AVES 



607 



and the surrounding tissue. Urine passes from the collecting tubules 

 in the kidneys to the pelvis of each of the ureters, and through the 

 ureters into the cloaca. From here it passes to the exterior through 

 the anus with the feces. There is no urinary bladder. 



R. SUBCLAVIAN ARTERY 

 R. CAROTID A. 

 AORTIC ARCH 



BRACHIO-CEPHALIC A. 



BRACHIO-CEPHALIC A. 

 LEFT ATRIUM 

 PULMONARY ARTERIES 

 ENTRANCE OF L. SUBCLAVIAN V. 

 ENTR. OF PULMONARIES 

 SEMI-LUNAR VALVES 

 L.ATRIO- VENTRICULAR VALVE 

 RIGHT ATR.-VENTR. VALVE 

 ENTR. POSTCAVAL VEIN 

 RIGHT PRECAVAL VEIN 

 RIGHT ATRIUM 

 INTER-VENTRICULAR WALL 



RIGHT VENTRICLE 

 LEFT VENTRICLE 



Fis:. 323. — Horizontal section of heart of chicken to show chambers and valves 

 from ventral view. (Drawn by Titus Evans.) 



Nervous System 



The bird has a relatively well-developed brain and nervous sys- 

 tem. In an average chicken the brain weighs about 10 grams. 

 The cerebrum, the anterior division of the brain, shows fair develop- 

 ment but no convolutions, while the olfactory lobes which are acces- 

 sory to it are very small. The optic lobes of the midbrain are well 

 developed and the cerebellum is exceptionallj' developed. The cere- 

 bellum consists principally of a median lobe called the vermis or 

 worm. The ventral part of this division of the brain forms the roof 

 of the fourth ventricle of the medulla oblongata. 



The sense organs of sight and hearing show improvement when 

 compared with the reptiles. The eye is large, rounded laterally, and 

 rather flattened anteroposteriorly. It has the usual layers of outer 

 sclerotic coat continuous with the coriiea in front, the pigmental 



