GENERAL SURVEY OF THE FROG 5 



tongue forward and feel the floor of the mouth behind the tongue. It is stiffened 

 by a cartilaginous plate, the body of the hyoid, whose anterior end is hollowed 

 out to receive the base of the tongue, and gives off a pair of slender curving 

 processes, the anterior horns of the hyoid, which extend posteriorly to the ears. 

 Scrape away the membrane from the floor of the mouth in order to see these 

 structures more clearly. Posterior to the body of the hyoid in the median line 

 is a circular hardened elevation, the laryngeal prominence, which bears in its 

 center an elongated slit, the glottis. Where does the glottis lead ? At the back 

 of the mouth cavity, roof and floor converge to a large opening, the beginning of 

 the esophagus, the first portion of the digestive tract. In the male frog, the 

 slitlike opening of the vocal sac is present on each side of the floor near the edge 

 of the jaw, on a level with the glottis. On the use of the vocal sacs in croaking, 

 see Holmes (p. 167). 



Draw the floor and roof of the mouth. 



D. BODY WALL, COELOME, MESENTERIES 



i. Structure of the body wall (see Holmes, chap, iv, pp. 73-80). Remove 

 the skin slowly from the trunk of the frog, noting carefully at what places the 

 skin is attached to the underlying parts by means of weblike partitions. (While 

 doing this note also the blood vessels to the skin described in the next paragraph, 

 and in the median dorsal line, the sensory nerves passing in pairs from the skin 

 into the vertebral column.) The space under the skin is divided by these parti- 

 tions into compartments, called the subcutaneous lymph spaces, or lymph sacs, 

 which in life are filled with a fluid, the lymph. Compare your observations with 

 Holmes (Fig. 78, p. 281), and read what Holmes says about the lymphatic 

 system. The lymph is similar in composition to and derived from the blood, 

 except that it contains no red blood corpuscles. It is in direct contact with the 

 living substance to which it supplies food and oxygen, and from which it removes 

 waste products. The frog and its relatives differ from other vertebrates in this 

 enormous development of huge lymph spaces, not only under the skin, but also 

 throughout the body. This structural feature is probably associated with the 

 amphibious habits of these animals. 



In removing the skin, note the extensive supply of blood vessels to the skin, 

 and particularly the following two large vessels: the musculo-cutaneous vein, 

 which runs posteriorly in the muscles of the ventro-lateral region of the body 

 wall, and then turns and passes to the skin in the partition between the abdominal 

 and lateral lymph sacs; and the cutaneous artery, which emerges in front of the 

 shoulder and supplies the skin of the dorsal side. This relatively large develop- 

 ment of skin blood vessels is due to the respiratory function of the skin, to be 

 discussed more fully later. 



The removal of the skin exposes the muscles of the body wall and the skeleton 

 which they inclose and to which they are attached. The principal parts of the 



