TISSUES, ORGANS, AND SYSTEMS. 107 



are the chief instruments of the animal motions. Their elements are 

 developed by the coalescence of round or stellate cells, Avhose contents 

 change into a homogeneous, semi-fluid substance, and then break up 

 into fibrils. Once formed, the muscular fibres grow by the elongation 

 and thickening of their elements, and in their complete condition they 

 enjoy a very energetic nutrition, -which is especially manifested by the 

 multiform products of their decomposition, as well as by the circum- 

 stance that their powers are exhausted in a short time when the circula- 

 tion is suspended. Wounds of the muscles never heal by transversely 

 striated muscular substance ; but an adventitious formation of this tissue 

 appears to occur sometimes, though rarely. 



Transversely striated muscular tissue is found in the following parts: 



1. In the muscles of the trunk and extremities ; of the globe of the 

 eye^ and all those of the ear. 



2. In the muscles of many organs ; as the larynx, j^harynx, tongue, 

 and oesophagus (upper half), the end of the rectum [sphincter cxternus, 

 levator ani), the genital organs {hulbo-ischio-cavernosus, urethralis trans- 

 versus, transversi perincei, cremaster, muscular fibres of the round liga- 

 ments of the uterus, in part). 



3. In certain parts of the vascular system, e. g. in the heart and in 

 the walls of the great veins which open into it. 



The muscular fibres of animals are not all composed of bundles of 

 transversely striated fibrils, but present a series of other forms, which 

 may best be grouped in the following manner : — 



1. Muscular tubes, with homogeneous, semi-solid, not transversely stri- 

 ated contents (most Mollusks, Worms, and Radiata). 



2. Muscular tubes with a membrane, a semi-fluid, homogeneous, cor- 

 tical layer in contact with it, and a fluid or granular, frequently trans- 

 versely striated or nucleated central substance. (Muscles of Petromy- 

 zon in part, certain muscles [of the lateral line and of the spiracles] of 

 the plagiostome and osseous Fishes. Muscles of the Hirudinidce, Lum- 

 hricidce, of Paludina in part, and of Carinaria.) 



3. Similar muscular tubes with a transversely striated cortical layer 

 without distinct fibrils. (Many muscular fibrils of the Hirundifiidce, and 

 of the muscles of Fishes enumerated under 2.) 



4. Muscular fibres without any internal cavity, with a sarcolemma 

 and transversely striated contents, which do not break up into fibrils, 

 but frequently into discs (Bowman), Salpce, some Radiata, many Arti- 

 culata. 



5. Similar muscular fibres, which readily break up into fibrils. (Most 

 Vertebrata, certain muscles of Insects.) 



6. Simple isolated cells, whose contents are changed into a trans- 

 versely striated substance, which either fills the whole cell or forms only 

 a thin layer upon its membrane. Here my observations lead me to place 



