348 Staple]/ and Lewis : 



On the large intestines the longitudinal muscle bands are 

 about half the length of the entire gut (man). From the short- 

 ening that these bands produce the sacculations of the bowels 

 result. These bands commence at the setting on of the appendix 

 from Avhich point they diverge on to the caecum and thence to 

 the colon. The external muscle coat can be profitabl}^ studied 

 with the use of Ti'eves' classical " four types of caecum." (Fig. 



Type A is a cone-shaped caecum Avhich Deaver calls infantile. 

 It represents about two per cent, of the caeca of the civilised 

 races. This A type is a caecum which has not formed an 

 appendix ; the muscle bands terminate at the narrowing of the 

 cone into the muscle sheath enveloping the end of the caecum. 

 It is a caecum in the immediately pre-appendicular stage. When 

 the muscle bands that terminate at the narrowing of the cone 

 come into activity, they deliver their pull at a point which 

 becomes the appendicular outlet of the caecum. With the re- 

 traction of the caecal muscle bands, the caecum is shortened 

 above the appendix, and with this shoi-tening an increased 

 breadth is established, due to a bellying-out of the circular 

 fibres. Two factors operate to shut off the appendix from active 

 digestive function — the pull of the longitudinal muscles delivered 

 at the junction of appendix and caecum serves to produce a con- 

 striction and to shut off the appendix from the caecum proper — 

 the sacculation of the caecum produced by the bellying-out of the 

 circular muscle, assisted in the upright position by gravitation, 

 leaves the appendicular opening as a small hole in a big area of 

 caecal fundus. The caecal fundus does not act as a guide of 

 ingesta to the appendix (Fig. 9) ; it so acts that food contained 

 within the caecum engages the appendicular opening with great 

 difficulty. 



1/ the muscle bands continue to the apex of the caecum, as 

 they do in the baboon, tio appendix results, because the whole 

 caecum is drawn ttp and no part shut off from the caecutn to 

 form a veriform appendix (Fig. 10). The lemur (Fig. 11) re- 

 presents a type of caecum which, if atrophy continued in that 

 species to the advanced stage shown in man or in the wombat, 

 would develop a caecum carrying a vermiform appendix, pro- 

 viding atrophy continued along the present lines indicated by 



