464 THE CAT. [chap. xiii. 



in the cat. As has been said, it unites dorsally with many vertebrte 

 in one large mass, while ventrally it is widely open. The acetabu- 

 lum is perforated. 



The ^fibuIa is but a needle of bone fused below with the tibia, but 

 the most surprising thing is, the apparent absence of a tarsus alto- 

 gether. 



Directly below the tibia there is only one long bone, which repre- 

 sents three metatarsals united in one and supporting three digits, while 

 at the lower end of its inner side is a small separate metatarsal bone 

 for the fourth digit. In fact, in the rook, the tarsus has partly 

 united with the tibia, and partly with the metatarsus, so that the 

 joint is placed not between the tibia and the tarsus, but in the 

 middle of the tarsus itself, which has become blended with adjacent 

 parts — the proximal part of the tarsus anchylosing with the tibia, 

 while its distal part anchyloses with the tarsus. ^ 



As in the cat, so here, there are only four digits, but in the rook 

 it is the representative of the outermost or little digit, not of the 

 hallux, which is absent. Counting from within outwards, the 

 numbers of the phalanges are two, three, four and five, a difierent 

 number for every digit. 



The spinal marrow has the lower parts of its canalis centralis 

 expanded, while at the same time in the brain the ventricles are 

 smoll and there is no fifth ventricle. 



The smaller hemispheres are also not connected by any corpus 

 callosum, and instead of corpora quadrigemina there are two lobes 

 called optic, placed one on each side in a lateral and depressed 

 position. The two Eustachian tubes open into the alimentary canal 

 by a single aperture. 



The great artery, the aorta, sends out only one branch, which 

 arches over the right bronchus. The right auriculo-ventricular 

 valve is muscular and not merely membranous as in the cat. 



The lungs are not freely suspended in the thorax, but are fixed 

 to the back of its cavity, and the bronchi do not divide dichoto- 

 mously within the lungs. Moreover, they communicate with air- 

 sacs, which not only extend within the body but even enter certain 

 of the bones, which are thus themselves filled with air. _ Thus the 

 circulating and respiratory structures are such as to give to their 

 functions an even greater perfection than in the cat. This perfec- 

 tion is needed for the almost constant activity of the bird, and the 

 enormous muscular power it has to exert in flying. Where the 

 trachea divides into the two bronchi there is a special arrange- 

 ment of parts called the si/rinx, or " lower larynx," the upper larynx 

 (wliich corresponds with the larynx of the cat) being much smaller. 



The ureters do not open into the bladder, but into a chamber or 

 reservoir (the cloaca), into which the bladder also opens, while the 

 cloaca itself opens on the surface of the body. It is also into this 

 cloaca that the posterior end of the alimentary tube opens, and not 

 on the surface of the body. Into it also open the ducts from the 

 testes in the male, and the (here single) Fallopian tube of the 



