NECTURUS 8 1 



of the skull called membrane bones ; these, however, are so closely 

 joined with the other bones of the skull that they will be considered 

 with them. 



The endoskeleton consists of the bones and cartilages which 

 form the framework of the body. It may be divided into ( i ) the 

 axial skeleton, which includes the skull and the vertebral column 

 and the ribs, and (2) the appendicular skeleton, which includes the 

 framework of the two pairs of appendages, or limbs. Median ap- 

 pendages are not present. 



We shall first study the appendicular skeleton. Each pair of 

 appendages is composed of the two limbs and the girdle by which 

 they are joined with the trunk, the girdle of the anterior append- 

 ages being the pectoral, or shoulder, girdle, that of the posterior 

 appendages being the pelvic girdle. 



The Anterior Extremities. The pectoral girdle consists of a pair 

 of delicate triradiate structures, composed principally of cartilage, 

 which lie on the ventral and lateral sides of the trunk. Each half 

 of the girdle supports an arm and consists of a single skeletal piece 

 which is embedded in the muscles of the trunk and is not joined 

 with the other half or with the axial skeleton ; at about the middle 

 of it is a depression, the glenoid fossa, which receives the head of 

 the humerus and forms the shoulder joint. 



Each half of the pectoral girdle is made up of a dorsal por- 

 tion, the scapula, and two ventral portions, the coracoid and 

 the procoracoid. The scapula is composed of two distinct parts : 

 a narrow, ventral portion, which is of bone, and a broad, dorsal, 

 cartilaginous portion. The procoracoid is a narrow strip of car- 

 tilage extending directly forward; the coracoid is a broad plate 

 of cartilage posterior to the procoracoid and extending medially. 

 The coracoids of the two sides overlap each other, the left one 

 usually being ventral to the right one. The coracoid lies beneath 

 the pectoral muscle, and the procoracoid beneath the procoraco- 

 humeralis muscle. 



A sternum is present, but is very slightly developed. It consists 

 of two or more irregular, very delicate cartilages, which appear in 

 the myocommas beneath the medial ends of the coracoids in the 

 ventral body wall. 



