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232 THE HEART. 



In some animals others are found which are only striated at their peri- 

 phery, and present a beaded appearance, the component cells being much 

 larger and more distinct ; these, which are known as Purkinje's fibres, 

 appear to represent a condition of arrested development of the ordinary 

 cardiac fibres. Plain muscular fibre-cells have also been described in 

 some situations ; and, in some animals, collections of adipose tissue are 

 met with beneath the endocardium. The membrane is usually more 

 opaque on the right side than on the left ; and 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 columns carnese of the 

 ventricles. 



DIMENSIONS AND WEIGHT OF THE HEART. 



Size. — It was stated by Laennec, as the result of his experience, that 

 the heart in its natural condition was about equal in size to the closed 

 hand 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 anterior to the posterior 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 on the general weight of the body and on the sex. 



Its mean weight is about 9 or 10 oz. According to Eeid's tables, 

 the average weight in the adult male is as high as 11 oz., and in 

 the female 9 oz. ; while according to Peacock the average of the male 

 is 9f oz., and that of the female 9 oz. 



The weiglit of the heart maintains some general iiropoi-tion to that of the body. 

 According to Tiedemann this is about 1 to 160 ; by Clendinning it was found to 

 be 1 to 158 in males, and 1 to 149 in females ; and by Eeid to be 1 to 169 in a 

 series of thirty-seven males and 1 to 176 in twelve females ; but in the healthy 

 males dying suddenly the ratio was as 1 to 173. 



It was shown by Clendinning that the heaii; continued to increase in weight 

 up to an advanced period of life, but at a comparatively slower rate subsequently 

 to the age of twenty -nine years. Subjoined is a tabular statement of some of the 

 average results derived from the observations of these authors. 



Entirely in accordance with these observations upon the increase of the heart's 

 weight according to age. it has been found by Bizot that this organ continues to 

 enlarge in all its dimensions as life advances, viz.. in the length, breadth, and 

 thickness of its walls. The greatest increase was detected in the substance of the 

 left ventricle, and the ventricular septum.* 



* Reid, in the Lend, and Edin. Monthly Joiimal of Med. Science, April 184-3 ; T. P.. 

 Peacock, in the same journal, in 1846, and reprinted .separately, with additional observa- 

 tions, in 1854 ; Clendinning, in the Medic. Chin Transact., 1S3S ; Bizot, Mem. de la 

 Soc. Medic. d'Observation de Paris, torn. i. p. 262. 1836. 



