MUSCLE. 



77 



absent. It is a single layer serving as the periosteum of the cement on 

 one side and of the alveolus on the other and being intimately joined to both 

 bones by Sharpey's fibers. It is named the alveolar periosteum [peridental 

 membrane]. Its numerous blood vessels are branches of those which enter 

 the apical foramen together with vessels from the gums and perhaps from 

 the mandible and maxilla. Its nerve endings are the terminations of 

 branches from the dental and alveolar nerves. Lymphoid tissue has been 

 found in the gums, but apparently it does not extend into the alveolar peri- 

 osteum. 



MUSCLE TISSUE. 



Contractility is a fundamental property of protoplasm. Muscle cells 

 are those in which the contractile function has become predominant. 

 They are elongated cells containing fibrils parallel with their long axes. 

 By the shortening of these fibrillated cells muscular action results. Em- 

 bryologically muscles arise either from mesenchyma or from epithelium. 

 Mesenchyma produces two types of muscle, smooth (non-striated, involun- 

 tary) and cardiac (the striated, involuntary mus- 

 cle of the heart). Mesodermal epithelium pro- 

 duces one type, the striated voluntary skeletal 

 muscles, ordinarily called striated. In the in- 

 vertebrates ectodermal and entodermal epithe- 

 lia also produce muscle cells. In mammals the 

 muscle fibers of the sweat glands are generally 

 recognized as ectodermal, and some in the iris 

 have been described as such; entodermal mus- 

 cles have not been observed. 



The three principal classes of muscles, 

 smooth, cardiac, and striated, may be described 

 in turn. 



SMOOTH MUSCLE. *? ::::: ; --^ ; ..: c.t. 



Smooth muscle develops around the large 

 lymphatic and blood vessels; around the intes- 

 tinal canal, including the principal gland ducts 

 opening into it and the large respiratory tubes; 

 also around the bladder and ureters, the uterus 

 and ducts of the genital system; and, finally, in 

 connection with the hairs, in the capsule of the spleen, and in other minor 

 places. In general terms, it forms the musculature of the viscera. 



The development of smooth muscle may be studied in a cross section 

 of an 18 mm. pig embryo (Fig. 88). The stratified entodermal epithelium 



epi. 



FIG. 88. FROM A CROSS SECTION- 

 OP THE OESOPHAGUS OP AN 



1 8 MM. PIG. 



Epithelium; b. m., basement 

 membrane; c. t., connective 

 tissue; c. m., circular smooth 

 muscle cut lengthwise; n. c., 

 nerve cells; 1. m., longitudinal 

 smooth muscle cut across. 



