152 



HISTOLOGY 



outside of the basement membrane comes a layer of circular muscle 

 fibers that are remarkable for the fact that no nuclei appear in their sub- 

 stances or directly among them. Closer examination will show that these 

 structures are not fibers, as are the non-striated elements of vertebrate 

 smooth muscle, but fibrils or small groups of fibrils, each one or two of 

 which belong to one of the large cells that lie in the layer of tissue just 

 outside of them. Each one of these structures is most probably a group of 

 several myo-fibrils, and not a fiber in the sense that such a structure is 



spoken of in the 

 vertebrate ani- 

 m a 1, for in- 

 stance. They 

 lie three or four 

 thick in the 

 larger vessels 

 and in a de- 

 creasing propor- 

 tion in the 

 smaller chan- 

 nels, until in the 

 smallest there 

 are none to be 

 found. The cel- 



l. mus. 



FIG. 135. Longitudinal section of part of the wall cf a blood vessel of 

 Cerebratulus lactatus. int., intima, a layer of cells with a very delicate 

 membrane; c.mus., bundles of circular muscle fibers; mus.c., cells to 

 which these muscle bundles belong; l.mus., longitudinal muscle fibers. 



lular layer found outside of the circular muscle-fibrils consists of the cell 

 bodies to which the muscle-fibrils belong, together with a few connective- 

 tissue cells and an occasional nerve cell. These form an epithelial-like 

 layer arranged radially and containing elongated nuclei. When the 

 vessel is contracted, the cells are elongated and columnar in form, and 

 when the blood distends the channel, they shorten and become cubical. 



The majority of the vessels possess no further covering except the 

 very slight amount of connective tissue found around and among the 

 muscle cells. The dorsal vessel and larger branches possess, in addition 

 to the tissues already described, an outer layer of longitudinal muscle 

 fibers. They appear in transverse sections of the vessel as roughly circu- 

 lar sections of the cylindrical cytoplasmic bodies of the muscle structures, 

 each containing a number of the same fibril groups that we have observed 

 in the other muscles of this animal. A single nucleus appears in each 

 section and these are probably, therefore, syncytia. In our longitudinal 

 sections this relation of nuclei and muscle-fibrils to the cell is not so plain, 

 but can be grasped by a comparison with transverse sections in the same 

 specimen. 



It will be noticed of this circulatory system that it is simple and un- 

 specialized in the fact that all its parts are substantially alike, that it is 



