374 THE HEART. 



(generally two) embedded in a more granular substance (fig. 327). The periphery 

 of the cells in question is composed of transversely striated substance, and they 

 appear to represent a condition of arrested development of cardiac muscular tissue. 

 They have been found in one instance in the human heart (Gregenbaur), but accord- 

 ing to Henle they do not as a rule occur after the first year in man. The endo- 

 cardium is thicker in the auricles (the left especially) than in the ventricles ; it is, 

 however, very thin on the musculi pectinati of the auricles and on the columnae 

 carneae of the ventricles. The endocardium does not receive any blood-vessels, and 

 the same is true for both the auriculo-ventricular and the semilunar valves. There 

 is a non-gangliated plexus of medullated nerve-fibres under the endocardium, to which 

 fine nerve-fibres pass, but their mode of termination is not known (Toldt). 



DIMENSIONS AND WEIGHT OF THE HEART. 



Size. It was stated by Laennec that the heart in its natural condition is about 

 equal in size to the fist of the individual. It is about five inches long, three and a 

 half in its greatest width, and two and a half in its extreme thickness from the 

 sternal to the diaphragmatic surface ; but linear measurements of a flaccid organ like 

 the heart must be subject to so many accidental variations as to render them of little 

 value. 



Weight. The weight of the heart in the adult is also subject to considerable 

 variation, ranging between rather wide limits, which depend mainly on the general 

 weight of the body and on the sex. 



The mean weight of the heart is about 11 oz. (310 grammes) in the male, and 



oz. (255 grammes) in the female. Its proportion to the body-weight is about 



1 : 205. 



The weight of the heart increases with the body-weight, but in a gradually diminishing 

 ratio, so that with a higher body-weight the proportional weight of the heart is usually 

 lower. At birth the heart weighs on an average 13-| drachms (24 grm.).and its proportion to 

 the body- weight is about 1 : 130. The absolute weight increases rapidly during the first 

 years of life, and then more slowly until the approach of puberty, when another period of 

 somewhat more rapid growth sets in. After adult age is reached the heart continues to 

 increase in weight slightly until about 70 years, when it begins to diminish. During the 

 latter part of foetal life the muscular walls of the two ventricles are of equal bulk ; but 

 after birth the right grows much less rapidly than the left, so that by the end of the second 

 year they are to one another as 1 : 2, a proportion which is thenceforward maintained. In 

 the foatus also the right auricle is the heavier ; during the first month after birth the two 

 become equal, but from the second year the right gains slightly again, and from puberty 

 onwards exceeds the left by about 5'5 per cent. (Miiller). 



Similarly, the volume- of the heart was found by Beneke to increase with age, at first 

 rapidly then gradually. Thus in the new-bom infant the average volume is 22 cubic centi- 

 meters ; at the fifteenth year it is 150 cc. to 160 cc. ; and at the twentieth year about 250 cc. 

 After this it increases but slowly up to the fiftieth year, by which time it has arrived at 

 280 cc. Subsequent to this there is a slight gradual diminution. Up to the age of puberty 

 the volume is about the same in both sexes, but after puberty it is 25 cc. to 30 cc. larger in 

 the male. On account of the obvious difficulties of the investigation these statements can 

 however only be regarded as approximate. 



The auricles are generally stated to be rather less capacious than the ventricles. The 

 right auricle is also said to be larger than the left. In the ordinary modes of death, the 

 right ventricle is always found more capacious than the left, probably owing to its being 

 distended with blood : the left ventricle, on the other hand, is found nearly empty, and 

 thus becomes more fully contracted. There are good reasons for believing that during life 

 scarcely any difference of capacity exists between the two cavities. 



The right auriculo-ventricular opening and the orifice of the pulmonary artery are both 

 found to be somewhat larger after death than the corresponding openings on the left side 

 of the heart. Their average dimensions may be stated as follows 1 : 



1 Calculated from Peacock's tables in " Weight and Dimensions of the Heart in Health and Disease, 

 reprinted from the Monthly Journ. of Med. Sci., 1854. 



