum (auriculoventricular) into two chambers, one of which, the auricle, lies at the base; the other, the ventricle, lies at the apex of the heart. There are thus right and left auricles and right and left ventricles. An external groove, partly filled with fat, separates the auricular portion of the heart from the ventricular part; this groove is known as the sulcus coronarius.
Each auricle or ventricle has a single set of blood-vessels either leaving it or entering it. Thus the blood enters the right auricle by the inferior and superior venæ cavæ ([Fig. 116], d and e) (præcava and postcava). It passes thence into the right ventricle, and from the right ventricle to the lungs by a single pulmonary artery ([Fig. 115], f). It returns to the left auricle by the numerous pulmonary veins ([Fig. 116], g, h, i), and passes thence to the left ventricle. From the left ventricle it passes to the body by a single aorta ([Fig. 115], g).