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GENERAL ZOOLOGY 



scattered muscle cells remain. These endothelial tubes are not 

 the true capillaries, the latter having an even more simplified 

 structure, the details of which are not completely known. 

 According to some descriptions, the capillary is a noncellular 

 membrane or 'perithelium which contains scattered contractile 

 cells and nerve endings. The diameter of the smallest capillaries 

 is so reduced that blood corpuscles are distorted in their passage 

 through them. The wall of the capillary is therefore extremely 

 thin which would seem to be of significance in connection with 



rapid exchanges by diffusion between blood 

 and tissue. 



Heart of the Frog. — The pericardium, the 

 sac enclosing the heart, bears the same 

 relation to the heart as the peritoneum 

 bears to the intestine. The pericardium is 

 really a double-walled sac, whose inner or 

 visceral wall is closely applied to the heart 

 and separated from the outer or parietal 

 wall by a space filled with pericardial fluid. 

 The two walls of the pericardium are con- 

 tinuous about the heart where the large 

 vessels are joined to it. The heart is an 

 organ composed almost entirely of muscle. 

 Cardiac muscle is striated and the structure 

 of the myofibrils is similar to that found in 

 skeletal muscle. Unlike skeletal muscle, whose fibers are usually 

 straight, cardiac muscle fibers form a branching network (Fig. 75). 

 The nuclei are located in the central axes of the fibers. In human 

 cardiac muscle the fibers are crossed by irregular intercalated 

 disks, located about midway between adjacent nuclei, which gives 

 the appearance of a cellular structure. Though striated, cardiac 

 muscle is involuntary in function. 



The frog's heart is composed of a single ventricle and two 

 auricles or atria. Viewed from the ventral side, the triangular 

 ventricle forms the posterior half of the heart and is marked off 

 from the atrial region in front by the coronary sulcus. The 

 ventral face of the atrial region is traversed by the broad truncus 

 arteriosus arising from the right anterior border of the ventricle 

 and dividing into right and left branches at the anterior edge of 

 the heart. On the dorsal side of the heart is a triangular sac, the 



Fig. 75. — Cardiac 

 muscle tissue of mammal. 

 C, intercalated disks. 



