VESTIGIAL PARTS. 65 



with the absence of the right jugular vein and carotid 

 artery in many birds. This was attributed by Owen to 

 the habit of birds sleeping with the head under the 

 wing. This view is not supported by facts. 



Man, in common with the four anthropomorphous 

 apes, has attached to the lower end of the caecum a 

 small thin tube, which may vary from two to eight 

 inches in length, known as the vermiform appendix 

 (fig- 33)- This tube agrees in structure with the rest 

 of the intestine, is covered with peritoneum, possesses 

 a muscular coat, and is lined with mucous membrane. 

 In the early embryo it is equal in calibre to the rest of 

 the bowel, but at a certain date it ceases to grow pari 

 passu with it, and at the time of birth appears as a thin 

 tubular appendix to the caecum. In the newly-born 

 child it is often absolutely as long as in the full-grown 

 man. This precocity is always an indication that the 

 part was of great importance to the ancestors of the 

 human species. 



Many mammals, closely allied to the anthropomor- 

 phous apes, possess very large caeca ; and in some of 

 these the terminal segment of the caecum, although not 

 represented as a thin, narrow tube, nevertheless resembles 

 the vermiform appendix in that it possesses a very large 

 proportion of the peculiar kind of tissue known as ade- 

 noid, or lymphatic. 



In man the vermiform appendix is a typical example 

 of a functionless part, and, like an idle person in a 

 community, is not infrequently a source of considerable 

 danger and suffering, and is responsible for a number 

 of deaths annually. The clanger may arise in three 



6 



