NON-STRIATED MUSCULAR FIBRE. 201 



Their size is usually much less than that of the fibres of Animal life ; but, 

 owing to the extreme variation in the flattening whicli they undergo, it is dif- 

 ficult to make a precise estimate of their dimensions. Those of the aliment- 

 ary canal are stated by Dr. Baly to measure from about the l-2500th to the 

 l-5600th of an inch; in the foot of the common Mussel, the Author has 

 found them 1o be as much as the 1-1 920th of an inch ; whilst in the respira- 

 tory sac of a Phallusia (an Ascidian Mollusk), their diameter is no more than 

 l-8400th. They sometimes present markings, which indicate a granular ar- 

 rangement in their interior ; and these markings have occasionally a degree 

 of regularity, which approaches that of the stria? on the striped Muscular fibres. 

 They frequently present nodosities at intervals, which are the nuclei of their 

 original component cells ; and, where these nuclei are not otherwise visible, 

 they may be brought into sight by acetic acid (Fig. 103, a). The plain or 

 non-striated fibres, like those of the other muscles, are usually arranged in a 

 parallel manner, into bands or fasciculi ; but these fasciculi are generally in- 

 terwoven into a net-work, not having any fixed points of attachment, but con- 

 tracting against each other. It is of this kind of structure, that the muscular 

 substance of the walls of the oesophagus, stomach, intestinal tube, bladder, 

 and uterus, is composed ; it occurs also in the bronchial tubes, in the ureters, 

 and most of the larger gland-ducts, and in the iris. In the Heart, are found 

 various forms of Muscular fibre ; some being distinctly striated, others quite 

 plain ; and others of intermediate character. The average size of the fibres 

 is less than that of the fibre, of which the voluntary muscles are composed ; 

 and the fasciculi, instead of being straight and parallel, are considerably in- 

 terlaced. This intermediate character accords well, as we shall hereafter see, 

 with the actions of the organ ; which correspond in their energy and rapidity, 

 with the contractions of voluntary muscles ; whilst they agree with those of 

 the non-striated kind, in being but little influenced by the nervous system. 

 The middle coat of the Arteries contains a contractile tissue, very similar to 

 that of unstriped muscle; and fibres of a similar nature are interwoven Avith 

 other fibrous tissues in the Skin, and especially in the Dartos, giving rise in 

 the former to the state termed culis anserina, under the influence of cold or 

 of depressing emotions ; and in the latter to the wrinkling of the scrotum. 

 There are certain points, at which the one system of fibres comes into close 

 connection with the other. This is the case, for example, in the oesophagus; 

 the upper part of which contains striated fibres, and is thrown into contrac- 

 tion by nerves ; whilst the muscular wall of the lower part seems entirely 

 composed of non-striated fibres, and acts for the most part independently of 

 the nerves. The point of transition varies in different animals ( 386) ; and 

 seems not to be constant among individuals of the human species. 



235. The Myolemma of the Muscular Fibre appears to be the part first 

 formed ; being distinctly visible long before any traces of fibrillae can be ob- 

 served in it. This tube seems to take its origin, like the ducts of Plants, in 

 cells laid end to end, the cavities of which coalesce, by the disappearance of 

 the partitions, at a subsequent period ; and the nuclei of these original cells 

 may be distinctly seen, for some time after the appearance of the strire, which 

 indicate the formation of the fibrillas in their interior. In an early stage of 

 the development of the fibres, indeed, these bodies project considerably from 

 their sides : in this respect, as well as in others, there is a close correspond- 

 ence between the temporary character of the Muscular fibre of Animal life, 

 and the permanent condition of that of Organic life. In the fully formed 

 muscle of Animal life, they are not perceptible, except when a peculiar me- 

 thod has been adopted for bringing them into view. This method consists in 

 treating the fibre with weak acids, which render the nuclei more opaque, 

 whilst the surrounding structure becomes more transparent. They are usually 



