288 ANATOMY OF THE DOMESTIC FOWL 



The fifth branch is given off from the second trunk and lies later- 

 ally. It is covered by the external head of the gastrocnemius, 

 passes along the vena saphena, and gives off a short main branch 

 to the inner side of the intertarsal joint. The main part passes 

 the tibial side of the joint, becomes subcutaneous, and finally 

 innervates the two plantar muscles. A lateral ascending inner 

 branch innervates the flexor perforatus digitorum, and, in company 

 with an outer branch, the external head of the gastrocnemius, and 

 also the flexor pedis perforatus. 



The plexus pedundus is formed from the spinal nerves coming out 

 of the plexus ischiadicus. These fibers emerge caudalward and 

 are directed horizontally. They frequently anastomose with each 

 other, especially on the pubic rim and on the outside of the plexus 

 ischiadicus. These branches are deeply imbedded in the kidney 

 substance and innervate the pubio-coccygeus, or depressor coccygis 

 lateralis, the ilio-coccygeus, the transversalis, the sphincter, and 

 other muscles of this region, and the skin of the anal region. 



THE BRAIN 



The Brain Coverings. The cerebro-spinal axis of birds is similar 

 to that of mammals. The meninges of the brain are three in number, 

 dura mater, arachnoid, and pia mater. The dura mater is the 

 thickest. It is constructed of white fibrous connective tissue, and 

 lines the cranial cavity. Thus it serves as an internal periosteum. 

 It is continuous with the spinal dura mater at the foramen magnum, 

 and is also prolonged as a sheath of the nerves. In birds of flight 

 where the air sacs and reservoirs are developed to the highest 

 state, Sappy finds that, " just as the medullary tissue is replaced by 

 air in the bones of birds, so might it be imagined that the sub-arach- 

 noid fluid is also replaced by air around the spinal cord," and ob- 

 servations justify the correctness of this statement. The .dura 

 mater measures exactly the volume of the marrow in birds; so that 

 there does not exist between the fibers and the nervous surface 

 any space for an accumulation of liquid. This anatomical fact is 

 sufficient to demonstrate the absence of subarachnoid fluid in the 

 bird. In denying the existence of this fluid, it ought to be added 

 that in this class of vertebrates, the spinal prolongation is covered 

 by a triple envelope; and that between the pia mater and the dura 

 mater is a thin transparent membrane, which is lubricated by a 



