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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



bone and associated muscles, its ventral and lateral walls, in the 

 thoracic region, by hard parts and muscles, by muscles alone in 

 the abdominal region. The body-cavity is lined ventrally and 

 laterally by a thin pigmented membrane, the plcuro-peritoneum, 

 which dorso- laterally becomes free, leaving a space, the sub- 

 vertebral lymph-sinus, beneath the vertebral column. The two 

 lateral halves of the pleuro-peritoneum run to the middle line, 

 forming a floor to this sinus, and then unite to make a double 

 sheet, the mesentery, the halves of which diverge, surround the 

 alimentary canal, and then become continuous. The alimentary 

 canal is thus suspended from the dorsal wall of the body-cavity, 

 and, strictly speaking, is outside it. The same may be said of the 

 other contained organs. 



The walls of the oesophagus, stomach, and intestines are com- 

 posed of four coats, which are, commencing from the outside : 

 (1) serous coat; (2) muscular coat; 

 (3) sub-mucous coat ; (4) mucous mem- 

 brane. 



(1) The serous coat is in reality the 

 pleuro- peritoneal investment of the gut. 

 It consists of a layer of simple squamous 

 epithelium, with a thin underlying stratum 

 of connective tissue in which run the 

 blood-vessels, lymphatics, and nerves 

 that supply the other coats. 



(2) The muscular coat, best developed 

 in the stomach, is divided into an outer 

 longitudinal and an inner circular layer. 

 It is made up of unstriated muscle- 

 fibres. These are spindle-shaped cells 

 (Fig. 52), each containing an elongated 



Fig. 52. UNSTRIATED nucleus > which dovetail together by their 

 MUSCLE - FIBRES (from tapering ends. 

 Landois and Stirling), (3) The sub-mucous coat is also best 



SStatedTuf tools'- devel P ed in *e st mach > and C0nsists 

 section. of loose connective tissue traversed by 



numerous blood-vessels and lymphatics. 



(4) The mucous membrane is the part which is raised into 

 folds internally. It is exceedingly glandular, and is made up of 

 (a) a thin external layer of unstriated muscle, the muscularis 



