HISTOLOGY 



145 



Striated fibers are bound together in bundles enwrapped by a con- 

 nective-tissue perimysium. Thick muscles consist of several or many 

 such bundles wrapped together. 



Cardiac muscle has striations which resemble those of somatic muscle 

 but the fibers are relatively short and they are branched. The sarcolemma 

 is less strongly developed than in somatic fibers. A peculiar feature of the 

 cardiac fiber is the presence of conspicuous transverse bands, the inter- 

 calated discs (Figs. 106 and 107), which are quite distinct from the ordinary 

 striations. Their significance is not known. 

 A cardiac fiber develops from a syncytium of 

 mesenchyme cells so that the adult fiber is the 

 product of several mesenchyme cells. 



Human car- 

 . very small 



Nervous Tissue 

 The proper relating of an animal to its 

 environment depends upon a nervous system 

 whose outposts must be at the surface of the 

 animal, therefore in, or at least very near, 

 the external epithelium. Central nervous 

 organs, presumably in evolution and certainly pj^. 107.- 

 in ontogeny derived from ectoderm, retreat to diac muscle; 



. . o • 1* 1, portion seen under much 



a protected deep position. Stimuh set up by higher magnification than 



impact of external agencies must be carried to that used in Fig. 106. d, 



1 . 1 • intercalated disc; Z, Krause s 



deep central organs which must, m turn, cause membranes which lie trans- 

 appropriate muscular or other reaction. For versely at regular intervals 

 .. . r ^^ r ^ along each myofibril, bisect- 



complete coordination of all parts of a large -^^^ ga,ch light band. The 

 complex animal it is necessarv that deep-seated distinction between light and 



.... 11' 1 • J • 1. dark bands does not appear in 



visceral activities be controlled in accord with ^^e figure. (From Bremer, 

 the external situation, and equally necessary Text-bookof Histology; after 

 that external actions fit the internal state. 



Therefore the animal must have a system consisting of receptors 

 connected by conductors (nerves) to a coordinating central nervous 

 organ whence other conductors extend outward to the effectors which 

 are such organs as muscles, glands and the electric organs of fishes. 

 Receptors are distinguished as exteroceptors which are stimulated by 

 external agencies, interoceptors which pick up visceral stimuli, and pro- 

 prioceptors which lie within the motor mechanism of the body-wall — the 

 muscles, tendons and joints. The central nervous organ of an automobile 

 is the driver. His own eyes and ears serve as exteroceptors for the moving 

 car. Certain interoceptors register on the instrument board the condition 

 of the gasoline and water which are concerned with the metabolism of the 

 car. Then there are proprioceptors which, registering in the speedometer 

 and oil gauge, give information concerning the performance and intrinsic 

 condition of the motor mechanism. 



