MONOTREMATA. 



387 



cardiac, pulmonic, cesophageal, and gastric 

 nerves. The spinal accessory nerve (jig. 180, 

 c) is thicker than the pneumogastric, and has 

 the usual distribution. 



The brachial plexus is formed by the five 

 posterior cervical and the first dorsal nerves. 

 The third cervical nerve is shown at g,Jig- 180 . 

 The median nerve perforates the inner condyle 

 of the humerus. 



The lumbar plexus is formed by the two 

 posterior dorsal, the two lumbar, and the first 

 sacral nerve. 



The great ischiadic nerve divides into the 

 peroneal and tibial branches before it quits the 

 pelvis. The crural nerve is shown at h (jig. 

 180). 



DIGESTIVE SYSTEM. 



The Ornithorhynchus, which subsists on 

 aquatic insects, larvae, mollusks, and other 

 small invertebrates which conceal themselves 

 in the mud and banks of rivers, is provided 

 with a mouth which most nearly resembles the 

 flat and sensitive bill of a lamellirostral bird. 

 The singularly modified jaw-bones, already de- 

 scribed, are invested by a smooth coriaceous inte- 

 gument, (fig. 173, A, E, a,) devoid of hair, but 

 perforated by innumerable minute foramina. At 

 the base of the jaws this integument is produced 

 into a free fold, which overlaps the hairy covering 

 of the cranium immediately behind it. The 

 integument covering the upper mandible ex- 

 tends beyond the margins of the bone, and 

 forms a tumid, smooth, and highly sensible 

 lip ; the narrower and shorter under jaw is 

 more closely invested : the oral or upper sur- 

 face of the lateral part of the under jaw sup- 

 ports a series of about twenty nearly transverse 

 folds, increasing in breadth as they approach 

 the angle of the jaw : the corresponding sur- 

 face of the upper jaw is smooth. 



The two anterior horny teeth in both jaws 

 are elongated, narrow, with their outer part 

 raised into a trenchant edge in the lower jaw. 

 The two posterior teeth (Jig. 173, h, and F) in 

 both jaws are flat, with two broad and slight 

 excavations, corresponding with the two parts 

 into which each molar may be divided in the 

 young animal. 



Immediately on the outside of the posterior 

 part of each molar in the lower jaw, is the orifice 

 of an oblong cheek-pouch (jig. 180, F, F), about 

 two inches in length, and half an inch in diame- 

 ter : the pouch is continued backwards, and 

 is lined with a hard dry cuticle. 



The tongue (Jig- 186) consists of two 

 parts, the normal, anterior, narrower portion 

 (e), and a broad, raised posterior lobe (f), 

 analogous to the intermolar eminence of the 

 tongue in certain Rodents. This part is pro- 

 duced anteriorly into a free projecting apex in 

 the Ornithorhynchus, and is rendered still 

 more remarkable in that animal by being 

 armed with two short thick horny spines (g, g), 

 projecting forward. 



The anterior part of the tongue is beset with 

 rather coarse papilla?, and extends into the 

 posterior interspace of the incisive teeth, but 

 with the apex more than an inch distant from 



Fig. 186. 



the anterior aperture of the 

 mouth. The raised pos- 

 terior lobe of the tongue 

 must impede the passage 

 of unmasticated food to 

 the pharynx, and doubtless 

 tends to direct it on each 

 side into the cheek-pouch- 

 es; whence the Ornitho- 

 rhynchus may transfer its 

 store at leisure to the mo- 

 lar teeth, and complete its 

 preparation for degluti- 

 tion. An air-breathing 

 warm - blooded animal, 

 which obtains its food by 

 the capture of small aqua- 

 tic animals, while sub- 

 merged, must derive great 

 advantage from the struc- 

 ture which enables it to 

 transfer them quickly to 

 a temporary receptacle, 

 whence they may be ex- 

 tracted and masticated 

 while the animal is floating 

 on the surface or at rest in 

 Tongue and larynx of Jt s burrow. 



The S ft palate ' S thick ' 

 broad, and divided poste- 

 riorly into three fimbriated lobes. 



The pharynx is narrow, and is singularly en- 

 compassed by two posterior processes of the 

 thyroid cartilage (Jig. 189, c, c). 



The oesophagus becomes slightly dilated near 

 the diaphragm, below which it expands into a 

 moderate-sized membranous stomach (Jig. 187, 

 /), which is chiefly remarkable for the close 

 approximation of the cardiac and pyloric ori- 

 fices. The intestinal canal is moderately wide, 

 five feet three inches and a half in length, and 

 provided, at a distance of four feet three inches 

 from the pylorus, with a small and slender 

 cojcum (w). 



The small intestines are chiefly remarkable 

 for the extent of the mucous coat, which is 

 disposed in numerous folds or valvulce conni- 

 ventes : these are transverse at the beginning 

 of the duodenum, but are placed more or less 

 obliquely in the rest of the small intestine ; 

 they are about two lines broad, and placed 

 very close together in the duodenum, but 

 diminish in breadth and number as they ap- 

 proach the coecum coli. There are about fif- 

 teen longitudinal folds in the first half of the 

 colon ; the remainder of the intestine has a 

 smooth inner surface. There is no valvula coli. 

 The rectum (2) terminates at the anterior and 

 dorsal part of the vestibular compartment of 

 the cloaca. In fig- 191 a probe (6') is passed 

 through this termination. On each side of its 

 termination there is an oblong glandular pro- 

 minence, about four lines in length and two 

 in breadth, on which there are about ten ori- 

 fices of glands, which Meckel considers as 

 analogous to the anal glands of other quadru- 

 peds. 



The long, slender, tubular mouth of thr 



2 c 2 



