AVES. 



91 



the usual intercostal artery in mammalia and passing before the head of the rib, the 

 other being continued down from the vertebral behind the same process, and both 

 becoming joined into one to form the intercostal artery. 



In birds there are the same leading parts as in mammalia, as the voluntary motive, 

 the involuntary, the true visual and sensitive centres and tracts. The voluntary motive 

 centres are contained in the exterior region of the brain, and the tracts pass as the 

 cms of the brain underneath the optic tract, from the motive centres to the anterior 

 portion of the oblong medulla. The oblong medulla is large, as in mammalia, and a 

 considerable portion of it consists of the involuntary centre. The involuntary centre 

 has its appropriate convolution, which is placed a little posterior to that of the true 

 visual tract, and occupies the same place in the oblong medulla as in mammalia ; it 

 becomes intimately united with the <3ptic tract. The true visual tract occupies the 

 same place as in mammalia, but is situated nearer to the margin of the anterior lobe 

 and more horizontally ; its tract is inserted into the optic tract near the optic com- 

 missure. Each sensitive centre occupies the posterior surface of the oblong medulla, 

 and the floor of the fourth ventricle forming the ventricular cords ; it extends upwards, 

 and communicates with the optic lobe, the thalamus, and the parietes of the lateral 

 ventricle. 



The optic tract begins from the surface of the thalamus ; it receives a large 

 addition from the optic lobe ; it is broad and short ; the pedicle of the striated septum 

 joins the optic commissure in the same manner as part of the anterior crus of the fornix 

 in mammalia. The true visual tract terminates in the optic tract ; the involuntary 

 centre also becomes connected with it ; at the base, in some instances, it receives an 

 addition from a geniculate body ; it meets the opposite optic tract in the optic com- 

 missure, becomes connected with the eminence surrounding the infundibulum, and then 

 terminates in the perfect optic nerve. 



The striated septum answers in many respects to the character of the fornix. 

 From its conjunction with the convolutions, and from having in its composition so 

 much grey matter, it has higher functions than the fornix as a consensual agent 

 between the several senses. The fornix, however, contains masses of grey matter in 



N 2 



