228 THE FROG— THE PLATANA 



membrane, and gives rise dorsally to the stalk of the pineal body. 



Its thickened sides are the optic thalami, and its cavity is the 



third ventricle. 

 (d) The optic lobes, a pair of ovoid bodies. 

 {e) The cerebellum, a narrow transverse band immediately behind the optic 



lobes. 

 (/) The medulla oblongata, behind the cerebellum. It gradually tapers 



towards its posterior end, where it is continuous with the spinal cord. 



On its roof the pia mater forms a very vascular choroid plexus. Its 



cavity is the fourth ventricle. 

 Make a sketch of the brain. (See Fig. 84, page 232.) 



Seventh Day. (Revision) 



With a second specimen revise the second, third, and fourth day's work, 

 paying particular attention to the vascular system. 



Eighth Day 



Nervous System (continued). 



B. The Peripheral Nervous System. 



1. The Spinal Nerves. 



Carefully remove the abdominal viscera so as to expose the spinal nerves. 

 These appear as white cords on each side of the vertebral column and are as 

 foUows : — 



(a) The first or hypoglossal nerve arising between the 1st and 2nd verte- 



brae. It runs forwards on the under surface of the head, beneath the 

 mylo-hyoid muscle, and supplies the tongue. It was noted in the 

 third day's work. 



(b) The second and, third nerves unite immediately after issuing from the 



spinal cord to form the brachial plexus, which supplies the muscles of 

 the shoulder, and then extends along the arm. 



(c) The fourth, fifth, and sixth nerves are small, and supply the muscles and 



the skin of the body- wall. 



(d) The seventh, eighth, and ninth nerves unite to form the sciatic plexus 



which gives rise to the large sciatic nerve extending along the thigh 

 and dividing above the knee into the peroneal and tibial nerves to 

 the leg and the foot. The seventh nerve, before it joins the plexus, 

 gives rise to an ileoh5^ogastric nerve supplying the skin and muscles 

 of the abdomen. Just beyond its union with the sciatic plexus it 

 gives rise to the cruralis nerve to the skin and muscles of the outer 

 surfaces of the thighs. The sciatic nerve also gives rise to the semi- 

 membranosus nerve (already noted in the third day's work) which 

 supplies the semimembranosus muscle. 



