PHYSIOLOGY, AND PATHOLOGY OP THE CHIMPANZEE, 333 



in the neck it becomes narrower and is inserted into the outer 

 surface of the mastoid process. The muscle raises a prominent 

 ridge on the anterior wall of the air-sac. Many vessels and 

 nerves pierce the deep fascia at the lateral border of the cleido- 

 mastoid ; and the spinal accessory nerve passes into its deep 

 surface in the upper third. 



When an incision is made through the platysma and deep 

 fascia the wall of the air-sac makes its appearance. The wall 

 varies in thickness in different parts, and its lining is smooth and 

 moist. It consists of a central part with two lateral diverticula. 

 The central part extends upwards to the hyoid bone, and down- 

 wards to the lower border of the manubrium sterni between the 

 tendons of origin of the steruo-mastoid muscles. Its anterior 

 wall is covered by the platysma, and the larynx, trachea and pre- 

 tracheal muscles shine through the thin posterior wall. The 

 lateral parts are very capacious, and have large circular orifices 

 under cover of the cleido-mastoids. When these are explored 

 the finger can pass along the greater part of the deep surface of 

 the pectoral muscles and the inner border of the deltoid ; it 

 palpates the entire length of the clavicle, the head of the humerus, 

 the glenoid cavity, and borders of the scapula. Many muscles, 

 nerves, and the carotid sheath form ridges in the walls of 

 the sac. 



The omo-hyoid (text-fig. 32 A) is more complex than in Man, 

 and it is more complex in my specimen than in others desci-ibed. 

 It consists of three bellies. The postero-mesial belly is tapering. 

 It arises from the back of the first costal cartilage along with the 

 sterno-thyroid muscle, with which it is considerably fused. The 

 anterior belly is tapering, and inserted into the lower border of 

 the hyoid bone at the side of sterno-hyoid. The postei-o-lateral 

 belly, which is the strongest, arises from the upper border of the 

 scapula close to the root of the coracoid process. All three bellies 

 meet in a Y-shaped junction, and a tendinous thread runs into 

 sterno-hyoid. 



The sterno-hyoid (text-fig. 30, S-H.M) arises from the back of 

 the upper part of the manubrium sterni, and is inserted into the 

 lower border of the hyoid bone. The opposite muscles first 

 diverge and then converge, and fibres pass between them on the 

 hyoid bone. The sterno-thyroid arises from the back of the 

 manubrium sterni and first costal cartilage and is inserted into 

 the upper part of the thyroid ala. Some fibres pass into the 

 thyro-hyoid muscle. 



The digastric muscle (text-fig. 30, D.M) is transitional between 

 Parson's first and third types. The anterior bellies are only 

 separate in front. They are fused behind where they arise from 

 the front of the body of the hyoid bone. Each belly is inserted 

 into the anterior two inches of each half of the mandible. The 

 posterior belly is bulky, but the tendon (text-fig. 30, P.B.D), which 

 enters the postero-lateral part of the anterior belly immediately 

 in front of the hyoid bone is long and slender. It arises from 



