812 DISSECTION OF THE THORAX. 



THE HEART AND ITS LARGE VESSELS. 



The heart is a hollow muscular viscus, and is the agent in the propul- 

 sion of the blood through the body. Into it, as the centre of the vascular 

 system, veins enter ; and from it the arteries issue. 



Form. When the heart is distended, its form is conical, but it is rather 

 flattened from before backwards. Its surfaces and borders have the fol- 

 lowing differences ; the anterior surface is slightly convex, whilst the pos- 

 terior is nearly flat : the left border is thick and round, but the right is 

 thin, sharp, and less firm. 



Size. The size varies greatly, and in general the heart of the woman 

 is smaller than that of the man. Its average weight is from ten to twelve 

 ounces in the male, and from eight to ten in the female. The measure- 

 ments may be said to be about four inches and three-quarters in length, 

 three inches and a half in width, and two inches and a half in thickness. 



Position and Direction. The heart lies beneath the lower two-thirds 

 of the sternum, ami projects on each side, but more on the left than the 

 right. Its axis is not parallel to, but is inclined obliquely across that of 

 the body ; and its position is almost horizontal with the base directed 

 backwards and to the right, and the apex forwards and to the left side. 

 The left margin of the viscus is undermost, whilst the right is foremost. 



In consequence of the direction of the heart in the thorax, only some 

 parts can be near, or in contact with the parietes : thus the right half and 

 the apex will correspond with the thoracic wall, though mostly with lung 

 intervening, whilst the base is directed away from the sternum and the 

 costal cartilages : and the left half will be undermost and deep in the 

 cavity. 



Limits (fig. 98.) The limits of the whole heart are the following : 

 the base is opposite the spinal column, and corresponds with four dorsal 

 vertebrae (5th to 8th). The apex strikes the wall of the thorax during 

 life just below the fifth rib, near the junction with the cartilage. 



The upper limit would be shown by a line across the sternum on a level 

 with the upper edge of the third costal cartilage. And the lower limit 

 would be marked by a line over the junction of the sternum with the 

 xiphoid cartilage, drawn from the articulations of the sixth and seventh 

 cartilages of the right side to the spot where the apex touches. 



Its lateral limits are the following. On the right it projects from one 

 to one inch and a half beyond the middle line of the sternum, and its in- 

 crease in this direction is constantly varying with the degree of distension 

 of the right half of the heart. On the left side the apex projects three 

 inches to three inches and a half from the centre of the breast bone. 



Component parts. The heart is a double organ, and is made up of two 

 similar halves. In each half are two hollow portions, an auricle and a 

 ventricle ; these on the same side communicate, and are provided with 

 vessels for the entrance and exit of the blood. On the surface are grooves 

 indicatory of this composition. Thus, passing circularly round the heart, 

 nearer the base than the apex, is a groove which cuts off the thin auricular 

 from the fleshy ventricular part. A longitudinal sulcus on each surface 

 marks the situation of a median partition between the ventricles : this sul- 

 cus does not occupy the mid space either on the anterior or the posterior 

 aspect, but it is nearer the left border of the heart in front, and the right 

 border behind ; so that most of the anterior sur'ace is formed by the right, 

 and the greater part of the posterior surface by the left ventricle. 



