INTESTINE AND BRAIN 



109 



p. 109) are characteristic of the group, and the latter hone is 

 peculiar to the Monotremata among mammals. So, too, is the 

 large coracoid. In the scapiila there is a spine which coincides 

 with the anterior border of that bone. The arrangement of the 

 nmscles in this region proves conclusively that this projection is 

 the homologue of the spine and the acromion of other mammals. 

 Here, again, we have a point of likeness to the Cetacea.^ In 

 the pelvis the acetabulum is perforate (in Echidna), as in 

 Saiiropsida. 



Considering the numerous very 

 archaic features which the general 

 structure of this group displays, 

 it is surprising to find how typi- 

 cally mammalian they are in 

 certain other peculiarities. The 

 mammalian diaphragm, one of 

 the distinguishing features of the 

 class, is perfectly normal in the 

 Monotremata. The alimentary 

 canal shows no great divergences 

 from the normal structure. The 



stomach is almost globular, with Fig._52.— Side view of right half of tlie 



a projecting pyloric region in 

 Orn ithorliynclius ; the intestine 

 is divided into a " small " and 

 " large " intestine by a slender 

 caecum. The liver has the sub- 

 divisions that this organ usually 

 shows in the Mammalia. How- 

 ever, the presence of the ventral mesentery and of the 

 abdominal vein in Echidna and Ornithorhynchus has already 

 been mentioned as a distinctive character. The peculiar and 

 apparently partly primitive valve of the right ventricle has been 

 described above (see p. 66). The brain is in most respects 

 mammalian in its characters, but naturally shows some important 

 differences. Dr. Elliot Smith, who has most recently studied 

 this question,^ is of opinion that the size of the cerebral hemi- 

 spheres is not at all reptilian ; indeed, it " greatly exceeds that of 



' Muscular insertions and attacliments do not, however, altogether support the 

 comparison. ^ Journ. Anat. Phys. 1899, p. 309. 



shoulder girdle of a young Echidna 

 (Echidna aculeata). x 1. a, Acro- 

 mion ; c, coracoid ; c&, coracoid border ; 

 cl, clavicle ; ess, coraco- scapular 

 suture ; ec, epicoracoid ; gh, glenoid 

 border ; gc, glenoid cavity ; ic, inter- 

 clavicle ; pf, postscapular fossa ; j3S, 

 presternum ; s, spine ; ss, suprascapu- 

 lar epii^hysis ; ssf, subscapular fossa. 

 (From Flower's Osteology. ) 



