These bones of the coccyx (French: coccyx), when united together, which is usually the case in advanced life, are supposed to resemble a cuckoo's bill and are therefore called coccygeal. Most commonly there are four of them, sometimes but three; in a few instances five have been found. They diminish gradually in size from above downwards, which gives them, when taken together, a pyramidal form. As they are placed in a continuous line with the inferior third of the sacrum, they form a slightly concave surface anteriorly, a convex one posteriorly.
The sacrum, much the largest piece of the vertebral column, is placed, when the body is in the erect position, at the superior and posterior part of the pelvis, beneath the last lumbar vertebra, above the coccyx, and between the ossa innominata, between which it is inserted, in some measure like a keystone into an arch.
As in the intervertebral articulations, so in the union of the first portion of the sacrum with the last lumbar vertebra, there are two sets of joints -viz. (a) Class. -False Synchondrosis. (b) Class. -Diarthrosis. Subdivision. -Arihrodia (a) a synchondrosis, between the bodies and intervertebral disc; and (b) a pair of arthrodial joints, between the articular processes. The union is effected by the following ligaments, which are common to the vertebral column: -(i) anterior, and (ii) posterior longitudinal; (iii) lateral or short vertebral; (iv) capsular; (v) ligamenta flava; (vi) supraspinous and (vii) interspinous ligaments. Two special accessory ligaments on either side, viz., the sacro-lumbar and the ilio-lumbar, connect the pelvis with the fourth and fifth lumbar vertebrae.
The heart is formed by the blending in the median fine of two longitudinal endothelial tubes lying ventral to the fore-gut of the early embryo. Each tube is partially surrounded laterally by the splanchnic mesoderm which forms a septum between the right and left sides of the coelomic cavity. The blended endothelial tubes form the endocardium. The splanchnic mesoderm in relation to the endocardium becomes the myoepicardium, and the double layer connecting the heart dorsally and ventrally with the somatic mesoderm becomes the (temporary) dorsal and ventral mesocardia. The somatic mesoderm of the heart region becomes the pericardium.
The pericardium is a cone-shaped, fibro-serous sac which surrounds the heart and contains a small amount of fluid [liquor pericardii]. Its apex is above at the root of the great vessels, and its base below, adherent to the diaphragm. Its connection with the diaphragm is in part to the central tendon and in part to the muscle, especially on the left side. It consists of an outer fibrous layer and an inner serous layer. The virtual space between the serous pericardium and the epicardium is commonly called the pericardial cavity.
The heart [cor] is a hollow organ principally composed of muscle, the myocardium. It is lined internally by endocardium which is continuous with the intima of the blood-vessels. Externally, it is covered by the epicardium, a serous membrane continuous with the serous lining of the pericardium. The form of the heart, when removed from the body without previous hardening, is that of a fairly regular truncated cone. The base [basis cordis] is poorly circumscribed but corresponds, in a general way, to the area occupied by the roots of the great vessels and the portion of the heart-wall between them. The base of the heart is held in position (Not necessarily fixed, for during systole the base performs a greater excursion than does the apex.) chiefly by the great vessels, which are attached to the pericardium; the remainder of the organ is capable of free movement within the pericardial cavity. The interior of the heart is longitudinally divided, into right and left cavities, by a septum passing from base to apex. Each cavity is subdivided into an atrium [atrium cordis] and a ventricle [ventriculus cordis], the former receiving the ultimate venous trunks and the latter giving rise to the main arteries. Thus the left atrium receives the four pulmonary veins, and the right atrium the superior and inferior vena cava and the coronary sinus; the aorta issues from the left ventricle and the pulmonary artery from the right. The ventricles, which constitute the major portion of the heart, may be recognized by their very thick walls. The atria have thinner walls and are less capacious than the ventricles; projecting from each is a diverticulum or auricle [auricula cordis]. The auricles (which receive their name from their resemblance to dog's ears) partially embrace the roots of the pulmonary artery and aorta.
The globe or ball of the eye is a composite structure of spheroidal form, placed in the fore part of the orbital cavity, and receiving the thick stem of the optic nerve behind. The recti and oblique muscles closely surround the greater part of the eyeball, and are capable of changing its position within certain limits: the lids, with the plica semilunaris and caruncle, are in contact with its covering of conjunctiva in front; and behind it is supported by a quantity of loose fat and connective tissue.
The parts which constitute the lachrymal apparatus are the following, viz.: - The gland by which the tears are secreted, situated at the upper and outer side of the orbit, together with its excretory ducts; the two canals into which the fluid is received near the inner angle: and the sac with the nasal duct continued from it, through which the tears pass into the inferior meatus of the nose.
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