182 \-ETERINAKY PATHOLOGY. 



uncommon, but large areas of liver tissue are probably never 

 regenerated, although some pathologists claim that they have 

 observed the regeneration of the major portion of a liver lobe 

 in the dog, cat and rabbit. Kidney cells, especially of the tubules, 

 are constantly regenerated, although the regeneration of an 

 entire tubule has never been observed. The testicular and ovar- 

 ian tissues are probably never regenerated except in the physio- 

 logic maintainance of spermatogenesis and oogenesis. 



Muscular tissue is imperfectly regenerated. Injuries of invol- 

 untary muscular tissue are usually repaired by the substitution 

 of fibrous tissue which may later be replaced by involuntary mus- 

 cular tissue, the latter being derived from the adjacent muscle 

 cells. Two or three days after an injury to a voluntary muscle 

 fibre, the nuclei near the injury divide and a multinucleated 

 protoplasmic mass is formed on the damaged fibre. These pro 

 toplasmic masses extend into the substituted fibrous tissue and 

 may split longtitudinallv into regular fibres but more frequently 

 they die and disintegrate. Destroyed heart muscle cells are 

 invariably replaced by fibrous tissue. 



Nerve cells are not regenerated, at least in adult animals, 

 although their processes, axones and dendrites, are regenerated in 

 peripheral nerves. After a nerve fibre is injured the axone 

 degenerates to the distal end and to the first or second node of 

 Ranvier proximally. A few days after the injury the axone, if 

 its continuity has not been destroyed, begins to elongate, ex- 

 tending peripherally, in the direction of least resistance, which 

 is in the old sheath. If the axone extends in the original sheath 

 the tissue deprived of its nerve supph^ may become perfectly 

 innervated. The rate of growth of an axone has been variously 

 estimated at from .1 mm. to 1mm. in twenty-four hours. Foot 

 lameness in horses that has been completely relieved by meta- 

 carpal and metatarsal neurectomies, sometimes reappear, in 

 from eighteen months to three years after the operation, thus 

 indicating that there has been reinnervation. If the proliferating 

 axone does not continue in the original nerve sheath it may 

 become entangled and coiled up in the scar tissue, of the wound, 

 thus producing sensitive scars and amputation neuromata. 



